Intermolecular decay mechanisms in helium nanodroplets · Intermolecular decay mechanisms in helium...

Intermolecular decay mechanisms in helium nanodroplets

A. C. LaForge∗1, M. Shcherbinin‡, F. Stienkemeier∗, R. Richter†, M. Mudrich‡

∗ Physikalisches Institut, Universität Freiburg, 79104 Freiburg, Germany† Elettra-Sincrotrone Trieste, Basovizza, 34149 Trieste, Italy

‡ Department of Physics and Astronomy, Aarhus University, 8000 Aarhus, Denmark

Figure 1. a) electron kinetic energy distributions corre-lated to K+-Rb+ ion-ion coincidences (black line). b)ion kinetic energy distributions correlated to K+-Rb+

ion-ion coincidences for K (black line) and Rb (redline). The photon energy is 21.6 eV. The droplet sizeis 50 000 He atoms.

As opposed to molecular systems where electrondynamics proceed only through intramolecular pro-cesses, weakly bound complexes create an environ-ment in which locally excited electrons can addition-ally interact with neighboring molecules leading tonew intermolecular decay mechanisms. Intermolec-ular Coulombic decay (ICD) [1] is a particularly in-teresting decay mechanism which occurs when localelectronic decay is energetically forbidden. ICD of-fers a new, ultrafast decay path, typically on the fem-tosecond timescale, where energy is exchanged witha neighboring atom leading to its ionization. Since itsdiscovery, ICD has been observed in a wide variety ofweakly-bound systems from He dimers to biological

systems such as water clusters [2]. Electron trans-fer mediated decay (ETMD) [3] is an additional in-termolecular decay mechanism where charge transferreleases energy leading to ionization of surroundingmolecules. Although generally considered a weakerdecay, ETMD, however, has gained new interest asit was predicted and shown to occur from the ionicground state where ICD is not allowed [4, 5].Here, we present a systematic study of intermoleculardecay mechanisms of mixed alkali dimers (K-Na, K-Rb, Na-Rb) in He nanodroplets using synchrotron ra-diation. By coincidence imaging techniques, we canfully characterize and energetically resolve the differ-ent decay paths with mass-correlated photoelectron/-ion spectra. In particular, we observed a new de-cay mechanism where the alkali dimer was doubly-ionized by the excited He atom. The process is simi-lar to that observed by Buchta et al. [6] where the en-ergy of an excited He atom is transferred to the neigh-boring atom leading to ionization. Here, we showthe process can even lead to double ionization of thedimer followed by Coulomb explosion. The photo-electron spectrum (Fig. 1a) shows the characteristicU-shaped distribution similar to single photon doubleionization [7] while the photoion spectrum (Fig. 1b)shows the energetic ionic fragments from Coulombexplosion. In addition, we observed electron transfermediated decay [4, 5] between alkali dimers and Heions. By varying the attached dimers, we can gaina better understanding of the interaction strength de-pending on the internuclear distance and the role ofnuclear dynamics in the process.

References

[1] L. S. Cederbaum et al. 1997 Phys. Rev. Lett. 79 4778

[2] T. Jahnke 2015 J. Phys. B: At., Mol. Opt. Phys. 48082001

[3] J. Zobeley et al. 2001 J. Chem. Phys. 115 5076

[4] V. Stumpf et al. 2014 Phys. Rev. Lett. 112 193001

[5] A. C. LaForge et al. 2016 Phys. Rev. Lett. 116 193001

[6] D. Buchta et al. 2013 J. Phys. Chem. A 117 4394

[7] R. Wehlitz et al. 1991 Phys. Rev. Lett. 67 37641E-mail: [email protected]

Wide-angle diffraction reveals 3D shapes of superfluid helium nanodroplets

Bruno Langbehn∗1, Yevheniy Ovcharenko∗,†, Daniela Rupp∗, Katharina Sander‡, Christian Peltz‡,Andrew Clark§, Riccardo Cucini¶, Paola Finetti¶, Michele Di Fraia¶, Denys Iablonskyi‖, Aaron C.

LaForge#, Verónica Oliver Álvarez de Lara§, Oksana Plekan¶, Paolo Piseri&, Toshiyuki Nishiyama4,Carlo Callegari¶, Kevin C. Prince¶,◦, Kyoshi Ueda‖, Frank Stienkemeier#, Thomas Fennel‡, and Thomas

Möller∗

∗ Institut für Optik und Atomare Physik, Technische Universität Berlin, 10623 Berlin, Germany† European XFEL GmbH, 22869 Schenefeld, Germany

‡ Institut für Physik, Universität Rostock, 18059 Rostock, Germany§ Laboratoire Chimie Physique Moléculaire, Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne,

Switzerland¶ Elettra - Sincrotrone Trieste, 34149 Trieste, Italy

‖ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan# Physikalisches Institut, Universität Freiburg, 79104 Freiburg, Germany

& CIMAINA and Dipartimento di Fisica, Università degli Studi di Milano, 20133 Milano, Italy4 Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan

◦ IOM-CNR TASC Laboratory, Basovizza, 34149 Trieste, Italy

SynopsisRecent pioneering experiments on superfluid helium nanodroplets revealed strong deformations of the particles’ shape projec-tions that were attributed to high angular momenta [1, 2]. We have used intense XUV pulses from the FERMI free-electronlaser to record diffraction patterns of helium nanodroplets up to large scattering angles that allow for a unique determinationof the droplets’ 3D shape. We find axisymmetric oblate, triaxial prolate and even two-lobed droplets, that are in line with theobserved shapes of non-superfluid spinning drops.

With the advent of free-electron lasers (FEL) de-livering femtosecond short-wavelength pulses, co-herent diffractive imaging methods have been devel-oped to gain insight into the structure of unsupportednanoparticles such as viruses or clusters. While ex-periments using light pulses in the X-ray regime aimat atomic resolution [3], full 3D information on theparticle shape and orientation from a single diffrac-tion pattern requires access to wide-angle scatteringsignal, available at longer wavelengths [4].In our experiment at the FERMI-FEL’s LDM end-station [5], diffraction patterns of single heliumnanodroplets were recorded using intense 100 fslight pulses at XUV photon energies ranging from19 to 62 eV.While the majority of the 45,000 bright scattering im-ages exhibit centrosymmetric rings, thus indicatingspherical droplet shapes, about 10% of the imagesshow diffraction patterns of non-spherical particles.In particular, a tilt of a deformed droplet out ofthe scattering plane produces features in the wide-angle diffraction pattern that break the point symme-try (cf. Fig. 1). In order to simulate these features, amulti slice Fourier transform (MSFT) algorithm wasemployed, similar to that described in Ref. [4]. Byassuming simple model droplet shapes and matchingthe MSFT simulations to our data, the droplets’ axesand volume could be retrieved. When compared to a

numerical model of non-superfluid rotating drops [6]our data show unexpectedly good agreement. Thisfinding raises questions on the role and implicationsof superfluidity and vortices in helium nanodropletswith high angular momenta.

Figure 1. Tilting a deformed droplet out of the scatter-ing plane (left) produces features in the diffraction pat-tern (right) that break the point symmetry.

References

[1] L. F. Gomez et al. 2014 Science 345 906-9

[2] C. Bernando et al. 2017 Phys. Rev. B 95 064510

[3] K. Ayyer et al. 2016 Nature 530 202-6

[4] I. Barke et al. 2015 Nat. Comm. 6 6187

[5] V. Lyamayev et al. 2013 J. Phys. B 46 164007

[6] K. A. Baldwin, S. L. Butler and R. J. A. Hill 2015 Sci.Rep. 5 7660

1E-mail: [email protected]

Laser-Induced Fluorescence Spectroscopy ofPTCDA and PDI in Helium Nanodroplets

S. Izadnia∗, M. Bohlen∗, A. LaForge∗, C. A. Rice∗, Y. Xu†1, W. Jäger†2, F. Stienkemeier∗3

∗ Institute of Physics, University of Freiburg, Hermann-Herder-Str. 3, 79104 Freiburg, Germany† Department of Chemistry, University of Alberta, Edmonton, Canada, T6G 2G2

Synopsis Electronic spectra of the S1 ← S0 transition of the perylenetetracarboxylic dianhydride (PTCDA) and perylenediimide (PDI) embedded in helium nanodroplets have been measured by laser-induced fluorescence. These two neutral speciespreserve the central perylene chromophore; however, the two oxygens in the aromatic ring of the former are exchanged forN–H. The spectral signatures of PTCDA and PDI are compared to each other and presented.

Perylene dyes are useful for the absorption of in-tense visible light, high structural stability, electronaccepting ability, and near-unity quantum yields. [1]The physical properties of these molecules makethem attractive for optoelectronic and photovoltaicdevices, thermographic processes, energy-transfercascades, light-emitting diodes, and near-infrared-absorbing systems. [2] By chemical substitution,these dyes can be engineered to absorb or emitthroughout the whole visible spectrum.

Perylenetetracarboxylic dianhydride (PTCDA)and perylene diimide (PDI) are two such moleculesthat belong to the class of perylene dyes (Figure 1).PTCDA monomer and oligomers embedded in he-lium nanodroplets has been previously studied bylaser-induced fluoroscence (LIF). [3, 4] The ori-gin band of the S1 ← S0 transition of PTCDAmonomer in a helium droplet is at 20 987.8 cm−1,while no clear origin in the electronic spectrum ofdimers and larger clusters is discernible. By ex-changing the two central oxygens on the ends ofPTCDA with N–H, one has the molecular geome-try of PDI. The monomer of PDI in a helium droplethas an origin band of the S1 ← S0 electronic transi-tion at 20 681.6 cm−1, being redshifted to PTCDA by≈ 300 cm−1.

Furthermore, in the electronic spectrum of PDI,there are absorption features redshifted to the ori-gin band. Some of these increase in intensity uponheating and are attributed to dimers of PDI; however,not all bands to the red are due to dimers formed inthe helium nanodroplet. An assignment of the elec-tronic spectrum of PDI and a comparison of PDI with

PTCDA will be presented.

Figure 1. Planar molecular structures of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) andperylene diimide (PDI). The central perylene chro-mophore is preserved in both PTCDA and PDI.

