WUTA08 Laboratori Nazionali di Frascati
Frascati, 8-10 October 2008
Probing free metallic and carbon clusters with VUV photons.
P. Piseri1,2,3 ([email protected]), G. Bongiorno1,2,3, T. Mazza1,2,3 , L. Ravagnan1,2,3, M. Amati1,2,3, M. Devetta1,2, C. Lenardi2,3,4, and P. Milani1,2,3, M. Coreno5,6, M. De Simone6, P. Rudolf 7, F. Evangelista7
1 Dipartimento di Fisica, Università degli Studi di Milano.2 CIMAINA, Università degli Studi di Milano.3 CNR-INFM4 Dipartimento di Farmacologia, Università degli Studi di Milano.5 CNR-IMIP, Area della ricerca di Roma 16 Laboratorio Nazionale TASC INFM-CNR7 University of Gröningen, The Nederlands
Laboratorio Getti Molecolari e Materiali Nanocristallini - LGMDirector: P. Milani ([email protected])
OutlineOutline
Core-level techniques became available for free-clusters with 3rd generation SR light sources
•The CESyRa experience
•Possibilities offered by the experimental setup
•Perspectives with next generation sources
ethane
ethylene acetylene
Simple hydrocarbons
Carbon
sp3
spsp2
Isolated carbon cluster with less than 30
atoms exist as purely sp carbon chains
(carbynes)
R.O. Jones, J. Chem. Phys. 110, 5189 (1999)
Diamond
Graphene
sp3
spsp2
Crystal structures
?
Carbon
a-C
ta-C• Hard• Semiconductor
• Soft• Conductive
Disordered phases:mixture of hybridizations
Chains and rings (sp)
N 32 atoms
E.A. Rohlfing et al. J. Chem. Phys. 81, 3322 (1984)
Pulsed Laser Vaporization source
Fullerenes and onions (sp2, sp3)
N > 32 atoms
Increasing the number of C atoms per cluster
N both even and odd N only even
High power density:annealing of the clusters up to their ground state
structure
Carbon clusters mass spectra
pulsed valve
insulating valve flange
anode
rotating cathode
thermalization cavity
graphite nozzle
ceramic body
Pulsed Microplasma Cluster Source (PMCS)developed at Laboratorio Getti Molecolari e Materiali Nanocristallini,
Department of Physics, University of Milano (Italy)
E. Barborini, P. Piseri, P.Milani, J. Phys. D, Appl. Phys. 32, L105 (1999)
1 mm
H. Vahedi-Tafreshi, et al. Journal of Nanoscience and Nanotechnology 6, 1140 (2006)
Cluster size (atoms)
Both odd and even clusters are detected
A-mode B-mode Residence time (s)
M. Bogana et al. NJP 7, 81 (2005)
The PMCS produces non-fullerenic clusters.
~40% of the whole mass distribution consists in odd clusters
Leaving the fullerene road…
ClCoh
ClClKin
E
NE /=ε
Gas-Phase Nanoparticle deposition, Gas-Phase Nanoparticle deposition, or: Cluster Beam Deposition (CBD) or: Cluster Beam Deposition (CBD) source
ε >> 1
Fragmentation
ε << 1
Memory effect
Low Energy Cluster Beam Deposition (LECBD)or
Supersonic Cluster Beam Deposition (SCBD)
L. Ravagnan et al. PRL 89, 285506 (2002)
Raman spectroscopy of ns-C films
1000 1200 1400 1600 1800 2000 2200 2400
Intensity [arb. units]
Raman shift [cm-1]1000 1200 1400 1600 1800 2000 2200 2400
Intensity [arb. units]
Raman shift [cm-1]
C band !!
ex situ T=300 K (RT)in situ T=300 K (RT)
First observation of a clear signature of sp bonds in a system of pure carbon !!
D+G band
sp-chains are destroyed by oxygen: in situ measurements are mandatory
1000 1200 1400 1600 1800 2000 2200 2400
Intensity [arb. units]
Raman shift [cm-1]
L. Ravagnan et al. PRL 89, 285506 (2002) L. Ravagnan et al. PRL 98, 216103 (2007)
Raman spectroscopy of ns-C films
ex situ T=300 K (RT)in situ T=300 K (RT)in situ T=150 K
Also the substrate temperature plays a crucial role!
D+G band
sp3
spsp2
a-C
ta-C• Hard• Semiconductor
• Soft• Conductive
Carbon
Disordered phases:mixture of hybridizations
Ternary phase diagram of the amorphouspure carbon system.
sp-rich a-C
* resonances are fingerprints of the specific molecular bonds
Auger electron
Gaseous acetylene and ethylene have * resonances at 285.9 eV and 284.7 eV respectively.
