WUTA08

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Probing free metallic and carbon clusters with
VUV photons.
P. Piseri1,2,3 (piseri_at_fisica.unimi.it),
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-INFM 4 Dipartimento
di Farmacologia, Università degli Studi di
Milano. 5 CNR-IMIP, Area della ricerca di Roma
1 6 Laboratorio Nazionale TASC INFM-CNR 7
University of Gröningen, The Nederlands Laborato
rio Getti Molecolari e Materiali Nanocristallini
- LGM Director P. Milani (pmilani_at_mi.infn.it)
WUTA08 Laboratori Nazionali di
Frascati Frascati, 8-10 October 2008
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Outline
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

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Carbon
sp3
sp
sp2
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Carbon
Diamond
Crystal structures
?
Graphene
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Carbon clusters mass spectra
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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)
H. Vahedi-Tafreshi, et al. Journal of Nanoscience
and Nanotechnology 6, 1140 (2006)
anode
insulating valve flange
ceramic body
pulsed valve
graphite nozzle
thermalization cavity
rotating cathode
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Leaving the fullerene road
Residence time (?s)
M. Bogana et al. NJP 7, 81 (2005)
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Gas-Phase Nanoparticle deposition, or Cluster
Beam Deposition (CBD)
source
e gtgt 1 Fragmentation
e ltlt 1 Memory effect
Low Energy Cluster Beam Deposition
(LECBD) or Supersonic Cluster Beam Deposition
(SCBD)
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sp-chains
But how can we embed sp structures in a
disordered phase of carbon?
Linear carbon chains survive only as small
isolated clusters if they are deposited they
cross-link undergoing to graphitization
T. Wakabayashi et al. J. Phys. Chem. B 108, 3686
(2004)
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Raman spectroscopy of ns-C films
DG band
First observation of a clear signature of sp
bonds in a system of pure carbon !!
sp-chains are destroyed by oxygen in situ
measurements are mandatory
L. Ravagnan et al. PRL 89, 285506 (2002)
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Raman spectroscopy of ns-C films
DG band
Also the substrate temperature plays a crucial
role!
L. Ravagnan et al. PRL 89, 285506 (2002)
L. Ravagnan et al. PRL 98, 216103 (2007)
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Carbon
sp3

• Hard
• Semiconductor

ta-C
Disordered phases mixture of hybridizations

• Soft
• Conductive

a-C
sp
sp2
Ternary phase diagram of the amorphous pure
carbon system.
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Beyond Raman NEXAFS spectroscopy
? resonances are fingerprints of the specific
molecular bonds
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CESYRA in situ NEXAFS of ns-C films
C.S. Casari et al. Phys. Rev. B 69, 75422 (2004)
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CESYRA in situ NEXAFS of ns-C films
The spectra evolves both in the ? and ? region.
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CESYRA in situ NEXAFS of ns-C films
We observe the decay of ?(C?C) and the increase
of the ?(C?C)
NEXAFS spectroscopy is capable of distinguishing
between sp and sp2 in a system of pure carbon!!
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CESyRa apparatus layout
Interaction part
Source part
High voltage supply
Beam diagnostic device (see inset)
Time of flight mass spectrometer
Turbo 2000 l/s
Turbo 500 l/s
Turbo 300 l/s
Turbo 300 l/s
Turbo 500 l/s
Beam dumping chamber and quartz monitor
microbalance (not shown)
Quartz and steel gate valves (not shown)
Cluster beam
skimmer
Cluster source
Light entrance flange
Mass flow controller
Feedthrough of the deposition substrate for
in-situ cluster assembled film analysis
Gas cell and deflection stage chamber
Beam diagnostic device chamber
Source expansion chamber
Interaction chamber

• cluster beams machine

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CESYRA TEY NEXAFS of isolated clusters
285.6 eV
TEY isolated clusters
The ? region is peaked at 285.6 eV the cluster
are predominantly made by sp carbon!
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CESYRA TEY NEXAFS of isolated clusters
285.6 eV
TEY isolated clusters
The ? region is peaked at 285.6 eV the cluster
are predominantly made by sp carbon!
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CESYRA PEY NEXAFS of isolated clusters
Delay time time elapsed between the discharge
and the detection of the photo-electron.
v cost
Residence time of the probed cluster in the
source.
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CESYRA2 PEY NEXAFS of isolated clusters
75 - 81 ms
Increasing delay time
15 - 21 ms
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CESYRA2 PEY NEXAFS of isolated clusters
75 - 81 ms
Increasing delay time
15 - 21 ms
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Electron Yield Spectra

• What kind of systems can we study?