References

[1] C. Huang et al. 2011 J. Org. Chem. 76, 2386

[2] T. Weil et al. 2010 Angew. Chem. Int. Ed. 49, 9068

[3] M. Wewer & F. Stienkemeier 2004 J. Chem. Phys.120, 1239

[4] J. Roden et al. 2011 J. Chem. Phys. 134, 054907

1E-mail: [email protected]: [email protected]: [email protected]

Vibrational Spectroscopy of Ions Trapped in Helium Nanodroplets: Application

to the Analysis of Biomolecules

Daniel A. Thomas† 1

, Eike Mucha†,§

, Ana Isabel González Flórez†,§

, Kevin Pagel†,§

, and Gert von

Helden† 2

† Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, Berlin 14195, Germany

§ Institute of Chemistry and Biochemistry, Freie Universität Berlin, Takustrasse 3, Berlin 14195, Germany

Synopsis The capture of trapped ions in helium nanodroplets allows for vibrational spectroscopy of cold ions

ranging from tens of Da to several kDa in mass. Here we take advantage of this method to gain detailed insight

into the structure of biomolecular ions, including peptides, proteins, and oligosaccharides.

Helium nanodroplets provide an ideal envi-

ronment for vibrational spectroscopy due to

their extremely low equilibrium temperature

(~0.4 K) and minimal matrix-induced spectral

perturbation. Whereas the capture of a neutral

molecule in a nanodroplet is accomplished us-

ing a pickup chamber containing analyte vapor,

ions can be captured in helium nanodroplets by

instead directing a nanodroplet beam through an

ion trap (Figure 1a). In addition to providing a

direct method to embed ions in helium

nanodroplets, such an approach has the added

advantage that significantly larger molecules

can be made available for uptake. Thus, large

biomolecules up to tens of kDa in mass can be

captured in helium nanodroplets and studied by

vibrational spectroscopy.

Figure 1. (a) schematic of instrument utilized for the

vibrational spectroscopy of ions captured in helium

nanodroplets [1]; (b) schematic of process leading to

cooling of ions and subsequent ejection upon ab-

sorption of resonant infrared photons.

An overview of the instrumentation utilized

for vibrational spectroscopy of biomolecular

ions is shown in Figure 1a [1]. Ions are generat-

ed by electrospray ionization and m/z selected

by a quadrupole mass filter before transfer to a

hexapole ion trap. A fraction of these ions are

then picked up by a pulsed beam of helium

nanodroplets traversing the trap. The helium

droplet with embedded ion possesses sufficient

kinetic energy to escape the potential well of the

hexapole trap and travel to the laser interaction

region. Ions are ejected from the helium droplet

by the absorption of resonant photons and de-

tected by a TOF MS. The measurement of ion

signal as a function of wavelength therefore

provides an infrared spectrum of the embedded

ions.

Figure 1b illustrates the process of ion up-

take and ion ejection upon absorption of reso-

nant infrared radiation. Following ion uptake,

the doped nanodroplet undergoes evaporative

cooling to return to a temperature of ~0.4 K,

and subsequent absorption of resonant photons

results in ion ejection. Although the exact ejec-

tion mechanism is not known, studies to date

suggest a nonlinear process in which ions are

ejected prior to complete droplet vaporization

[2, 3].

This experimental approach can be utilized

to capture both cationic and anionic molecules

ranging in size from tens of Da to several kDa

and subsequently probe their structure utilizing

vibrational spectroscopy. This work will discuss

the latest applications of the method to the

structural analysis of biomolecular ions, includ-

ing the structure of peptides and proteins in the

gas phase as well as the detailed fingerprinting

of complex oligosaccharide molecules.

References

[1] González Flórez A I et al. 2016 Angew. Chem.,

Int. Ed. 55 3295–9

[2] Zhang X, Brauer N B, Berden G, Rijs A M and

Drabbels M 2012 J. Chem. Phys. 136 044305

[3] González Flórez A I et al. 2015 Phys. Chem.

Chem. Phys.

1 E-mail: [email protected]


Large Helium Droplets from Pulsed Source

Deepak Verma* 1, Andrey F. Vilesov* 2

*Department of Chemistry, University of Southern California, Los Angeles, CA-90089, USA

Synopsis A modified electromagnetic pulsed valve has been used to produce large helium droplets. We discuss the

optimal cooling of the valve by a close cycled refrigerator. As a result low temperature of the nozzle of T < 5 K could be

reached during operation at 20 Hz. Production of He droplets containing up to 1011 atoms is reported.

Helium (He) nanodroplets have long been

used as cryogenic matrices for spectroscopy of ionic

and neutral clusters and complexes.[1] The

continuous cryogenic expansion at temperatures as

low as 5 K, has been used to produce large droplets

containing up to about 1012 atoms with diameter up

to about 2 μm.[2,3] On the other hand, it is much

more difficult to reach low temperature with a pulsed

electromagnetic valves due to ohmic heat release

during the operation. Pulsed helium droplet beam

yields much larger peak droplet density, which

correspondingly assures larger signals if pulsed laser

or electron beam techniques of interrogation are

applied. It is for these reasons that pulsed He droplet

beam would prove advantageous in experiments on

coherent diffractive imaging with free electron

lasers, as described in our recent

publications.[2,3,4,5]

Here we report on the formation of large

sized helium droplets, <NHe> = 105-1011, from a

modified Parker Series 99 pulsed nozzle valve at

stagnation pressures of 5 and 10 bar and

temperatures of 4-15 K with a nozzle diameter of 0.5

or 1 mm. The average sizes were obtained by

attenuation of the droplet beam with collisional

helium gas at room temperature.[6] Additionally, the

helium flux ejected from the nozzle was estimated

using a sensitive fast ionization gauge, which

showed ~100 times larger values as compared to a

continuous beam source, as previously reported.[7]

Ratio of M = 16 to M = 8 intensity peaks were

furthermore investigated at different nozzle

temperatures using a quadrupole mass spectrometer

(QMS) which utilizes an electron impact ionizer. At

lower nozzle temperatures, the ratio increases due to

new pathways of He4+ ion production in large

droplet. The pulsed valve setup comprises of the

nozzle part shielded within a copper enclosure

providing effective thermal contact between the

pulsed valve body and the second stage of the close

cycle refrigerator. The result is lower nozzle

temperatures down to ~ 4K, which is almost twice

lower than attainable temperatures in our earlier

setup.[6,7]

References

[1] J. P. Toennies and A. F. Vilesov 2004 Angew. Chem.,

Int. Ed. 43 2622

[2] L. F. Gomez et al. 2014 Science. 345 906

[3] C. Bernando et al. 2017 Phys. Rev. B 95 064510

[4] R. M. P. Tanyag et al. 2015 Struct. Dyn. 2 051102

[5] C. F. Jones et al. 2016 Phys. Rev. B 93 180510

[6] L. F. Gomez et al. 2011 J. Chem. Phys. 135 154201

[7] M. N. Slipchenko et al. 2002 Rev. Sci. Instrum. 73

3600



Ground state stability of quantum dipolar filaments in BECs

Fabio Cinti *, †, 1

*National Institute for Theoretical Physics (NITheP), Stellenbosch 7600, South Africa †Institute of Theoretical Physics, Stellenbosch University, Stellenbosch 7600, South Africa

Recent experiments on BECs with dipolar interac-

tions have observed a clear stabilization of the gas

(typically dysprosium or erbium) in a three-

dimensional setup [1,2]. Such remarkable results

rekindled interest in systems characterized by aniso-

tropic dipolar-interactions. Yet the microscopic

mechanisms that cause these interesting quantum

regimes is still under debate. Along these lines, we

are undertaking an investigation of cluster phases

made up of dipolar bosons [3]. Upon increasing the

strength of the dipolar interaction we find a wide

region where filaments of several bosons are stabi-

lized against short wavelength instabilities by quan-

tum fluctuations [4]. Most interestingly by compu-

ting the local superfluid fraction we conclude that

coherence is preserved up to strong interactions.

Quantum Monte Carlo simulations at finite temper-

ature confirm the stability of such filaments against

thermal fluctuations.

References

[1] H. Kadau et al., Nature 530, 194 (2016).

[2] L. Chomaz et al., Phys. Rev. X 6, 041039 (2016).

[1] F. Cinti et al., arXiv:1610.03119.

[2] F. Cinti, M. Boninsegni, to be submitted.

*, †, 1 E-mail: [email protected]

Gold doped helium nanodroplets: from atomic spectroscopy to localized surface

plasmon resonances in deposited nanoparticles

Florian Lackner*1, Roman Messner*, Alexander Schiffmann*, Maximilian Lasserus*, Martin Schned-

litz*, Daniel Knez†, Georg Haberfehlner†,‡, Gerald Kothleitner†,‡, Ferdinand Hofer†,‡ and Wolfgang E.

Ernst2*

* Institute for Experimental Physics, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria † Graz Centre for Electron Microscopy, Steyrergasse 17, 8010 Graz, Austria

‡Institute for Electron Microscopy and Nanoanalysis, Graz University of Technology, Steyrergasse 17, 8010 Graz, Austria

We present the first electronic excitation spectrum of gold atoms inside helium nanodroplets as well as a new

approach for the study of optical properties of deposited Au nanoparticles synthesized inside helium nanodroplets.

The Au-HeN excitation spectrum exhibits characteristic blue shifted and broadened features caused by the interac-

tion of the atom with the surrounding helium. For the study of deposited Au nanoparticles electron energy-loss

spectroscopy is applied, enabling the investigation of localized surface plasmon resonances of selected, individual

nanoparticles subsequently to their preparation in helium nanodroplets.

Gold nanoparticles are of great importance in

a variety of fields such as catalysis, analytical

chemistry, biology and even medicine because of

their outstanding plasmonic properties combined

with chemical inertness. Many of these applica-

tions are based on localized surface plasmon res-

onances.

We focus on the optical properties of Au atoms

and nanoparticles and present the first investiga-

tion of the electronic excitation spectrum of sin-

gle gold atoms isolated in helium nanodroplets as

well as an investigation of the localized surface

plasmon resonance in gold nanoparticles subse-

quently to their deposition on substrates.