A.P. Hitchcock et al. J. El. Spec. 10, 317 (1977)
Beyond Raman: NEXAFS spectroscopy
CESYRA: in situ NEXAFS of ns-C films
C.S. Casari et al. Phys. Rev. B 69, 75422 (2004)
270 280 290 300 310 320 330 340
Intensity [arb. units]
Photon energy [eV]
in situ T=300 K (RT)
We know from Raman that by heating the sample we induce the decay of the sp chains and a partial
reordering of the sp2 matrix.
1000 1200 1400 1600 1800 2000 2200 2400
Intensity [arb. units]
Raman shift [cm-1]
in situ T=300 K (RT) in situ T=350 K
270 280 290 300 310 320 330 340
Intensity [arb. units]
Photon energy [eV]
282 284 286 288 290 292 294 296
Intensity [arb. units]
Photon energy [eV]
CESYRA: in situ NEXAFS of ns-C films
The spectra evolves both in the * and * region.
in situ T=300 K (RT)in situ T=350 K
Normalization
282 283 284 285 286 287 288 289 290
Intensity [arb.units]
Photon energy [eV]
Difference (pre-edge)
284.7 eV
285.9 eV
270 280 290 300 310 320 330 340
Intensity [arb. units]
Photon energy [eV]
sp
sp2
CESYRA: in situ NEXAFS of ns-C films
We observe the decay of *(CC) and the increase of the *(CC):
NEXAFS spectroscopy is capable of distinguishing between sp and sp2 in a system of pure carbon!!
in situ T=300 K (RT)in situ T=350 K
Normalization
CESyRa apparatus layoutCESyRa apparatus layout
cluster beams machine
Mass flow controller
High voltage supplyTurbo 2000 l/s
Turbo 500 l/s
Turbo 500 l/sTurbo 300 l/s
Cluster source
Quartz and steel
gate valves (not
shown)
Beam diagnostic device (see inset)
Time of flight mass spectrometer
Beam dumping chamber and
quartz monitor microbalance (not shown)
Feedthrough of the deposition substrate
for in-situ cluster assembled film
analysis
skimmerCluster beam
Source expansion chamber
Gas cell and deflection
stage chamber
Beam diagnostic
device chamber
Interaction chamber
Source part Interaction part
Turbo 300 l/s
Light entrance flange
282 284 286 288 290 292 294 296
Intensity [arb. units]
Photon energy [eV]
270 280 290 300 310 320 330 340
Intensity [arb. units]
Photon energy [eV]
CESYRA: TEY NEXAFS of isolated clusters
Normalization
285.6 eV
ns-C film in situ (RT)TEY isolated clusters
The * region is peaked at 285.6 eV: the cluster are predominantly made by sp carbon!
282 284 286 288 290 292 294 296
Intensity [arb. units]
Photon energy [eV]
270 280 290 300 310 320 330 340
Intensity [arb. units]
Photon energy [eV]
ns-C film in situ (350 K)
CESYRA: TEY NEXAFS of isolated clusters
Normalization
285.6 eV
ns-C film in situ (RT)TEY isolated clusters
The * region is peaked at 285.6 eV: the cluster are predominantly made by sp carbon!
0 20 40 60 80 100 120
Electrin Yield [arb. units]
Delay time [ms]0 20 40 60 80 100 120
Electrin Yield [arb. units]
Delay time [ms]
PEY: binning of the electron yield for intervals of delay times
First exiting clusters:
short “annealing”
Last exiting clusters:
long “annealing”
Pulsed source discharge
CESYRA: PEY NEXAFS of isolated clusters
Delay time: time elapsed between the discharge and the detection of the photo-electron.
Residence time of the probed cluster in the source.
v ~ cost
CESYRA2: PEY NEXAFS of isolated clusters
270 280 290 300 310 320 330 340
PEY [arb. units]
Photon energy [eV]
Increasing delay time
15 - 21 ms
75 - 81 ms
Small change in the * region.
276 280 284 288 292 296 300
PEY [arb. units]
Photon energy [eV]
Increasing delay time
285.7 eV284.8 eV
CESYRA2: PEY NEXAFS of isolated clusters
15 - 21 ms
75 - 81 ms
10 20 30 40 50 60 70 801.10
1.15
1.20
1.25
1.30
1.35
I(285.8 eV)
/ I(284.7 eV)
Delay time [ms]
Electron Yield SpectraElectron Yield Spectra
What kind of systems can we study?
h
h
XAS on free clustersXAS on free clusters
e-
h
h’
e-
e-
++
Many different relaxation channels open up in free clusters.
h
h
e-
+
h
h’+
e-
e-
XAS on free clustersXAS on free clusters
e-
h
h’
e-
e-
++
Many different relaxation channels open up in free clusters.
