IPP Annual Report 2007 - Max-Planck-Institut für Plasmaphysik ...

Charged Water Clusters
Many aspects of the photoelectron
spectra of free molecules
are well understood. On the other
hand, electron spectroscopy of
liquids has only become possible
recently and is a field of rapidly
growing interest. Photoelectron
spectra of liquid water differ in a
number of respects from the mole-
cule: Features due to ionic state vibrations, a characteristic
of molecular conformational changes, are smeared out or
absent in the liquid. Ionization energies of all orbitals shift towards
lower values. The value of the vertical ionization energy
of the least strongly bound electrons in the liquid has been
measured some years ago. In our study of free water clusters,
we show how this literature value is smoothly approached
when ionization of clusters of increasing size is studied.
HOMO Energy Shift to Monomer (eV)
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.0
0.2
0.4 0.6
N-1/3 Figure 1 gives a graphical representation of the shift in photoionization
energy of water clusters. Experimentally, clusters
are produced by supersonic expansion of water vapour
into vacuum. Clusters with a rather broad size distribution
are thus produced, the mean of which can be varied by
changing the expansion conditions (pressure and nozzle
temperature). An empirical scaling law, derived from mass
spectrometry of cluster beams, was used for estimating this
mean size. The energy plotted in figure 1 is measured from
the adiabatic peak in the isolated molecule to the maximum
of the first photoelectron peak in the cluster.
The linear dependence of these energies vs. inverse size is in
line with expectation, if their main origin is the polarization
Electron Spectroscopy
Head: PD Dr. Uwe Hergenhahn
Photoelectron spectroscopy is a valuable tool
for the study of material properties, as seen for
example by its use in the investigation of fusion
reactor wall materials. In fundamental studies
of its application to materials in general we
have measured photoelectron and autoionization
spectra from free water clusters. The sizedependent
vertical ionization energies are shown
for the first time.
Cluster
Monomer
Liquid Water
0.8
1.0
Figure 1: Energy difference of the least strongly bound photoelectrons of
water clusters as a function of inverse cluster size. N is the mean value of
the number of molecules in the cluster. The asymptotic value for liquid
water is taken from the literature.
101
Autoionization of Clusters
screening of the positive hole in
the cluster. Initial state effects,
surface charging and changes
of the nuclear framework in the
ionization process are thus found
to be of minor importance. For
the latter, this is confirmed by
the fact that these shifts are
practically identical in heavy
water (D 2 O).
The influence of the chemical environment on autoionization
in weakly bonded systems has been studied recently,
and a new transition pathway, which is only possible
because of the cluster environment, identified (Intermolecular
Coulombic Decay, ICD). ICD is expected to be a
universal phenomenon, but so far has only been identified in
noble gas clusters. The presence of a large quantity of low
kinetic energy electrons coincident with primary inner
valence photoelectrons from water clusters is the first experimental
evidence for ICD in a hydrogen bridge-bonded system
(figure 2). The same set-up was used to record electronelectron
coincidence spectra of Ne clusters. Here, ICD is
well-known from our experiments using a conventional
(non-coincident) spectrometer. These earlier findings were
beautifully corroborated.
Coinc. Events /eV
Kinetic Energy (e1) (eV)
10000
8000
6000
4000
2000
0
16
14
12
10
8
6
4
ICD Spectrum
Scientific Staff
5 10 15 20
Kinetic Energy (e2) (eV)
250
S. Barth (1-3/07), V. Ulrich (1-7/07), M. Mucke (8-12/07),
M. Förstel (9-12/07), T. Lischke, A. M. Bradshaw, U. Hergenhahn.
Inner Valence
2
Coinc. Events / (eV x eV)
Photoelectron Spectrum
0 2000 4000 6000
Coinc. Events /eV
Figure 2: Intensity map of photoelectron-secondary electron pairs for inner
valence ionization of water clusters. The ionization energy was set to 45 eV.
The joint emission probability for each combination of kinetic energies of the
two electrons detected in coincidence is presented in the two-dimensional
map. Spectra shown in the top and righthand panel pertain to summation over
all secondary electron energies or all photoelectron energies, respectively.