A review of experiments on ICD. The ICD process (in general) Proposition: Demonstrate it in an...
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Transcript of A review of experiments on ICD. The ICD process (in general) Proposition: Demonstrate it in an...
A review of experiments on ICD
• The ICD process (in general)
• Proposition: Demonstrate it in an experiment
Graphics: V. Averbukh
• Make (or take) weakly bonded system– so far: clusters, liquid jet
• Excite (photons, electrons, other particles)– so far: single photon ionization with synchrotron radiation,
particle impact (few experiments)
• ( Make sure that system is in ICD active state )– so far: photoelectron-‘ICD decay product’ coincidence experiment
or: ‘resonant ICD’, ICD-like decay after resonant excitation
• Detect decay products of ICD (electrons, ions)– so far: electron spectroscopy, ion spectroscopy & various combinations
fluorescence (few experiments, only for resonant ICD)
• ( Add time information to the experiment )
Graphics: Till Jahnke
A b
rief
his
tory
of
ICD
How can we detect ICD ?
Uwe Hergenhahn, IPP
Detect:
Lifetime broadening of ionized state
ICD electron
Fragment ions from
Coulomb explosion
A b
rief
his
tory
of
ICD
A measured ICD electron spectrum
• NeN + hn NeN-1 Ne+ 2s-1 + eph- NeN-2 (Ne+ 2p-1 )2 + eICD
- + eph
-
S. Barth, U. Hergenhahn et al., Chem. Phys. 329, 246 (2006)
Experimental Prerequisites: Vacuum
• Mean free path dependent on pressure
Exp
eri
menta
l
p (mbar) 10-12 10-6 1
n (cm-3 ) 2.47 *104 2.47 *1010 2.47 *1016
R. A. Haefer: Cryopumping
~1/sn
s: cross section Electrons, soft X-ray photons: * 1000
Experimental Prerequisites: Weakly coupled aggregates
• Cluster jets (atoms/molecules condensed by expansion into vacuum)
Other experiments:• Liquid jets
(thin solvent filament sprayed into vacuum)
• (Surfaces)(e.g. adsorbates, ice, …)
Exp
eri
menta
l
in vacuum
Sample gas
Nozzle
Skimmer
Point of interaction (with synchrotron radiation)
Experimental Prerequisites: Weakly coupled aggregates
• Cluster jets (atoms/molecules condensed by expansion into vacuum)
Clusters larger than dimer appear mixed in size:
Exp
eri
menta
l
in vacuum
P.M. Dehmer and S.T. Pratt, J. Chem. Phys. 76, 843 (1986).
Exp
eri
menta
l
Experimental Prerequisites: Excitation
• Amount of energy needed:– Inner valence ICD of atomic and molecular clusters: some tens of eV– Core level ICD, Auger-ICD cascades: some hundreds of eV
Future (?):– Cascade processes in radiation chemistry: hundreds of keV to MeV– Anionic initial states: few eV– ICD-like processes in quantum dots: vis. or IR range
• (single) photon impact– energy transfer known
• particle impact– inelastic scattering, energy transfer hard to control
Exp
eri
menta
l
Experimental Prerequisites: Short wavelength light sources
1
1200
10
120
100
12
1000
1.2
10 keV
0.12
photon energy (eV)
wavelength (nm)
vis. UV X-ray
Laser Laser (HHG)
Plasma discharge
sourcesX-ray line sources
Free Electron Lasers
Synchrotron Radiation (from Storage Rings)
: under development
ourwork
Experimental Prerequisites: Charged particle spectroscopy
• Measure energy of single electrons/single ions– Strategy 1:
Deflect by electric and/or magnetic, measure bendedness of trajectory– Strategy 2:
Use fast enough stopwatch
• ad 1)– e.g. hemispherical electron analyser– most widespread, robust, commercially available, easy to operate
• ad 2)– e.g. ‘magnetic bottle spectrometers’, ‘reaction microscopes’– exotic, homemade, requires experienced team – some advantages in research on ICD
Exp
eri
menta
l
Exp
eri
menta
l
• Example: Hemispherical electron analyzer• Electrons detected in narrow energy interval
and narrow solid angle interval– (most electrons are thrown away)
• Energy in hemispheres is fixed (pass energy Ep),use lens to sweepover spectrum
Experimental Prerequisites: Electron spectroscopy
E. P. Benis and T. J. M. Zouros, Nucl. Instrum. Meth. A 440, 462 (2000)
Electronlens
Deflectinghemispheres
Electrondetector
Sample
Ekin = Ep
Exp
eri
menta
l
Corrections of the spectrum
• Subtraction of atomic signal
• (Measured) transmission function of analyzer
Monomer2s line
Ep = 5 eV
A b
rief
his
tory
of
ICD
A measured ICD electron spectrum
• NeN + hn NeN-1 Ne+ 2s-1 + eph- NeN-2 (Ne+ 2p-1 )2 + eICD
- + eph
-
• Note background: Secondary electrons from inelastic scatteringS. Barth, U. Hergenhahn et al., Chem. Phys. 329, 246 (2006)
A b
rief
his
tory
of
ICD
How can we detect ICD ?