e-h
h’
e-
+
Mixed clusters and cluster-molecule systems add more possibilities
Cluster beam
Photon beam
PEPICO TOF setupSignal to TDC stop channels 1-7 (100 ms range, 80 ps resolution)
Signal to TDC stop channel 8 (100 ms range , 80 ps resolution)
Electron detector
Multiple Ion detectors
TDC start signal from pulsed source discharge
Ex-post reconstruction of electron-ion coincidence spectrum (100 µs range) by software computing the stop1-
7-stop8 time differences
PxPy…CO setup
Events Time StructureEvents Time Structure
Recording the full information
0Delay from start (ms)1.00.5
Delay from start (ms)
tel,jtion,k
Actual He injection
~ 1 ms
He injection trigger 350 s
Discharge60 s
Ion detector array
x
y
z
light clusters
heavy clusters
electrons
Photon beam
Cluster beam
Electron detector
Cluster source
Determination of cluster velocity
Cluster velocity is obtained dividing the detector position by the mean detected time of flight at different detection time;
Complete timing information allows residence-time resolved velocity measurements.
Beam kinematics
€
v m( ) = vHe 1−m − mHe
m + mHe
⎛
⎝ ⎜
⎞
⎠ ⎟
k⋅σ ⎛
⎝ ⎜ ⎜
⎞
⎠ ⎟ ⎟
The velocity of a particle with mass m,seeded in a He supersonic expansion can be modeled by:
After k collisions with the He carrier gas.
is proportional to the collision cross section and is given by
β m=
Where β is 2/3 for a spherical shaped particle
Bu. Wrenger and K. H. Meiwes-Broer, Rev. Sci. Instrum. 68 (5), May 1997, 2027
€
v ttof( ) = vHe 1−α ⋅ ttof
2 − mHe
α ⋅ ttof2 + mHe
⎛
⎝ ⎜ ⎜
⎞
⎠ ⎟ ⎟
k⋅ α ⋅ttof2
( )β ⎛
⎝
⎜ ⎜ ⎜
⎞
⎠
⎟ ⎟ ⎟
Beam kinematics: velocity vs residence time
Data fitting by varying: vHe, k, β
A fitting parameter β~0.84 is found against β=2/3=0.667 as expected for dense spherical particles
Vapor
Further growth steps
Clusters have a fractal structure!
e-
+
h
h’+
e-
e-
e-
h
h’
e-
e-
++
h
h’
e-
e-
+
?
XAS on free clustersXAS on free clusters
What relaxation channels in complex clusters ?e-
h
h’
e-
+
Ion - Ion correlation spectraIon - Ion correlation spectra
n-Erlang distributions for the false coincidence background instead of exponential
PEPICO
PInCO
1st order inter-arrival time distribution
2nd order inter-arrival time distribution
4th order inter-arrival time distribution
Ion - Ion correlation intensityIon - Ion correlation intensity
Maps of nth-order correlated ions intensity
Space correlation
• More channel correlations
Ions per bunch
(channel) = 0 (channel) = 1(channel) = 2(channel) = 3
Ions per bunch
(channel) = 0 (channel) = 1(channel) = 2(channel) = 3
Ions per bunch
Channels 1-4 Channels 4-7
Ion Ion coincidence spectraIon Ion coincidence spectra Space resolved
Fragmentation yieldFragmentation yield Space resolved
x100
We have demonstrated the feasibility of X-ray absorption spectroscopy experiments on free carbon clusters transition metal clusters and oxide clusters
The experiment has been performed by coupling a supersonic cluster beam apparatus (based on a PMCS) with the Gas Phase beamline at Elettra
An “event reconstruction” approach is used to gain insight into the occurring relaxation channels
Improved TOF and position resolution are expected to bring better insight into the fragmentation process.
Independent structural determination of the free clusters is desirable for a validation of the aerodynamic acceleration model.
Conclusions and outlook
XPS
S. Peredkov, et al. Phys Rev B 75, 235407 (2007)
bulk
surface
XPS
S. Peredkov, et al. Phys Rev B 75, 235407 (2007)
Source of uncertainty for R∆Z = ±1
S. Peredkov, et al. Phys Rev B 76, 081402(R)
XPS
ACKNOWLEDGEMENTS:
Senior:Paolo Piseri, Cristina Lenardi,
Post-doc:Tommaso Mazza, Gero Bongiorno, Luca Ravagnan, Matteo Amati
Graduate/PhD-students:Michele Devetta, Flavio Della Foglia
UniMI: People at LGM (Group leader Prof. Paolo Milani):
GasPhase:Marcello Coreno, Monica De Simone, Lorenzo Avaldi, Kevin Prince
University of Gröningen (The Nederlands):Petra Rudolf, Fabrizio Evangelista
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