In contrast to Au-HeN, the lowest excited states

in Ag-HeN [1] and Cu-HeN [2] have already been

explored. Using resonant two photon ionization

spectroscopy, we recorded the lowest two elec-

tronic transitions in gold atoms inside helium

nanodroplets. In addition to the characteristic

broadening and blueshift, the bare Au mass chan-

nel exhibits a sharp line identified as a transition

originating from the 5d96s2 2D manifold, reveal-

ing that the helium environment opens up a re-

laxation channel which is forbidden in the free

atom.

When deposited on suitable substrates, metal na-

noparticles formed inside helium nanodroplets

can be studied by means of transmission electron

microscopy. Using electron energy-loss spec-

troscopy, localized surface plasmon resonances

of individual nanoparticles can be characterized

in a transmission electron microscope [3]. It is

important to note that this method not only gives

access to optically allowed dipole modes, but

also to bulk and quadrupole modes. We will pre-

sent first results for gold nanoparticles with di-

ameters less than 10 nm. Note that in contrast to

the works by the Vilesov group on Ag particles

in helium nanodroplets [4], our approach charac-

terizes the nanoparticles after deposition. Fur-

thermore, the flexibility of the helium droplet

isolation approach allows the deposition of nano-

particles on other substrates such as fused silica

in quantities that enable an investigation by

means of UV/vis spectrophotometry. After find-

ing suitable particle size regimes and deposition

rates, this method may serve as complementary

approach for the study of localized surface plas-

mon resonances in nanoparticles prepared by he-

lium droplets.

Our work on Au nanoparticles points out an ex-

citing new direction for the field of helium drop-

let assisted synthesis of nanoparticles. Consider-

ing that it has been shown that the method can be

used to form core-shell nanoparticles [5,6], we

think that it will become possible to produce and

characterize novel, plasmonic core-shell nano-

particles which cannot be formed with conven-

tional approaches.

References

[1] Loginov et al. 2007, J. Phys. Chem. A, 111,

7504-7515

[2] Lindebner et al. 2014, Int. J. Mass. Spec., 365,

255-259

[3] Scholl et al. 2012, Nature, 483, 421-427

[4] Loginov et al. 2012, Phys. Rev. Lett., 106,

233401

[5] Haberfehlner et al. 2015, Nature Communica-

tions, 6, 8779

[6] Thaler et al. 2014, Phys. Rev. B, 90, 155442



Capture of heliophilic atoms by quantized vortices in 4He nanodroplets

François Coppens∗ 1, Francesco Ancilotto†,‡ Manuel Barranco∗,§,¶ Nadine Halberstadt∗ Martí Pi§,¶

∗ Université Toulouse 3 and CNRS, LCAR-IRSAMC, 118 route de Narbonne, F-31062 Toulouse Cedex 09, France† Dipartimento di Fisica e Astronomia “Galileo Galilei” and CNISM, Università di Padova, via Marzolo 8, 35122

Padova, Italy‡ CNR-IOM Democritos, via Bonomea, 265 - 34136 Trieste, Italy

§ Departament FQA, Facultat de Física, Universitat de Barcelona. Diagonal 645, 08028 Barcelona, Spain¶ Institute of Nanoscience and Nanotechnology (IN2UB), Universitat de Barcelona, Spain

Synopsis We present a computational study, based on time-dependent Density Functional theory, of the real time trapping ofAr and Xe atoms by superfluid 4He nanodroplets either pure or hosting a quantized vortex line.

Recent experiments have shown evidence thatquantized vortices in superfluid helium induce co-alescence of solvated impurities, which becometrapped along their cores, eventually resulting in theformation of nano-sized filaments[1], thus pointingforward to a potentially new method of producingnanowires and nanotubes[2, 3, 4]. Moreover, dopingvortices with impurities has emerged as a valuablepractical tool to image the vortex presence and dy-namics, especially in the presence of vortex tanglesand vortex reconnections.

In this work we present results obtained withinTDDFT[5] for the collision and capture of Xe andAr atoms by a 4He1000 droplet at different kinetic en-ergies and impact parameters. Due to the relevanceof the interaction of foreign impurities with quan-tized vortices, special attention is paid to the time-dependent interaction of Xe and Ar atoms with he-lium nanodroplets hosting a vortex line.•We study the capture of Xe atoms by a 4He nan-

odroplet, both for head-on collisions and for differentimpact parameters, with velocities ranging from ther-mal values up to several hundreds m/s. The results ofperipheral collisions with different values of the im-pact parameter are used to estimate the cross sectionfor the Xe capture;• We study how a Xe atom dynamically inter-

acts with a droplet hosting a vortex line, under dif-ferent initial conditions resulting in different velocityregimes of the impurity as it collides with the vortexcore: (i) a Xe atom initially located in the interior ofthe droplet and close to the vortex core; (ii) a Xe atominitially at rest on the droplet surface sinking underthe effect of solvation forces; (iii) a head-on collisionof Xe and Ar atoms against the 4He nanodroplet.

We find that Xe and Ar atoms at thermal veloc-ities are readily captured by helium droplets, with acapture cross section similar to the geometric crosssection of the droplet.

Also, upon capture, during 50 ps the Xe (or Ar)atom wanders in the bulk of the droplet at velocitiesof a few tens of m/s. If the droplet hosts a vortex linealong the diameter of the droplet, the thermal impu-rity is captured by the vortex line (see also Ref. [6]).

We have found that the capture is helped by anadditional energy transfer from the impurity to thedroplet as compared to a capture by a non-vorticalstate[5]; indeed, large amplitude displacements of thevortex line take place in the course of the captureof the impurity by the vortex, constituting the mainsource of the kinetic energy lost by the impurity.

In short, what is of fundamental interest is thatat thermal energies, most of the impurity energy islost in the collision process. Then, if the impact pa-rameter is such that the impurity is captured, thereare two facts that contribute to the eventual meet-ing and capture of the Xe/Ar atom and the vortexline, namely that the vortex line is fairly robust (an-gular momentum conservation) and remains near thedroplet symmetry axis, and that the equilibrium posi-tion of Xe/Ar is in the bulk of the droplet.

References

[1] L.F. Gomez, E. Loginov, and A.F. Vilesov, Phys. Rev.Lett. 108, 155302 (2012).

[2] E. Latimer, D. Spence, C. Feng, A. Boatwright, A.M.Ellis, and S. Yang, Nano Lett. 14, 2902 (2014).

[3] Ph. Thaler, A. Volk, F. Lackner, J. Steurer, D. Knez,W. Grogger, F.Hofer, and W.E. Ernst, Phys. Rev. B 90,155442 (2014).

[4] V. Lebedev, P. Moroshkin, B. Grobety, E.B. Gordon,A. Weis, J. Low Temp. Phys. 165, 166 (2011).

[5] Coppens, F., Leal, A., Barranco, M. et al, J LowTemp Phys (2016). doi:10.1007/s10909-016-1690-xand references therein.

[6] I.A. Pshenichnyuk and N.G. Berloff, Phys. Rev. B 94,184505 (2016).


Dilute magnetic droplets of a bosonic erbium quantum fluid

G.Faraoni*† 1, L.Chomaz* 2, S. Baier*, D. Petter*, J. H. Becher*§, R. van Bijnen‡, M. J. Mark*‡ , and

F. Ferlaino*‡

* Institut für Experimentalphysik,Universität Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria

† Dipartimento di Fisica e Astronomia, Universitá degli Studi di Firenze,Via Sansone 1, 50019 Sesto Fiorentino

(Florence), Italy §Physikalisches Institut, Universität Heidelberg, Im Neuenheimer Feld 226, 69120 Heidelberg, Germany

‡Institut für Quantenoptik und Quanteninformation, Osterreichische Akademie der Wissenschaften, 6020 Innsbruck,

Austria

Synopsis We report on the investigation of droplet formation in quantum fluids of bosonic erbium.

Due to their large magnetic moment and exotic electronic configuration, erbium atoms are an

ideal platform for exploring the competition and interplay between inter-particle interactions of differ-

ent nature, in particular isotropic contact interactions and anisotropic and long-range dipole-dipole

interactions. When these two interactions are made of opposite sign and almost balance each other, a

novel many-body quantum phase appears: a high-density quantum droplet-like phase. Here, an ordi-

nary Bose-Einstein condensate (BEC) transforms into a liquid-like state, where atoms are bound by

high correlations and beyond-mean field effects. These surprising phases have been recently observed

in experiments with dysprosium (Dy) and erbium (Er), and extensively studied in theory. We present

the quantum droplet phenomena from the Innsbruck perspective and discuss our experimental results

using an ultracold gas of Er atoms.

† 1 E-mail: [email protected]

[email protected] 2 E-mail: [email protected]

Fingerprints of angulon instabilities in spectra of matrix-isolated moleculesIgor Cherepanov∗1, Mikhail Lemeshko∗2

∗ IST Austria (Institute of Science and Technology Austria), Am Campus 1, 3400 Klosterneuburg, Austria

Synopsis We use the concept of the angulon quasiparticle to simulate ro-vibrational spectra of light molecules trapped insuperfluid 4He nanodroplets. In this work, we demonstrate that previously predicted angulon instabilities can explain theanomalous broadening of individual spectral lines measured for CH3 and NH3 in experiment. Transitions to the instabilities areaccompanied by a creation of one phonon with non-zero angular momentum.

The recently introduced angulon model [1] basedon a quasiparticle approach to the problem of a ro-tating molecular impurity immersed in a many-bodymedium has already affirmed its applicability to 4Henanodroplets. It showed a good agreement withexperimental data on renormalization of rotationalconstants for a wide range of both light and heavymolecules [2], [3] and also allowed to describe dy-namical properties of molecules in nanodroplets thatwere studied in femtosecond laser-induced alignmentexperiments [4]. A quasiparticle approach primarilysimplifies a theoretical consideration of such com-plex systems and makes it possible to understand themain peculiarities of interactions of molecules withsuperfluid helium environment.

One of the most curious predictions of the angu-lon theory is the existence of so-called angulon insta-bilities [1], taking place for the angulon states withnon-zero angular momentum. A transition to the in-stability corresponds to a transfer of angular momen-tum λ from the molecule to helium. Here we demon-strate that these instabilities appear exactly for thoselevels which are involved in the transitions showingthe anomalous broadening (>50 GHz) in the experi-mental ro-vibrational spectra of the ν3 band of CH3[5] and NH3 [6] molecules.