Uwe Hergenhahn, IPP
Detect:
Lifetime broadening of ionized state
ICD electron
Fragment ions from
Coulomb explosion
Lifetime broadening due to ICD
Any discrete energy level degenerate with a continuum acquiresa finite width (‘lifetime broadening’)
A b
rief
his
tory
of
ICD
Lifetime broadening due to ICD
• Substantial lorentzian broadening for the 2s bulk component • Lifetime: 6±1fs (bulk), > 30 fs (surface)• Cooperation University Uppsala (G. Öhrwall, O. Björneholm, …) with IPP (S. Marburger, U. Hergenhahn, …)G. Öhrwall et al., PRL 93 173401 (2004)
A b
rief
his
tory
of
ICD
Coulomb explosion due to ICD: The COLTRIMS technique
Uwe Hergenhahn, IPP
Horst Schmidt-Böcking, Reinhard Dörner, Joachim Ullrich, …
A b
rief
his
tory
of
ICD
Coulomb explosion due to ICD: Results (Till Jahnke et al.)
T. Jahnke et al., PRL 93 163401 (2004)
• Experimental strategy– Search for electron, ion, ion coincidence
– Require ion pairs with p1 = -p2
• Plot kinetic energy of ions vs. kinetic energy of electrons
– Ions: ‘Kinetic energy release’ (KER)– Energy conservation
A b
rief
his
tory
of
ICD
How can we detect ICD ?
Uwe Hergenhahn, IPP
Detect:
Lifetime broadening of ionized state
ICD electron
Fragment ions from Coulomb explosion
Ele
ctro
n,
Ele
ctro
n c
oin
cidence
An electron-electron coincidence experiment on ICD
e1
e2 Energy
e 1 Ene
rgy
mapping
e2
Ele
ctro
n,
Ele
ctro
n c
oin
cidence
Results for Ne clusters, N = 45
Ne 2sphotoline
ICDelectron spectrum
electron-electroncoincidence map
Ele
ctro
n,
Ele
ctro
n c
oin
cidence
Energy sharing in electron, electron experiments
Ekin, e1
Ekin, e2
diagonal features:equal energy sharing,inelastic electron scattering
horizontal features:sequential processesICD
Ele
ctro
n,
Ele
ctro
n c
oin
cidence
Results for Ne clusters, N = 630
Ne 2sphotoline
ICDelectron spectrum
electron impactionization (intracluster)
• For larger clusters, inelastic electron scattering becomes clearly visible
ICD
Ele
ctro
n,
Ele
ctro
n C
oin
cidence
NeAr clusters (hn = 52 eV)
• ArMNeN + hn ArMNeN-1 Ne+ 2s-1 + eph-
ArMNeN-2 (Ne+ 2p-1 )2 + eICD- + eph
-
ArM-1 (Ar+ 3p-1 ) NeN-1 (Ne+ 2p-1 ) + eICD- + eph
-
Ne 2s in coinc.with Ne ICD
Ne 2s in coinc.with mixed ICD
Ekin = 1.2-1.6 eV
Ekin = 7 - 8 eV
Ele
ctro
n,
Ele
ctro
n C
oin
cidence
NeAr clusters (hn = 52 eV)
• Outer valence PES shows cluster composition
• Relative yield of NeAr ICD vs. NeNe ICD differs (effect of cluster composition)
Ele
ctro
n,
Ele
ctro
n C
oin
cidence
Connecting ICD to cluster structure
– plot (relative) intensity of NeAr ICD vs. Ar content– compare to simulations for various cluster structures– find best match
[email protected] clusters
Results for water clusters
M. Mucke, UH et al., Nature Phys. 6, 143 (2010).
[email protected] clusters
Current work: Efficiency of ICD (aau)
• Coincidence detection allows to rigorously establish efficiency of ICD• Idea:
– Count photoelectrons– Count ICD electrons = number of coincidence events– Efficiency = (# of coinc.) dividec by (# of photoelectrons)
• Real world: finite detection efficiency g:
(P: coincidence count, p: non-coincident counts, a: ICD efficiency)
• Apparatus factors g can be determined from measurement of (e.g.) atomic Auger decay, e.g. Xe 4d (NOO)