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XAS on free clusters
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XAS on free clusters
Mixed clusters and cluster-molecule systems add
more possibilities
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Signal to TDC stop channels 1-7 (100 ms range, 80
ps resolution)
Multiple Ion detectors
PEPICO TOF setup
PxPyCO setup
Ex-post reconstruction of electron-ion
coincidence spectrum (100 µs range) by software
computing the stop1-7-stop8 time differences
TDC start signal from pulsed source discharge
Cluster beam
Photon beam
Signal to TDC stop channel 8 (100 ms range , 80
ps resolution)
Electron detector
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Events Time Structure
Delay from start (ms)
Actual He injection 1 ms
Delay from start (ms)
0
1.0
0.5
tion,k
tel,j
He injection trigger 350 ?s
Discharge 60 ?s

• Recording the full information

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Events detection efficiency

• PEPICO

Electron-ion coincidence count rate (s-1)
Fraction of detected events
Detector number
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Events detection efficiency
Charge-state

• PEPICO

Average charge-state
guessed electron- detector efficiency 20
Fraction of detected events
Detector number
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Fraction of detected events
Photon Energy (eV)
Cluster residence time (ms)

• Average charge state does not evolve
  significantly !

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Determination of cluster velocity
Ion detector array
Photon beam
light clusters
heavy clusters
Cluster source
Cluster beam
electrons
Electron detector

• 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.

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Beam kinematics
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.
Bu. Wrenger and K. H. Meiwes-Broer, Rev. Sci.
Instrum. 68 (5), May 1997, 2027
? is proportional to the collision cross section
and is given by
Where ? is 2/3 for a spherical shaped particle
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Beam kinematics velocity vs residence time
A fitting parameter ?0.84 is found against
?2/30.667 as expected for dense spherical
particles
Data fitting by varying vHe, k, ?
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Determination of cluster v(m) distribution
deconvolution of t.o.f. spectra in different
charge states according to the detection
efficiency
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2nd condition different charge state must arise
from a same original mass distribution
Channel 2
Channel 3
Channel 4
Channel 5
Channel 6
Channel 7
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Determination of cluster v(m) distribution
deconvolution of t.o.f. spectra in different
charge states according to the detection
efficiency
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Beam kinematics velocity vs residence time
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XAS on free clusters
?
What relaxation channels in complex clusters ?
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Ion - Ion correlation spectra
PEPICO
1st order inter-arrival time distribution 2nd
order inter-arrival time distribution 4th order
inter-arrival time distribution
PInCO

• n-Erlang distributions for the false coincidence
  background instead of exponential

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Ion - Ion correlation intensity

• Maps of nth-order correlated ions intensity

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Space correlation

• More channel correlations

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Ion Ion coincidence spectra

• Space resolved

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Fragmentation yield

• Space resolved

x100
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Conclusions and outlook

• 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.

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XPS
bulk
surface
S. Peredkov, et al. Phys Rev B 75, 235407 (2007)
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XPS
S. Peredkov, et al. Phys Rev B 76, 081402(R)
(2008)
S. Peredkov, et al. Phys Rev B 75, 235407 (2007)
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XPS
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ACKNOWLEDGEMENTS
UniMI People at LGM (Group leader Prof. Paolo
Milani)
Senior Paolo Piseri, Cristina Lenardi,
Post-doc Tommaso Mazza, Gero Bongiorno, Luca
Ravagnan, Matteo Amati Graduate/PhD-students Mic
hele Devetta, Flavio Della Foglia
GasPhase Marcello Coreno, Monica De Simone,
Lorenzo Avaldi, Kevin Prince
University of Gröningen (The Nederlands) Petra
Rudolf, Fabrizio Evangelista