Fig.1 shows the angulon spectral function illus-trating a set of angulon states. Angulon levels are la-beled as L jkλ , where L is the total angular momentumof the system, j and k give the angular momentumof the molecule and its projection on the molecularz-axis respectively and λ is the angular momentumof helium excitations. The instability appears in thearea inside the black frame. The transition to this in-stability (indicated by the blue arrow) from the lowerangulon level leads to the creation of one phononwith λ = 3 while the molecule undergoes the forbid-den transition to the molecular state 11. The valueα = 1.12 reproduces the following line width of theRR1(1) line: 43 GHz for CH3 and 46 GHz for NH3.

Thus, this work provides a strong evidence thatthe angulon instabilities have already been observed

in experiments on molecules in superfluid helium.

Figure 1. The angulon spectral function for CH3 as afunction of the dimensionless energy E = E/B (B is arotational constant) and the α parameter regulating thestrength of the CH3-helium interaction. The blue arrowcorresponds to the transition to the instability observedin the experiment and assigned as the RR1(1) line.

References

[1] R. Schmidt, M. Lemeshko 2015 Phys. Rev. Lett. 114203001

[2] M. Lemeshko 2017 Phys. Rev. Lett. 118 095301

[3] Y. Shchadilova 2017 Physics 10 20

[4] B. Shepperson et al. 2017 arXiv:1702.01977v1

[5] A. M. Morrison et al. 2013 J. Phys. Chem. A 11711640

[6] M. N. Slipchenko, A. F. Vilesov 2005 Chem. Phys.Lett. 412 176

1E-mail: [email protected]: [email protected]

Time resolved excited state dynamics of 1-Iodonapthalene molecules studied

inside helium droplets

James D. Pickering1*, Lars Christiansen*, Henrik Stapelfeldt*

* Department of Chemistry, Aarhus University, 8000 Aarhus C, Denmark

Synopsis Time-resolved measurements on the excited state of 1-iodonaphthalene have been measured for isolat-

ed molecules and molecules inside helium droplets.

The dynamics of electronically excited 1-

iodonaphthalene (INaph) molecules inside he-

lium droplets have been studied using time-

resolved ion yield spectroscopy (TRIY). A

120fs 266nm pump pulse was used to excite the

Inaph from the ground state to the excited state.

The population in the excited state was moni-

tored by measuring the ion yield of singly ion-

ised molecules as a function of time delay be-

tween the pump pulse and a non-resonant 40fs

800nm probe pulse. By keeping the intensity of

the probe pulse low enough not to ionise from

the ground state, we are selective to only the

excited state. The decay in ion yield from mol-

ecules excited outside and inside helium drop-

lets reveal different timescales, thus the excited

state decay inside helium droplets is modified

by the environment. These experiments pave

the way for better understanding the electroni-

cally excited states in the time domain where

little is known compared to the frequency do-

main.

The TRIY measurements performed here are a

first step towards more elaborate time resolved

excited state experiments. For isolated mole-

cules time-resolved photoelectron spectroscopy

has proven to be an extremely powerful tech-

nique to study the evolution of excited states of

molecules [1]. By using the photoelectron as an

observable more information can be extracted.

In particular if the molecules are prealigned and

the photoelectron angular distributions are

measured [2].

References

[1] A. Stolow and J. G. Underwood, Advances in

Chemical Physics 2008, 139:497-584

[2] Katharine L. Reid, Annu. Rev. Phys. Chem. 2003.

54:397–424

E-mail: [email protected]

Time resolved relaxation dynamics of Ak atoms

attached to He nanodroplets: the Rb 5p case

Johannes von Vangerow*2, Frank Stienkemeier* and Marcel Mudrich‡1

* Phys. Institut, Universität Freiburg, Hermann-Herderstr. 3, 79104 Freiburg, Germany ‡ Department of Physics and Astronomy, Ny Munkegade 120, 8000 Aarhus C, Denmark

We study dynamics of Rb atoms attached to the surface of He nanodroplets and excited to the droplet perturbed 5p states by means of femtosecond (fs) pump-probe photoelectron and photoion imaging spectroscopy and time of flight mass spectrometry. Relaxation from ∏3/2 to ∏1/2 is observed to occur with a time constant of ~1ns, causing dopant ejec-tion.

In this poster presentation, experimental results addressing droplet induced relaxation following electronic excitation of Rb atoms attached to He nanodroplets will be presented. In a two-color fs pump-probe scheme, the pseu-dodiatomic ∏1/2, ∏3/2, and ∑1/2 states correlating to the 5p states of Rb are excited. Subsequently, emerging Rb and RbHen=1,2 products are ion-ized. Velocity Map Imaging of photoions and photoelectrons allows to detect charged photo-fragments angularly and energetically resolved. Photoelectron spectra suggest the presence of an efficient relaxation mechanism from the ∏3/2 state to the ∏1/2 state as proposed by Brühl et al. [1]. The time constants of the different occur-ring processes as well as the relation between electronic relaxation and exciplex formation will be discussed. This experimental study is

complementary to theoretical work based on time dependent density functional theory. It is an extension of recent combined theoretical and experimental work focusing on the real time excited state dynamics of doped He nanodroplets[2]. In addition, results referring to the hot topic talk “Highly excited molecular iodine inside helium nanodroplets” will be presented.

References

[1] F. R. Brühl, R. A. Trasca and W. E. Ernst, J. Chem. Phys., 115, 10220 (2001). [2] J. v. Vangerow, F. Coppens, A. Leal, M. Pi, M. Barranco, N. Halberstadt, F. Stienkemeier and M. Mudrich, J. Phys. Chem. Lett., 8, 307-312 (2017).

1 E-mail: [email protected] 2 E-mail: [email protected]

Reactive Scattering between Metastable Helium and Magneto-OpticallyTrapped Lithium Atoms

J. Grzesiak, S. Hofsäss, F. Stienkemeier, M. Mudrich, K. Dulitz1

Department of Physics, University of Freiburg, Hermann-Herder-Str. 3, 79104 Freiburg, Germany

The experimental study of Penning ionization re-actions, i.e., the reactive scattering of metastable raregas atoms with neutral species, has recently attracteda lot of attention through the observation of orbitingresonances at low collision energies [1].Our project is aimed at a detailed study of reactivePenning collisions between quantum-state-selectedlithium atoms (Li) and metastable helium atoms(He*) in order to elucidate the influence of spin po-larization on the reaction rate. Our setup consists ofa cryogenically cooled source for the production ofvelocity-tunable, supersonic beams of metastable he-lium atoms and a magneto-optical trap (MOT) for ul-

tracold Li atoms which serves as a stationary scat-tering target. By selectively switching off the MOTlaser beams, we can selectively populate one of thetwo hyperfine components of the 2S1/2 electronicground state of Li. In this contribution, we willpresent first experimental results for reactive Penningcollisions between He* and Li at different collisionenergies, including a detailed discussion of the su-personic beam and MOT characteristics.

References

[1] A. B. Henson, S. Gersten, Y. Shagam, J. Narevicius,E. Narevicius 2012 Science 338 234


Small clusters attached to He nanodroplets under EUV photoexcitation:

Photoelectron-photoion coincidence spectroscopy

In this talk, we will present in brief a few representative results from our investigations of small

atomic and molecular clusters attached to He nanodroplets interrogated by coincident

photoelectron-photoion spectroscopy at the Elettra synchrotron. When photoexcited between

20…30eV across the atomic ionization threshold of He, we find intriguing behaviour of the systems

attached to the host He droplets both by Penning transfer of energy from the single-photon excited

He matrix to the dopant system, as well as by charge-transfer from He to the dopant system when

photon energies were sufficient to directly ionize the droplet. Using alkali atomic dopants attached

to the droplet surface as well as small molecules immersed in the droplet, we explore the fascinating

dynamics of these complex atomic systems.

References:

1) D Buchta, S R Krishnan, et al., The Journal of chemical physics 139 (8), 084301 (2013)

2) Ibid., The Journal of Physical Chemistry A 117 (21), 4394-4403 (2013)

3) A C LaForge et al., Physical review letters 116 (20), 203001.

On the submersion of alkali metals in prestine and dopedhelium-nano droplets

Lorenz Kranabetter∗1, Alexander Kaiser∗, Michael Renzler∗, Matthias Daxner ∗, Andreas W. Hauser ¶,Wolfgang E. Ernst ¶, Albrecht Lindinger †, Robert E. Zillich ‡, Andrew M. Ellis §, Paul Scheier ∗2;

∗ Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, A-6020 Innsbruck, Austria¶ Institut für Experimentalphysik, Technische Universität Graz, A-8010 Graz, Austria

† Institut für Experimentalphysik, Freie Universität Berlin, 14195 Berlin, Germany‡ Institut für Theoretische Physik, Kepler Universität Linz, A-4020 Linz, Austria

§ Department of Chemistry, University of Leicester, Leicester LE1 7RH, United Kingdom

Superfluid helium nanodroplets (HND) haveproven to be an interesting environment for study-ing ion-molecule reactions and that is especially truefor alkali metal compounds. The electronic structureof alkali metals is the reason for this special interest,because it is introducing a repulsive potential – thePauli repulsion – into the interaction. Therefore allalkali metals atoms are classical heliophobes and donot stay submerged inside a HND but do reside in asmall dimple on the surface of the droplet, even smallcluster of alkali metals are not solvated in the helium.Theoretical prediction by Stark and Kresin suggestedthat there should be an upper limit of atoms, after thislimit is met the cluster is gaining an energy advan-tage if it submerges into the HND [1]. In order to testthis theory we doped a beam of HND with differentalkali metals and subsequently ionized them via elec-tron bombardment. Then a mass to charge distribu-tion is measured with a high-resolution time of flightmass spectrometer. By scanning through the ioniza-tion energies – because there are two different mainchannels for surfaced and submerged atoms or clus-ter – one can obtain information about the positionof the cluster via their respective ion yield. To optaininfomation about the impact of a second species in-side the HND C60 was added to he helium dropletsafter it was doped with the alkali meatal atoms. Forthe submersion of the alkali metal into prestine HNDthe theory and the experiment are in good agreementto each other and for sodium and photasium there is

evidence that suggests a lower limit for the criticalcluster size of 21 respectively 80 [2, 3]. The exper-imental data for the doped HND seem to establishthe picture that every alkali cluster or atom with thesole exception of cesium can be submerged into thedroplet in presence of a Buckminster fullerene [4].

