Tartu. March 2004

G. Öhrwall, University of Uppsala, Sweden

X-ray photoemission studies of free molecular clusters

using synchrotron radiation

Tartu. March 2004

Why clusters?Bridge between the isolated atom and the infinite solid

Size-dependent physical and chemical properties

Microscopic origin of macroscopic properties

Applications?

Tartu. March 2004

Cluster productionions, e-

SR

Skimmer

Turbo-pump

P≈10-3 mbar

P≈1-10 bar

Nozzled≈100 mm <N> = <N>(T, D, p, k)

Tartu. March 2004

+<N>≈2000XPS of Ar clusters

2p3/2 XPS

+ ++

--

-

++

+

++

--

-+

+

PRL 74, 3017 (1995), JCP 104, 1846 (1996)

Tartu. March 2004

CO2 cluster XPS

400

300

200

100

0

-2.0-1.5-1.0-0.50.00.51.0

Rel. Binding Energy (eV)

CO2 XPS O 1s, hν=560 eV 1 , C s hν=310 eV

ΔEvert=0.83 eV

Same shift forC 1s and O1s

van der Waalsbonded - mainlyfinal state relaxationCore hole screening

Tartu. March 2004

C2H5OH cluster XPS2500

2000

1500

1000

500

0

-4-3-2-1012Relative Energy (eV)

Ethanol O1s XPShν=560 eV

1 Ethanol C s XPShν=350 eV

Total spectrum Cluster contribution Molecular contribution

Δ =1.3 E eV

Δ =0.95 E eV Δ =0.90 E eV

Tartu. March 2004

Core level shifts forethanol clusters

Chemical shiftsΔE(O 1s)=1.3 eVΔE(C 1s, intermediate)=0.95 eVΔE(C 1s, methyl)=0.9 eV

Tartu. March 2004

Core level shifts forethanol clusters

Chemical shiftsΔE(O 1s)=1.3 eVΔE(C 1s, intermediate)=0.95 eVΔE(C 1s, methyl)=0.9 eV

Weak initial state effects - hydrogen bonding

Tartu. March 2004

Core level shifts forethanol clusters

Chemical shiftsΔE(O 1s)=1.3 eVΔE(C 1s, intermediate)=0.95 eVΔE(C 1s, methyl)=0.9 eV

Weak initial state effects - hydrogen bondingChemical shift predominantly relaxation effect

Tartu. March 2004

Core level shifts forethanol clusters

Chemical shiftsΔE(O 1s)=1.3 eVΔE(C 1s, intermediate)=0.95 eVΔE(C 1s, methyl)=0.9 eV

Weak initial state effects - hydrogen bondingChemical shift predominantly relaxation effectDifferent screening implies different coordinationfor O and C atoms - depends on geometry

Tartu. March 2004

500

400

300

200

100

0

294 293 292 291 290

Binding Energy (eV)

No Seeding

1 bar Ar

2 bar Ar

CH3OH

C1s XPShν=310 eVθ=54.7°

Δ =0.4 E eV

Δ =1.1 E eV600

500

400

300

200

100

0

541 540 539 538 537 536 ( )Binding Energy eV

CH3OH

1 O s XPShν=570 eVθ=54.7°

No Seeding

1 bar Ar

2 bar Ar

Δ =0.8 E eV

Δ =1.4 E eV

CH3OH cluster XPS

Size dependent vertical shift

DifferenceC1s-O1s similarto ethanol:0.3-0.4 eV

Tartu. March 2004

Molecule Cluster

+

++

+

++

++

Locailzed or delocalized final states?

Tartu. March 2004

ΔE=22X=4X

+1 core ionized state

Molecule

ΔE=X

Cluster

+2 valence ionized state

ΔEAuger=4X-X=3X

XPS and Auger shifts

Tartu. March 2004

Ar cluster Auger

200 202 204 206 208 210 212 214

Inte

nsi

ty (

arb

. U

nit

s)

Kinetic Energy (eV)

Ar LMM<N>≈200hν=310 eV

Cluster spectrum(surface and bulk) modelled as shiftedand broadenedversion of atomicAuger spectrum.

ΔE=3X works forsurface and bulk!

Tartu. March 2004

470 480 490 500 510KE (eV)

Molecule theory

Ice theory

Ice exp

Cluster expMolecule exp

?ΔE≈3eV

ΔE≈8eV

HH22O Auger O Auger Li

egner

and C

hen

JCP 8

8,

2618 (

198

8)

Localizedpicture insufficient.

Misinterpretedsolid AES?

Tartu. March 2004

Ultra fast dissociation inresonant Auger decay

SR

|i> (ground state)

|i> (intermediate state)

|f> (final state)

Dissociation can occur onsame time scale as corehole life time - few fsfor k-shell in second row elements.