• Example: Ne
• Result: aau = 0.99 ± 0.11
auauph
auph
EEp
EEP
)(
1
)(
),(
M. Förstel, UH et al., JES 191, 16 (2013).
[email protected] clusters
Efficiency of ICD in water clusters• Somewhat more complicated: inner valence monomer and cluster feature
overlap
• (c: degree of condensation, f: inelastic scattering losses, x: deviation monomer-cluster cross section)
• Large systematic error bars due to peak-background separation (bg from inelastic scattering)
• Result: aau < 1 , aau (H2O) < aau (D2O)
auauph
auph cfx
c
EcEp
EEP
1
)(
1
)(
),(
Energy (eph)
Ene
rgy
(eIC
D) ICDscattering
Furt
her
experi
ments
on IC
D
Auger decay - ICD cascades: Theory (Robin Santra et al.)
Uwe Hergenhahn, IPP R. Santra & L.S. Cederbaum, PRL 90 153401 (2003)
Ne atom,doubly ionized states
Ne dimer,doubly ionizedstates
triple ionization threshold
triple ionization threshold
Decay by ICD
Numerous experimental results for rare gas clusters (K. Ueda et al., R. Dörner et al.)
Furt
her
experi
ments
on IC
D
Auger decay - ICD cascades: Another picture (water)
Uwe Hergenhahn, IPP
For this species, experiment still missing !
Furt
her
experi
ments
on IC
D
How important is ICD ?
• Proportion of states that can decay by ICD (very approximate)– singly to doubly ionized (photoionization): around 20 %– doubly to triply ionized (Auger-ICD cascade): 20 to 50 %– singly to doubly ionized (resonant Auger-ICD cascade): over 50 %
• Resonant excitation: Select site of excitation
Produce site selective excitations, all of which decay by emitting an ICD electron
Uwe Hergenhahn, IPP
singly ionized
doubly ionized
binding energy0
+
+
+
outer v. inner v.
IntermolecularCoulombicDecay
States thatcannot candecay by ICD
Furt
her
experi
ments
on IC
D
Resonant Auger-ICD cascades
Gokhberg et al., Nature (doi: 10.1038/nature12936)
Results also byJahnke et al, Ueda et al., O’Keefe et al.
Oth
er
experi
ments
on IC
D
ICD initiated by impact of a fast ion beam
Uwe Hergenhahn, IPP
Kim, Jahnke, Dörner, et al , PNAS 108, 11821 (2011)
Total ion kinetic energy [eV]
• Low kinetic energy electrons:
• Conservation of energy
ICD initiated by ion impact
Oth
er
experi
ments
on IC
D
Water desorption initiated by ICD
Uwe Hergenhahn, IPP
Electron Transfer Mediated Decay – ETMD(3)
Zobeley, Cederbaum et al., JCP 115, 5076 (2001). Müller & Cederbaum, JCP 122, 094305 (2005).
ETMD (2):two sites involved
ETMD(3):three sites involved
process driven byelectron correlation
ETMD vs ICD (II)
Decay via charge transfer (ETMD) can compete with decay via energy transfer (ICD) if
• Decay via energy transfer is energetically not allowed (this talk)
• Decay via energy transfer is forbidden by selection rulesSakai, Ueda et al., PRL 106, 033401 (2011)see also Jahnke et al., PRL 99, 153401 (2007).
IP
+
IP'
center neighbours
Li+
+
H2O H2OH2O
• Charge transfer is needed becauseat vacancy site there are no electronsprediction: Müller & Cederbaum, JCP 122, 094305 (2005).
ArKr ETMD(3): primary photoionization
ArN KrM + hn ArN-1 Ar+(3s-1) KrM + eph-
ArN KrM-2 [ Kr+(4p-1) ]2 + eETMD- + eph
-
Ar outer valence
Kr outer valenceAr inner valence
• ArKr Clusters by coexpansion of Ar and Kr• Structure: Kr core, Ar shell• Search for autoionization of Ar 3s
IP Ar+(3s-1) (cl.) Kr+-Ar-Kr+ Kr-Ar+-Kr+
28.4 eV 27.9 eV 29.7 eV x
Lundwall, Björneholm, et al., PRA 74, 043206 (2006).Pernpointner et al., JCP 129, 024304 (2008).
ArKr ETMD(3): ExperimentKr
Ar
Ar 3s photoelectrons
ETMD
hn = 32 eV
Förstel/Hergenhahn et al.,PRL 106, 033402 (2011).
• Successful experimental strategies to detect ICD so far
– Electron spectroscopy– Electron, (Electron | Ion) coincidence spectroscopy
(better discrimination against inelastic scattering background)
• Samples so far
– Rare gas dimers,– Larger rare gas clusters, water cluster– Liquid jet targets
• Future (wishes)– Greater variety in both sample preparation and detection techniques