Figure 1. Normalized ion yield versus the electron en-ergy for different alkali C60 compounds [4].

References

[1] C. Stark and V. Kresin 2010 Phys. Rev. B81 085401

[2] L. An der Lan et. al 2012 Phys. Rev. B85 115414

[3] L. An der Lan et. al 2011 J. Chem. Phys. 135 044309

[4] M. Renzler et. al 2016 J. Phys. Chem. 45 181101

1E-mail: [email protected]: [email protected]

Collective effects in atomic and helium droplet systems revealed byphase-modulated pump-probe spectroscopy

Lukas Bruder∗1, Ulrich Bangert∗, Marcel Binz∗, and Frank Stienkemeier∗

∗ Physics department, University of Freiburg, 79104 Freiburg, Germany

Synopsis Phase-modulated pump-probe spectroscopy is used to selectively detect multi-atom resonances in atomic vapors anddoped helium droplet beams. A distinct phase signature is observed for the individual resonances. The observation of theseeffects is surprising since interparticle interactions should be insignificant in these systems.

Time-resolved nonlinear spectroscopy hasopened many new directions to study ultrafast dy-namics in complex quantum systems [1, 2, 3]. Whilemost applications have been to the condensed phase,we are focusing on dilute gas phase samples, inparticular, on doped helium droplet beams. To ac-count for the small densities in our samples, wehave recently significantly improved the sensitivityin our femtosecond pump-probe scheme by adaptinga phase modulation technique combined with phase-synchronous lock-in detection [4].

Incorporating harmonic lock-in detection, wehave further extended this technique to enable selec-tive probing of higher-order nonlinear signals. Withthis, we observed for the first time the collective ex-citation of up to four atoms in a dilute atomic vapor(ρ ≈ 108 cm−3) [5]. The origin of these signals is dis-cussed since interatomic interactions are very smallin these systems [6]. Similar signatures are observedin doped helium droplet beams, where interactionsbetween dopants sitting on different droplets shouldbe even less significant.

The detected one- to four-atom resonances scaleall linearly with the sample density which may seemcounter intuitive. However, this behavior was alsoobserved for other collective effects such as inter-atomic coulombic decay (ICD) [7, 8].

Most striking is the distinct phase signatures ob-served for the collective resonances in our experi-ments. High resolution measurements (∆ν ≈ 1 GHz)have revealed that this phase behavior is connectedto the fundamental hyperfine sub-levels in the sys-tem. In Fig. 1 we show data obtained for a rubidiumvapor. Here, the one-atom transitions 5S1/2→ 5P3/2,5S1/2 → 5D5/2 exhibit the same phase signature. Incontrast, the two- to four-atom response clearly re-veals different phases for the individual hyperfine

transitions.

x10

5S1/2 5P3/2

2x 5S1/2 5P3/2

3x 5S1/2 5P3/2

4x 5S1/2 5P3/2

5S1/2 5D5/2

Figure 1. Spectra for one- to four-atom excitation (la-beled 1H-4H) in a rubidium vapor. The 1H spectrumshows the 5S1/2→ 5P3/2 transition with the onset of thehyperfine sub-structure. In the 2H spectrum the two-photon excitation 5S1/2 → 5D5/2 of a single atom canbe seen as well as the collective two-atom excitation2×5S1/2→ 5P3/2. The 3H and 4H show the same col-lective excitation for three and four atoms, respectively.

References

[1] T. Brixner et al. 2005 Nature 434 625

[2] N.S. Ginsberg et al. 2009 Acc. Chem. Res. 42 1352

[3] A. Nemeth et al. 2010 J. Chem. Phys. 132 184514

[4] L. Bruder et al. 2015 Phys. Chem. Chem. Phys. 1723877

[5] L. Bruder et al. 2015 Phys. Rev. A 92 053412

[6] S. Mukamel 2016 J. Chem. Phys. 145 0411022

[7] A.I. Kuleff et al. 2010 Phys. Rev. Lett. 105 043004

[8] A.C. LaForge et al. 2014 Sci. Rep. 4 3621


Collinear phase-modulated femtosecond pump-probe experimentsusing a low repetition-rate laser system

Marcel Binz∗1, Lukas Bruder∗, Ulrich Bangert∗, Katharina Schneider∗, and Frank Stienkemeier∗

∗ Institute of Physics, University of Freiburg, Hermann-Herder-Str. 3, 79104 Freiburg im Breisgau, Germany

Synopsis A 5 kHz repetition-rate laser system is used to perform phase-modulated pump-probe spectroscopy on rubidiumvapor. We confirmed the applicability of high-frequency lock-in modulation with such low repetition-rate lasers and show thatmuch higher modulation frequencies than laser repetition-rates can be used without losing performance. This undersamplingtechnique can be advantageous for any other lock-in measurements with low sampling rates by shifting the lock-in demodulationto a higher frequency for more efficient noise filtering.

The helium nanodroplet isolation (HENDI) tech-nique is a well-established powerful method to per-form spectroscopic studies at very low temperatures.Due to the low target densities in doped droplet beamexperiments, coherent time-resolved spectroscopy ofsuch systems has remained a challenging task. In thiscontext, we are investigating the phase-modulationtechnique established by Marcus et al. [1]. Thecombination of continuous acousto-optical phase-modulation with lock-in detection greatly improvesthe signal-to-noise ratio (SNR) and the sensitivity inthis scheme [2]. However, the method was thoughtto be suitable only for high repetition-rate laser sys-tems (> 200 kHz) to be able to sufficiently sample theimparted acousto-optic modulation.

Recently, we have successfully implemented thistechnique in a pump-probe scheme with femtosec-ond laser pulses at 5 kHz repetition-rate. As a simplemodel system, we investigated the 52S1/2→ 52P1/2transition in a rubidium vapor. We found, what seemsat first very counterintuitive, that much higher mod-ulation frequencies than laser repetition-rates canbe used without losing performance (see Fig. 1).This effect, which we call phase-synchronous un-dersampling, shows promise for the implementa-tion of the phase-modulation scheme in even lowerrepetition-rate XUV laser sources by shifting thecarrier frequency far away from the low frequencynoise spectrum. Besides this application, the phase-synchronous undersampling scheme can be trans-ferred to any other lock-in measurement where it is

desirable to demodulate at higher frequencies thanthe sampling rate in the experiment.

- 1 0 1 2 3 4 5 6 2 0 2 1 2 2 4 0 4 1 4 20

2 55 07 5

1 0 01 2 51 5 01 7 52 0 0

m o d u l a t i o n f r e q u e n c y [ k H z ]

SNR

ν n y q

u n d e r s a m p l i n g r e g i m e

Figure 1. Dependence of the SNR in the pump-probemeasurements with the 5 kHz repetition-rate laser onthe modulation frequency. The dashed line representsthe Nyquist frequency (νnyq), which is given by half thelaser repetition-rate. For modulation frequency higherthan νnyq, the acousto-optic modulation is sampled byless than two laser shots per period. We call this theundersampling regime. Surprisingly, the SNR does notshow a decrease when reaching this regime.

References

[1] A. H. Marcus et al. 2006 J. Chem. Phys. 125 194303



Building Carbon Bridges on and between Fullerenes in Helium Nanodroplets

S. A. Krasnokutski†1, J. Postler*, M. Goulart*, M. Kuhn*, A. Kaiser*,

A. Mauracher*, M. Renzler*, D. K. Bohme‡, P. Scheier*2

* Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, A-6020 Innsbruck, Austria,

† Laboratory Astrophysics Group of the Max Planck Institute for Astronomy, University of Jena, 07743 Jena, Germany,

‡ Department of Chemistry, York University 4700 Keele Street, Toronto, Ontario, Canada M3J 1P3.

Synopsis: We report mass spectrometric investigations and the observation of sequential encounters of fuller-

enes with C atoms in helium nanodroplets. Density functional theory calculations were also performed to inter-

pret the experimental results.

Ever since the discovery of fullerenes by

Kroto et al. [1] and even more since the recent

laboratory confirmation of fullerene presence in

the interstellar medium [2], fullerene formation

and reaction schemes have been subject of a

lively debate. Here we report the observation of

sequential encounters of fullerenes with C at-

oms in an extremely cold environment. The ex-

periments were performed with helium

nanodroplets at 0.37 K doped with C60 mole-

cules and C atoms derived from a pure source

of C atoms. They were subsequently exposed to

electrons at a controlled energy and investigated

using a high-resolution time-of-flight mass

spectrometer. The mass spectra revealed the

formation of carbenes of the type C60(C : )n

with n up to 6. Bridge-type bonding of the C

adatoms to form the known dumbbell

C60=C=C60 was also observed.

To interprete the experimental findings, we em-

ployed density functional theory calculations at

the B3LYP/6-31g(d) level that elucidated the

carbene character of the C60(C : )n species and

their structures.

Figure 1. Graphic representation of molecular struc-

tures formed by single carbon addition in He clusters

doped with fullerene molecules.

References

[1] Kroto, H. W. et al. 1985 Nature 318 162;

[2] Campbell, E. K. et al. 2015 Nature 523 322.

Error! Reference source not found. E-

mail: [email protected] Error! Reference source not found. E-

mail: [email protected]

Photofragmentation of C60+Hen

M. Kuhn1, M. Renzler, J. Postler, S. Ralser, S. Spieler, M. Simpson, M.K. Beyer, R. Wester, A. Lin-

dinger*, J. Cami†, A.G.G.M. Tielens‡, H. Linnartz‡ and P. Scheier2

Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Austria *Institut für Experimentalphysik, Freie Universität Berlin, Germany

†Department of Physics and Astronomy, The University of Western Ontario, Canada

‡Leiden Observatory, Leiden University, Netherlands

Recently several laboratories developed new

helium tagging techniques to enable action spec-

troscopy for complex molecular ions [1]. One pos-

sibility to tag a molecule with helium is to use heli-

um nanodroplets as a matrix. Even though the in-

tensity of an ion in the mass spectrum with only one

He atom attached is lower than in other methods,

the high number of attachable helium atoms renders

the method as an important alternative.