Ultra fast dissociationgives rise to featuresconstant in kinetic energy

Tartu. March 2004

Ultra fast dissociation in CH3Br clusters

7674727068

Photon Energy (eV)

CH3Br NEXAFS

Cluster Molecule

4a1 resonanceknown to give riseto ultra fastdissociation(Nenner & al., J. ElectronSpectrosc. Relat.Phenom. 52, 623 (1990))

Br 3d5/2

-> 4a1

Tartu. March 2004

CH3Br cluster RAS300

250

200

150

100

50

0

25 20 15 10

Binding Energy (eV)

CH3Br molecule

1200

1000

800

600

400

200

0

25 20 15 10

Binding Energy (eV)

CH3Br Cluster & Molecule

hν=70.6 eV( 3Br d5/2 -> 4a1 )

hν=61 eV( 3 )below Br d

Features from ultra fast dissociation

hν=70.6 eV( 3Br d5/2 -> 4a1 )

hν=61 eV( 3 )below Br d

Δ =0.8 E eVΔ =0.8 E eV

UFD featuresas intense inmolecules andclusters - notsurface effect!

Tartu. March 2004

SummaryThird generation synchrotron radiation offers new possibilities to study free clusters

Core level PES on clusters gives information on local surrounding - surface/bulk, geometry

Localization/delocalization of two-hole final states in AES

Possible to observe femtosecond nuclear dynamics in core excited state in “solid”

Tartu. March 2004

Acknowledgements

Maxim Tchaplyguine MAX-labJoachim Schulz

Olle Björneholm Uppsala UniversityMarcus LundwallAndreas LindbladTorbjörn RanderSvante Svensson

Tartu. March 2004

AcknowledgementsDept. of Physics, UppsalaOlle Björneholm Marcus LundwallSvante Svensson Andreas LindbladRaimund Feifel Torbjörn Rander

MAX-lab, LundMaxim Tchaplyguine Andreas LindgrenStacey Sorensen

Financial SupportKAW, SSF

Tartu. March 2004

Cluster beam size

Atomic Ar 3p-lines Cluster Ar 3p-lines

Scienta SES-200 detector image

Slit 25 mmMagn.=5x

Atomic Arwidth ≈5 mmCluster Arwidth >1 mm5 cm from nozzle

KineticEnergy

Pos.

Tartu. March 2004

CO2 cluster valence PES

300

250

200

150

100

50

0

20E 18 16 14 12

Binding Energy (eV)

X 2Πg

A2Πu

B2Σu

+

C2Σg

+

CO2+ PES hν=60 eV

ΔEad=1.6 eV

ΔEvert=0.85 eV

ΔEad=1.6 eV

ΔEvert=1.04 eV

ΔEad≈1.2 eV

ΔEvert≈0.5 eV

ΔEvert=0.69 eV

ΔEad=1.15 eV

Shifts depend weakly on electronic state

Vertical shiftssimilar to corelevel shifts(screened 1-holestates)

Tartu. March 2004

H2O cluster valence PES

20 18 16 14 12 10Binding Energy (eV)

hν=60 eV

Cluster+Mol.

Mol.

A-state ((3a1)-1)more affected bycluster formationthan X or B.

X

AB

Tartu. March 2004

Clustering from a binary gas mixture

Pure expansion: <N> = <N>(T, D, p, k)

Mixed A B expansion: <”N”>= <”N”>(T, D, p, kA, kB, rA/B)

Present experiment: • T, D, p fixed • kA, kB, rA/B varied

Tartu. March 2004

Valence PES (UPS)Core-level PES (XPS)NEXAFSPE(PI)nCO TOF-MS

Homogenousmixing

Radial segregation

Radiallayering

Non-mixing

Tartu. March 2004

Relative binding energy (eV)

Ar 2p3/2

Kr 3d5/2

0.0 -1.0

Surface: more Ar, less Kr

Bulk: less Ar, more Kr

+

+

--

-+

+

+

-+

+

--

-+

++

+

+--

Ar/Kr clusters from 1.8% Kr in Ar XPS @ 50 eV Ek

Tartu. March 2004

Structure of Ar/Kr mixed clusters

Ar/Kr radial gradient

Surface: more Ar, less Kr

Bulk: less Ar, more Kr

Tartu. March 2004

Ar/Xe clusters XPS @ 50 eV Ek

Xe 4d5/2

Ar 2p3/2

100%

3.2%

2.1%

2.7%

5.3%

0%

Tartu. March 2004

O2 cluster XPS1400

1200

1000

800

600

400

200

0

546 545 544 543 542 541

Binding Energy (eV)

O2 O 1s XPShν=570 , eV θ=54.7°

Cluster and Molecule Molecule

Δ =1.1 E eV

Δ =0.6 E eV

2Σ ,g u

4Σ ,g u

Exchange splittingsame in moleculeand cluster

Tartu. March 2004

O2 cluster NEXAFS

Valence orbitals less affected by cluster formation than Rydberg states.

800

600

400

200

0

545540535530

Photon Energy (eV)

O2 01s NEXAFS

Not normalized to photon flux Molecule Cluster

σ*

π*

Recorded RASon top of σ*

Tartu. March 2004

O2 cluster RAS

250

200

150

100

50

0

530520510500490480470

Kinteic Energy (eV)

O2 O 1s -> σ* , RAS θ=54.7°

539.9 Clu eV 539.2 Mol eV

Δ =0.8 E eV

Cluster spectrumcontains featuresconsistent with ultra fast dissociation