Department of Physics and Astronomy, Queens University, Belfast, BT7 1NN, N. Ireland, UK

1
Photoion-Pair Formation in H2O and D2O in the EUV
spectral region
CA Hunniford, R Browning. SWJ Scully, KF Dunn and
CJ Latimer
Department of Physics and Astronomy, Queens
University, Belfast, BT7 1NN, N. Ireland, UK
Ion-pair production is an extremely important
method for the investigation of excited and
superexcited states in neutral molecules. Doubly
excited Rydberg states, lying between the single
and double ionization thresholds, provide
fundamental information on electron correlation.
In addition, investigations involving isotopic
substitution can provide information on the decay
dynamics of these states. Furthermore, water
radiolysis is known to be an important process
causing cell mutations and death 1. In this
context there is a need for cross section data on
fragmentation processes, including the following
anion production channels-
states built on the (2a1)-1 ion core indeed
this is what permits the superior resolution
possible in this work. The production of OH- ions
(figure 4) is found to be solely associated with
excitation to states in the energy region 1720
eV converging on the (1b2)-1 ion states.
Considerable vibrational structure is also
observed, extending in the form of superexcited
states, well into the continuum.
Figure 2 Schematic diagram of the apparatus
All three possible negative ions O-, OH- and H-
ions have been observed and appear at their
thermochemical thresholds 2. Figure 3 shows the
cross sections for O- production in the range 18
33 eV. Two major features are apparent a
small one at 19.5 eV and a large composite
feature in the region 23 31 eV. The former
shows only a small isotope effect, and can
readily be associated with the single-hole
one-electron states built on the (1b2)-1 band of
ion states centered at 18.5 eV. The large
composite feature appears in a barren region in
the photoelectron spectrum and is clearly due to
formation via doubly excited states. Two such
states at 25 and 28 eV, as well as states built
on the (2a1)-1 ion state at 32.3 eV, have
recently been observed in the fluorescence
spectra of fragment H atoms 3. However fitting
Gaussian functions to our data clearly shows that
there are at least three such doubly excited
states at energies of 25.2, 27.1 and 29.0 eV.
The strong isotope effect shows that the
fragmentation process may be governed by
autoionization. Interestingly few O- ions are
observed to come from excitation to Rydberg
(H2O) ? O- (2H) h? H2O ?
? OH- H (H2O)
? H- (OH)
Figure 4 OH- and OD- formation in H2O and D2O.
Calculated energies of Rydberg states are also
shown.
Anion production has also been observed to
increase strongly in the double ionisation region
above 33.5eV where 8 dication states (see figure
5) are known. Our data for H- and D- formation
are particularly interesting on account of the
exceptionally large isotope effects observed
(cross section ratio H-/D- 15).

Figure 1 Schematic diagram of the inner and
outer valance orbitals in the water molecule
 
These experiments were carried out at the
Daresbury Laboratory (UK) synchrotron, beamlines
3.1 and 5D. The VUV photon beam was crossed by a
low pressure gas jet of target molecules. Cross
sections for the production of negative ions were
obtained by extracting them with an electric
field and analyzing them using a triple
quadrupole mass spectrometer.
Figure 5 H- and D- formation in H2O and D2O. The
known energies of 8 dication states are also
shown.
References 1 GH Olivera et al Phys Med Biol 43
2347 (1998) 2 J Berkowitz in VUV and Soft X-ray
Photoionization, Eds U Becker and D A Shirley,
Plenum Press, New York 1996 3 M Kato, T
Odagiri, K Kodama, M Murata, K Kameta and N
Kouchi J Phys B 37 3127 (2004)
Figure 3 O- formation in H2O and D2O. The lines
labelled T indicate the thermochemical thresholds
and D1, D2 and D3 are the doubly excited states
observed for the first time in this work