One molecule of interest for this method is C60,

which is subject of extensive research in our work-

ing group [2]. Therefore, one of our helium cluster

sources was adapted to combine photofragmenta-

tion with mass spectrometry. Experimental results

obtained by this setup show a remarkable linear

absorption redshift of 0.07 nm per added He up to

C60+He32 (see Figure 2), where all faces of C60 are

occupied with one He atom (Figure 1). Combined

with MD simulations and DFT calculations the re-

sults indicate a phase transition from solid to liquid

to superfluid for the solvation of C60+ in helium.

Furthermore, our results confirm that C60+ may be

the carrier of at least four diffuse interstellar bands

as previously reported by John Maiers group [1].

The presently utilized method can be easily ap-

plied for various other ionic species such as polycy-

clic aromatic hydrocarbons or other fullerenes solv-

ated in helium or hydrogen. It also allows fast scans

through large frequency ranges searching for un-

known resonances of ionic complexes that might be

relevant in the interstellar medium.

Figure 1. C60+ He complex boils of Helium after absorp-

tion of an IR photon

Figure 2. Center positions of the absorption spectra

of C60+Hen around 959 and 965nm

References

[1] Campbell et al. 2015: Laboratory confirmation of

C60+ as the carrier of two diffuse interstellar bands. Na-

ture 523, p322-323. doi:10.1038/nature14566

[2] Kuhn et al. 2016: Atomically resolved phase transi-

tion of fullerene cations solvated in helium droplets. Na-

ture Communications 7. doi:10.1038/ncomms13550



From rare gas to hydrogen-bonded clusters:Size distributions of supersonic beams from a pulsed valve using the titration

technique

Matthias Bohlen∗1, Aaron LaForge∗, Rupert Michiels∗, Nicolas Rendler∗, Mykola Shcherbinin†, andFrank Stienkemeier∗

∗ Molecular- and Nanophysics Group, Physics Institute, University of Freiburg, Hermann-Herder-Str. 3, D-79104Freiburg, Germany

† Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark

Synopsis We present recent improvements and characterizations of a versatile pulsed valve, which can produce supersonic gaspulses down to 20 µs duration at repetition rates of several hundred Hz. Cluster size distributions are measured by the so calledtitration technique, which we applied to argon, water, and ammonia clusters.

Pulsed valves offer many advantages over contin-uous beam sources such as higher beam densities andreduced gas load. Recently, we developed a pulsedvalve in collaboration with UBC, Vancouver, whichproduces supersonic gas pulses down to 20 µs du-ration at repetition rates up to several hundred Hz.The pulsed-valve driver can be adjusted for optimalvoltage amplitude, duration and repetition rate.

Figure 1. Home-built control unit for adjusting the pa-rameters (pulse length, voltage and repetition rate) ofthe CRUCS valve.

Moreover we established different versions of thevalve to accomodate different needs, such as im-proved heat conductance, or chemical resistance. Theoverall design and geometry of the valve is retained,making it versatile to produce rare-gas and evenhydrogen-bonded clusters.

Additionally, cryogenic cooling of the valve canbe utilized to produce helium nanodroplets. To es-

timate the cluster size distribution, we use a titra-tion technique [1], which has accurately determinedcluster sizes of continuous supersonic beams. Here,we report on a systematic study of cluster size dis-tributions by varying expansion parameters. Thetechnique has been applied to argon, ammonia, andwater clusters, and the results are compared to mod-els of Hagena [2] and Bobbert [3], respectively.

Figure 2. Front and Side View of the CRUCS Valvewith Visualisation of size

References

[1] L. F. Gomez, E. Loginov, R. Sliter, A. F. Vilesov 2011J. Chem. Phys. 135 154201

[2] O. F. Hagena 1981 Surface Science 106 101-116

[3] C. Bobbert, S. Schütte, C. Steinbach, U. Buck 2002Eur. Phys. J. D 19 183-192


Mass spectrum processing of TriMethylAluminium embedded in liquid Helium

Matúš Sámel* 1, Martin Kuhn† and Paul Scheier† 2

* Faculty of mathematics, physics and informatics, Comenius University, 842 48 Bratislava, Slovakia,

† Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, A-6020 Innsbruck, Austria.

Synopsis In this contribution we present the positive mass spectrum of TriMethylAluminium (TMA), which is one of the

possible precursor candidates for FEBID. We have also determined and discussed the magic numbers of most promi-

nent cluster series. We suppose that these cluster numbers are more abundant because of favourable structural

conditions, but subsequent calculations have to be carried out to verify our assumptions.

Focused Electron Beam Induced Deposition

(FEBID) is a direct-writing technique with na-

nometer resolution. After few decades of con-

tinuous development FEBID has reached a

stage, when this technique is particularly attrac-

tive for several areas in basic and applied re-

search [1, 2]. However, nature of precursors af-

ter collisions with electron beam and their sub-

sequent fragmentation or clustering play a ma-

jor role in FEBID process, therefore further re-

search of precursors, among other things, is

needed.

The positive mass spectrum presented in this

contribution has been taken on the “ClusTOF”

apparatus at the University of Innsbruck. In

brief, Helium nanodroplets are produced by ad-

iabatic expansion, then pass through the pick-up

chamber, where investigated sample in gas

phase is embedded into the droplets. The doped

droplets subsequently pass through the ioniza-

tion region and finally reach the time-of-flight

mass spectrometer, where the produced ions are

detected in a form of mass spectra [3, 4]. In this

contribution we will discuss a positive mass

spectrum of TriMethylAluminium (TMA), with

chemical formula of Al(CH3)3.

Let’s emphasize, that in processed spectrum

we have identified hundreds of peaks forming

tens of cluster series and in this contribution, we

will present just the most prominent series.

The Fig. 1 contains signal derivative to clus-

ter number plot of the most prominent cluster

series of the positive spectrum. The series be-

longs to clusters of [TMA]n-1[DMA]1, which

corresponds to loss of one methyl ligand from

intact clusters of [TMA]n. The magic numbers

for n = 3, 6, 9, 12 and 15 are clearly observable.

Each signal peak of these numbers does not

necessarily have the highest signal intensity in

spectrum, but it is almost as prominent as previ-

Figure 1 The signal derivative of most prominent

TMA cluster series as a function of cluster number

ous one. Moreover, a significant signal drop

occurs at subsequent number. We assume that

molecules of TMA under used experimental

conditions are more likely to form clusters with

mentioned cluster magic numbers, because of

the favourable structures, probably with lower

potential energy.

So far however, these are just our assump-

tions, therefore theoretical simulations have to

be carried out to verify them in order to bring

deeper insight and understanding into nature of

TMA and eventually more sophisticated FEBID

applications.

This project has received funding from the

European Union's Horizon 2020 research and

innovation programme under grant agreement

No 692335.

References

[1] M. Huth et al. 2012 Belstein J. Nanotech. 3 597

[2] J. Teresa et al. 2016 J. Phys. D: Appl. Phys. 49

243003

[3] A. Lan et al. 2011 J. Chem. Phys. 135 044309

[4] M. Harnisch et al. 2015 J. Chem. Phys. 119

10919



Theoretical study of the dynamics of superfluid helium nanodropletsdoped with potassium

Maxime Martinez∗1, François Coppens∗, Manuel Barranco∗†, Nadine Halberstadt∗, Martí Pi†,

∗ Laboratoire des Collisions, Agrégats, Réactivité , IRSAMC, UMR 5589, CNRS et Université Paul Sabatier-Toulouse 3,118 route de Narbonne, F-31062 Toulouse Cedex 09, France

† Departament FQA, Facultat de Física, and IN2UB, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Spain

Synopsis The goal of this work is to study the photodesorption dynamics of a single atom of potassium from the surface ofsuperfluid helium nanodroplets using the most recent and powerful theoretical simulation tools.

During the past few years, several real time dy-namics experiments have been conducted on super-fluid helium nanodroplets doped with alkali atomsusing femtosecond pump-probe laser techniques [1].Alkali atoms are particularly interesting as dopantsbecause of their very weakly attractive interactionwith helium, which makes them reside in a dimple atthe droplet surface [2, 3]. Upon photoexcitation theyusually desorb [3], except the heavy alkalis Rb andCs excited close to the gas phase D1 transition [4, 5].This is due to the strong repulsion between the ex-cited state electronic orbital, which is much morediffuse and spread out than in the ground state, andthe surrounding helium. The process can be rathercomplex, since the nanodroplet can absorb and dissi-pate part of the recoil energy as density waves and/oratom evaporation. In addition, the alkali atom canbring along one or a few helium atoms and desorb asan exciplex [1, 6].

From a theoretical point of view, the light massof helium makes it a challenge to study the real timedynamics of this process because of quantum ef-fects. The Helium density functional theory (He-DFT) approach and its time-dependent version (He-TDDFT) are very efficient semi-empirical methodswhich work with the helium density rather than theN-helium wave function, like quantum chemistryDFT does with electron density. They have proven tobe the only way to date to simulate both the stabilityand the dynamics of a droplet with a size comparableto experiment [7, 8].

We study the 4p ← 4s and 5s ← 4s photo-excitation and desorption dynamics of a heliumdroplet doped with potassium using He-TDDFT. Themethod is the same as the one already used for other

alkalis [9, 10]. Potassium presents the additional in-terest that its dynamical behavior is at the border be-tween classical and quantum mechanical dynamics.We will present and discuss the results for a classicaldescription of potassium photo-desorption dynamicsand give a preliminary discussion of the quantum ef-fects.

References

[1] M. Mudrich and F. Stienkemeier 2014 Int. Rev. Phys.Chem. 10.1080/0144235X.2014.937188

[2] F. Ancilotto, E. Cheng, M. W. Cole and F. Toigo 1995Z. Phy. B 10.1007/BF01338398

[3] F. Stienkemeier, J. Higgins, C. Callegari, S. I.Kanorsky, W. E. Ernst and G. Scoles 1996 Z. Phys.D 10.1007/s004600050090

[4] G. Auböck, J. Nagl, C. Callegari and W. E. Ernst 2008Phys. Rev. Lett. 10.1103/PhysRevLett.101.035301

[5] M. Theisen, F. Lackner and W. E. Ernst 2011 J. Chem.Phys. 10.1063/1.3624840

[6] F. Stienkemeier and K. K. Lehmann 2006 J. Phys. B:At. Mol. Opt. Phys. 10.1088/0953-4075/39/8/R01

[7] M. Barranco, R. Guardiola, E. S. Hernández, R.Mayol, J. Navarro and M. Pi 2006 J. Low Temp. Phys.10.1007/s10909-005-9267-0

[8] F. Ancilotto, M. Barranco, F. Coppens, J. Eloranta, N.Halberstadt, A. Hernando, D. Mateo and M. Pi 2017to be published

[9] A. Hernando, M. Barranco, M. Pi, E. Loginov, M.Langletb and M. Drabbels 2012 Phys. Chem. Chem.Phys. 10.1039/C2CP23526A

[10] J. von Vangerow, F. Coppens, A. Leal, M. Pi, M.Barranco, N. Halberstadt, F. Stienkemeier and M. Mu-drich 2017 The Journal of Physical Chemistry Letters10.1021/acs.jpclett.6b02598


IsotopeFit – data extraction and processing software for cluster mass spectra

Michal Ďurian* 1, Stefan Ralser†, Johannes Postler† and Paul Scheier† 2¶

* Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava 84248, Slovak Republic

† Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, A-6020 Innsbruck, Austria

Synopsis We are developing a software capable of aiding in evaluation of complex cluster mass spectra. Theo-

retical knowledge of characteristic isotopic pattern of each fragment is used to create a matrix of all fragments

contained in the mass spectrum. Least squares routine is employed to find abundances of these fragments. Cur-

rently, efforts are concentrated on making the software efficient enough to run on common computers with rea-

sonable performance and execution times.

Mass spectroscopy of clusters often produces

fairly complex spectra, complicating the extrac-

tion of useful data, such as magic numbers,

cluster sizes and species identities. To alleviate

encountered difficulties, a dedicated software

tool – IsotopeFit – has been developed [1] in

MATLAB. Its core ability is finding the abun-

dancy of each fragment that contributes to the

mass spectrum. Currently, our main goal is to

make calculations more resource efficient to

tackle even more complex spectra and most im-

portantly, to make even a regular laptop capable

of running the software without limiting the us-

er’s productivity. Therefore, a new perfor-

mance-optimized version is being developed in

C/C++, which will be published under one of

the open-source licenses.

Abundance (𝐴𝑖) extraction procedure begins,

after background subtraction and mass axis cal-

ibration, by calculating isotopic patterns 𝑝𝑖(𝑚)

for all fragments specified by user. Afterwards,

a convolution core 𝜅(𝑚), typically a Gaussian,

is applied, thus creating a characteristic spectral

fingerprint of the fragment. These subsequently

constitute the design matrix �� of the calculated

mass spectrum 𝑠𝑐𝑎𝑙𝑐(𝑚):

𝑠𝑐𝑎𝑙𝑐(𝑚) = ∑ 𝐴𝑖 . 𝑝𝑖(𝑚) ∗ 𝜅(𝑚)

𝑖

= ��. 𝐴

Finally, a non-negative linear least squares fit-

ting routine yields the abundance vector 𝐴 by

minimizing the residuum between 𝑠𝑐𝑎𝑙𝑐(𝑚) and

the experimental spectrum being evaluated.

More detailed description of underlying con-

cepts can be found in [1].

From a technical point of view, the first non-

trivial routine is the creation of the design ma-

trix ��𝑀×𝑁 . Typical dimensions range between

105 – 106 × 103 − 104 . Due to the fact, that

columns, representing individual fragment fin-

gerprints, are being calculated from lighter to

heavier ones, we obtain a tall-and-skinny sparse

matrix with non-zero values dispersed roughly

around the diagonal. Sparseness allows for low-

er memory requirements.

Second, and the most important technical

point is the preparation of input data for the

least squares fitting routine, which absolutely

requires optimizations due to the dimensions

and properties of the design matrix. We take

advantage of the fact that calculating QR-

decomposition of the design matrix and then

using the upper triangular matrix 𝑅 for the least

squares routine does not change the resulting

abundance vector 𝐴, while also having a matrix

with dimensions of only 𝑁 × 𝑁. Moreover, the

QR-decomposition can be calculated by blocks,

which, with the use of Givens rotations imple-

mentation, allows for parallelization [2].

In its current state, the program is capable of

running all necessary subroutines, although with

many optimizations and checks still not in

place, most notably in the least squares fitting

routine. At present, a standard least squares al-

gorithm is used, which is to be replaced with

non-negative least squares routine [3], which

provides a speed boost and better numerical stabil-

ity for our case.

This project has received funding from the

European Union's Horizon 2020 research and

innovation programme under grant agreement

No 692335.

References

[1] S. Ralser et al. 2015 Int. J. Mass Spectrom.

379 pp. 194-199 [2] S. Hammarling, C. Lucas 2008 MIMS

ISSN 1749-9097

[3] R. Bro, S. D. Jong 1997 J. Chemom. 11 pp. 393-

401



Resonant Ignition of Doped Helium Droplets

Michael Kelbg * 1, Lev Kazak , Josef Tiggesbäumker, Karl-Heinz Meiwes-Broer

* Universität Rostock, Institut für Physik, AG Cluster und Nanostrukturen

Resonant absorption of ultrashort pump-probe laser pulses in doped helium droplets is studied for different drop-

let sizes and dopants.

The process of resonantly ionizing doped

helium droplets is studied using ultrashort laser

pulses. In a pump-probe process the initially

transparent helium droplet turns into a strong

absorber of infrared light due to the initial

charging of the dopant. The second pulse is then

resonantly absorbed at the time the core reaches

matching charge density. Seeding the droplet

with cluster forming Xenon is compared to

evenly distributed Magnesium. Furthermore, the

size-dependency of the helium droplet and the

amount of dopant atoms are discussed using

high charge states as indicator for optimal con-

ditions.

Figure 1. Shift of the yield of different Xenon-charge

states for delays of 700 fs (top), 500 fs (middle) and 300

fs (bottom)

1 email: [email protected]

Ion chemistry in helium droplets doped with adamantane

M. Goulart†, M. Kuhn

†, L. Kranabetter

†, A. Kaiser

† , J. Postler

†, M. Rastogi

†, A. Aleem

‡, B. Rasul

§, D. K. Bohme1*, and P. Scheier2*

†

† Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Technikerstrasse 25, A-6020

Innsbruck, Austria

‡ LINAC Project, PINSTECH, P.O. Box Nilore, Islamabad 44000, Pakistan

§ University of Sargodha, 40100 Sargodha, Pakistan

*Department of Chemistry, York University, Toronto, Ontario M3J 1P3, Canada

The technique employing helium droplets

as nano reactors has received flurry of research

after Scheidemann et al. in 1990, demonstrated

the potential of helium droplets to capture for-

eign species [1]. The technique is intriguing in a

way that it delivers an extraordinarily cold envi-

ronment, which can be attributed to the super-

fluid matrix. Further, the ultra-high vacuum fa-

cilitates inert and exceptionally clean environ-

ment. Additionally, this technique is promising

since it opens the door for clean synthesis of

nanoparticles without involving solvents, lig-

ands or additives [2, 3].

The aim of this contribution is to deliver in-

sights into the electron interactions with pristine

and doped helium droplets and subsequent ion

molecule reactions for cations, dications, and

trications of large clusters of adamantane [4].

Energetic He species (He*, He+, or He*–) were

involved in the ionization, through one or more

collisions with helium droplets.

Adamantane has been the choice of molecule

as it has the reputation of being the building

block of diamondoids and perhaps diamonds.

The aggregation of adamantine has been ex-

plored in its ionized form near 0 K in helium

droplets.

Magic number (m/z ratio) as obtained by

high-resolution mass spectrometry, exhibited

the packing of adamantane molecules into clus-

ter structures of special stability involving pre-

ferred arrangements of these molecules. No re-

lation between abundance anomalies and charge

state could be observed. The spectrum revealed

some dehydrogenation of adamantane and its

clusters. Nevertheless, no major transformations

into adamantoids or microdiamonds could be

seen.

References

[1] A. Scheidemann et al. 1990 Phys. Rev. Lett. 64

1899

[2] G. Haberfehlner et al. 2015 Nat. Comm. 68779

[3] J. Postler et al. 2015 J. Phys. Chem. C 11920917

[4] M.K. Goulart et al. 2016 J. Phys. Chem. C

6b11330

1E-mail: [email protected].

2E-mail: [email protected].

Probing the formation of PAH interstellar dust grains

M. Goulart∗, T. Kurzthaler∗, L. Kranabetter∗, M. Kuhn∗, A. Kaiser∗,P. Martini∗, A. Lidinger‡, A.M. Ellis†, P. Scheier∗1

∗ Institut für Ionenphysik und Angewandte Physik, University of Innsbruck, 6020, Austria‡ Institut für Experimentalphysik, FU-Berlin, 14195, Germany

† Department of Chemistry, University of Leicester, LE1 7RH, UK

Synopsis The first solvation shell of coronene clusters solvated by helium atoms or hydrogen molecules was analyzed to obtaininformations about the most conformational patterns.

The infrared emission lines commonly observedin various astronomical sources, such as planetarynebulae, edges of ionized regions or around youngstars can be associated to polycyclic aromatic hy-drocarbons (PAH’s). These organic molecules arevery common in the universe [1] and can aggregateto form soot-like particles or dust grains in interstel-lar clouds. The reaction pathways that occur on thesurface of dust grains can be used to explain the abun-dances of many species in the interstellar medium(ISM) [2].

Superfluid helium nanodroplets (HND) can beused to recreate the conditions of the ISM, mainlydue to their special properties to pick up virtually anyatom or molecule that collides with them and coolthem down to 0.4 K [3], making these cryogenic ma-trix a perfect laboratory to simulate this conditionswhere PAH aggregation takes place.

In order to study the conformational aspects ofa dust grain, a new mass spectrometric method wasimplemented. HNDs were first doped by individualpickup with coronene molecules and subsequentlywith Helium atoms or hydrogen molecules. The neu-tral doped droplets were then ionized by electronbombardment and analyzed by a reflectron time-of-flight mass spectrometer. By investigating the behav-ior of the first He or H2 solvation shell, it is possibleto obtain information concerning the packing of thePAHs. Anomalies in the intensity of the mass spectrapeaks, known as magic numbers, are used to iden-tify the most likely aggregation patterns of the PAHmolecules.

Figure 1. Graphical representation of a coronenemolecule solvated by He atoms.

References

[1] Tielens A.G.G.M., et al. Astrochemistry: From molec-ular clouds to planetary systems. in Symposium ofthe International Astronomical Union. 2000 Sogwipo,Cheju, Korea:Astronomical Society of the Pacific.

[2] Hollenbach D. and E.E. Salpeter, Surface recombi-nation of hydrogen molecules. Astrophysical Journal,1971.163(1):p.155-164

[3] Toennies, J.P and A.F. Vilesov, Superfluid heliumdroplets: A uniquely cold nanomatrix for moleculesand molecular complexes. Angewandte Chemie-International Edition, 2004.43(20):p.2622-2648.


Electron Interactions with Doped Neon Clusters

Rebecca Meißner*†1, Georg Alexander Holzer*, Michael Neustetter*, Anita Ribar*, Stephan Denifl*2

* Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, A-6020 Innsbruck, Austria

† CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829- 516

Caparica, Portugal

The study of positive and negative ion formation in doped inert gas clusters opens a door to further insights into

cluster properties. This includes for example in electron attachment the stabilisation of the parent anion by the

cluster environment. Furthermore, the understanding of the electron attachment process itself can be extended to

the low temperature range. Here, a study of neon clusters doped with CO2 is presented and compared to earlier

results for pure CO2 clusters and helium droplets doped with CO2.

Atomic and molecular cluster matrices dif-

fer strongly in their characteristics such as tem-

perature and phase. Studies of neutral helium

clusters revealed a fluid up to an even superflu-

id behaviour independently of the degree of

doping [1]. In contrast, neon and hydrogen

clusters change from a fluid-like to a solid-like

behaviour after the embedment of already a few

dopants [2].

Besides their differences, all (semi-) fluid,

extremely cold (0.4-10K) rare gas clusters

share two main similarities. Firstly, a repulsive

force towards an excess electron raises an elec-

tron bubble in which the charge is localised and

thus the electron travels around the cluster [3].

Secondly, the cluster surface represents an elec-

tric barrier for an incoming electron, requiring

enough kinetic energy of the electron to over-

come it.

Electron attachment to doped clusters ex-

tends the knowledge of the process itself to the

very low temperature range. Besides solvation,

temperature can be an important factor in the

efficiency of the process and the formation of

fragments. Additionally, the cluster environ-

ment stabilises otherwise unstable anions

formed upon electron attachment. Thus, specif-

ic cluster characteristics can be studied [4].

In the current setup, neon clusters are doped

with CO2. Carbon dioxide is a fundamental

molecule and electron attachment has already

been studied in gas phase, in pure CO2 cluster

beams and in helium droplets [5]. This allows

the comparison of stable anion formation

among them and provides first hints to the en-

ergy barrier for an electron entering the neon

cluster.

The experimental setup comprises a high reso-

lution hemispherical electron monochromator,

a cluster source, a quadrupole mass analyser

and a channeltron detector. The cluster source

is based on the principle of supersonic expan-

sion, using a 10µm pinhole nozzle cooled to

65-100K and an expansion pressure of around

20bar. Results of positive and negative ion

formation were obtained and will be presented

in the frame of this conference.

Figure 1. Schematics of the experimental setup.

Acknowledgements

This work was supported by FWF (P24443)

and by Fundação para a Ciência e a Tecnologia

through the Radiation Biology and Biophysics

Doctoral Training Programme and scholarship

grant PD/BD/114452/2016 to RM.

References

[1] S. Denifl 2013 Eur. Phys. J. Special Topics 222,

2017

[2] R. von Pietrowski et al. 1997, Z. Phys. D 40, 22

[3] M. Rosenblit et al. 1995, Phys. Rev. Lett. 75, 4079

[4] E. J. Al Maalouf et al. 2016 Eur. J. Phys. J.D 70,

148

[5] J. Postler et al. 2014 J Phys Chem A 118(33), 6553



Correlated decay mechanisms in weakly bound acene molecules attached

to neon clusters

Sharareh Izadnia* 1, Aaron LaForge* 2, Frank Stienkemeier*3

* Institute of Physics, University of Freiburg, Hermann-Herder-Str. 3,

D-79104 Freiburg, Germany

Charge/excitation transfer along with the corresponding decay/loss mechanism are fundamental aspects in light

harvesting, organic photovoltaics, and optoelectronic devices. In our studies, we probe aggregates of organic

molecules isolated on neon clusters in order to understand collective processes of electronically excited systems.

Collective processes in weakly interacting sys-

tems offer a unique means to study energy and

charge transfer processes. In particular, singlet

fission is a unique decay mechanism where a mole-

cule excited to its singlet state can partially transfer

its energy to a neighboring ground state molecule,

and thereby create two molecules excited to a triplet

state. As such, singlet fission can increase the effi-

ciency of organic electronics and photovoltaics by

creating multiple charge carriers from one single

photon [1,2,3].

Figure 1. Neon cluster doping schematic.

Here, we show the experimental observation of flu-

orescence lifetime reduction of tetracene, pentacene

and anthracene by directly tuning the number of

molecules placed on the surface of neon clusters.

Such complexes are ideally suited to probe the role

of e.g. the number and the intermolecular distance

of interacting molecules. We attribute these effects

to singlet fission. Furthermore, we observe in the

same systems, Dicke superradiance [4], which de-

scribes the effect of an ensemble of excited

molecules collectively emitting radiation as a

coupled quantum lifetimes and an enhancement in

the radiative intensity system. This leads to a

reduced effective lifetime and an enhancement

in the radiative intensity. Also depending on the

substance and the aggregate, the experimental

results indicate that triplet-triplet annihilation is

another process that can influence the system.

Figure 2. Depiction of singlet fission.

References

[1] M. B. Smith and J. Michl, Chem. Rev., vol. 110,

11, pp. 6891,6936, 2010.

[2] Reusswig, P. D.; Congreve, D. N.; Thompson, N.

J.; Baldo, M. A. Applied Physics Letters,101,

113304, 2012.

[3] M. B. Smith and J. Michl. Annual Review of

Physical Chemistry, 64, 361-386, 23298243,2013.

[4] M. Müller, S. Izadnia, S. M. Vlaming, A. Eisfeld,

A. LaForge, and F. Stienkemeier, “Phys. Rev. B, vol.

92, p. 121408, 2015.




Two-dimensional Electronic Spectroscopy of Controlled Isolated Systems

Ulrich Bangert1, Lukas Bruder, Marcel Binz, and Frank Stienkemeier.

Institute of Physics, University of Freiburg, Hermann-Herder-Str. 3, 79104 Freiburg im Breisgau

Synopsis Phase-modulated pump-probe spectroscopy is capable of detecting non-linear signals even from dilute samples. Wecurrently implement a phase-modulated two-dimensional electronic spectroscopy setup to measure 2D spectra of doped heliumnanodroplets. The 2D spectra will be used to investigate coherent processes in complex molecules.

Two dimensional electronic spectroscopy(2DES) is a powerful tool to study coherences andcorrelations on ultrafast time scales. Until now,2DES has been limited almost exclusively to con-densed phase studies. Our aim is to apply 2DES tocontrolled isolated systems by using doped heliumnanodroplet beams. Helium nanodroplets providethe dopant with a cold environment and minimal per-turbation, which are ideal conditions to study thebehavior of an individual system in a well-controlledenvironment.

However, the target density in doped heliumdroplet beams is several orders of magnitude lowerthan in bulk condensed phase samples. Furthermore,2DES depends on the third order response of thesample to the incident light. Together, this leads toparticularly small signals. We adapt a phase mod-ulation technique combined with lock-in detectionto overcome this issue [1]. This technique has al-ready shown significant sensitivity improvements forcoherent pump-probe spectroscopy in helium nan-odroplets [2].

One advantage of 2D spectroscopy is that in the2D spectra homogeneous and inhomogeneous broad-ening mechanisms are readily disentangeled. For thisreason, 2D spectroscopy has been used to charac-terize the dynamics of solvents such as the ultrafastrearrangement dynamics of water [3]. In a similarway, 2DES may be used to get more insights into thehelium droplet properties when doped with differentspecies.

We currently implement the experimental setupand do initial characterization measurements.

Figure 1. Schematic of the experiment without phasemodulation. A train of four fs-pulses is used to excitethe dopant within the droplets. The excited state popu-lation is then measured by photo ionization or fluores-cence. The resulting signal is Fourier transformed withrespect to the inter pulse delays τ and t yielding 2D-spectra at different times T .

References

[1] P. Tekavec et al. 2007 J. Chem. Phys. 127 214307


[3] M.L. Cowan et al. 2005 Nature 434 199


Pendular state spectroscopy of molecular ions in Helium nanodroplets

V. Oliver∗1, X. Zhang†, M. Drabbels∗

∗ Laboratory of Molecular Nanodynamics, Institute of Chemical Sciences and Engineering, Swiss Federal Institute ofTechnology in Lausanne (EPFL), CH-1015 Lausanne, Switzerland.

† Laboratory of Molecular Physical Chemistry, Institute of Chemical Sciences and Engineering, Swiss Federal Instituteof Technology in Lausanne (EPFL), CH-1015 Lausanne, Switzerland.

Molecular alignment methods have always beenof interest in order to obtain information about struc-ture and bond orientation. The pendular state orien-tation method can achieve substantial orientation ofmolecules in an experimentally simple way [1], [2].Modest electric fields must be applied to achieve thisorientation. However, the magnitude of the appliedfield relies on cooling since the field must overcomethe molecule’s tendency to rotate freely. Using su-perfluid helium as a medium leads to lower rotationaltemperatures, further facilitating the molecular align-ment. Even though this method has been formerly

used for neutral molecules [3], [4], no studies onmolecular ions have been done previously. This con-tribution covers the first results obtained using thistechnique.

References

[1] H.J. Loesch et al. 1990 J. Chem. Phys. 93:7 4779-90

[2] B. Friedrich et al. 1991 Z. Phys. D 18 153

[3] F. Dong et al. 2002 Science 298:5596 1227-30

[4] M.Y. Choi et al. 2005 Phil. Trans. R. Soc. A 363 393-413


Intermolecular decay mechanisms in helium nanodroplets · Intermolecular decay mechanisms in helium...

Documents

Transcript of Intermolecular decay mechanisms in helium nanodroplets · Intermolecular decay mechanisms in helium...