doi:10.1016/j.nimb.2005.03.123 
Copyright © 2005 Published by Elsevier B.V.
Bond rearrangement caused by sudden single and multiple ionization of water molecules
I. Ben-Itzhak
, 
, A. Max Sayler, M. Leonard, J.W. Maseberg, D. Hathiramani, E. Wells, M.A. Smith, Jiangfan Xia, Pengqian Wang, K.D. Carnes and B.D. Esry
J.R. Macdonald laboratory, Department of Physics, Kansas State University, Manhattan, KS 66506, USA
Available online 29 April 2005.
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Abstract
Bond rearrangement, namely the dissociation of water into 
following ionization by fast proton and highly charged ion impact, was investigated. Single ionization by fast proton impact exhibits a strong isotopic effect, the dissociation of 
being about twice as likely as 
, with HDO+ → HD+ + O in between. This suggests that the bond rearrangement does not happen during the slow dissociation, but rather during the very fast ionization, and thus 
should also be produced when the water molecule is multiply ionized. We observed that the 
and 
production in 1 MeV/amu F7+ + H2O collisions are 0.209 ± 0.006% and 0.0665 ± 0.003%, respectively, of the main double-ionization dissociation product, H2O2+ → H+ + OH+. This ratio is similar to the triple to double ionization ratio in similar collisions with atomic targets thus suggesting that the bond-rearrangement fraction out of each ionization level is approximately constant. Similar dissociation channels in the heavier water isotopes, which are expected to be smaller, are under study. Finally, the fragmentation of HDO exhibits very strong isotopic preference for breaking the OH bond over the OD bond.
Keywords: Bond rearrangement; Fragmentation; Isotopic-effect; Bond-breaking asymmetries; Water molecules
PACS: 34.50.Gb; 82.30.Lp; 82.30.Qt
Fig. 1. The yield of 
relative to H2O+ molecular ions produced by 1 MeV/amu highly charged ion impact as a function of the pressure in the target cell, which is about a factor of 200 larger than the pressure measured by the chamber ion gauge. The arrow on the figure show the range of pressures for which the residual gas contribution is negligible.
Fig. 2. The yield of 
relative to single ionization of water as a function of the projectile velocity. Proton impact – open symbols, electron data (from [2]) – full symbols (see text).
Fig. 3. The contour plot of the OH–OH+ cut through the H2O+ PES. The expected peak of the overlap between the initial and final nuclear wave functions is marked as a color-filled contour plot centered around RO–H = 2 a.u. (red-high, blue-low, the same scheme is used in all following contour plots).
Fig. 4. The contour plot of the R–HH+ cut through the H2O+ PES (see text). Note that the expected peak of the overlap between the initial and final nuclear wave functions (marked as in Fig. 3) is partly on the steep slope along the H–H+ bond (see text).
Fig. 5. The contour plot of the OD–DD+ cut through the D2O+ PES. Note that the expected peak of the overlap between the initial and final nuclear wave functions (marked as in Fig. 3) is partly on the steep slope along the D–D+ bond (see text).
Fig. 6. Coincidence time-of-flight spectrum of water molecules ionized by 1 MeV/amu F7+ collisions.
Fig. 7. Density plots of the coincidence time-of-flight spectrum of water molecules ionized by 1 MeV/amu F7+ collisions. (a) Raw data, (b) simulated random pairs (normalized to H++H2O+ yield, see text) and (c) data after subtraction of random pairs, i.e. (a)–(b). The true ion-pair channels labels are “boxed”, while the random ones are unboxed. The solid lines mark the expected behavior of two-body fragmentation, i.e. a slope of −1 for q1 = q2 = 1 (see text).
Fig. 8. Density plots of the bond-rearrangement dissociation channels of double and triple ionization. The solid lines mark the expected behavior of two-body fragmentation, i.e. a slope of q1/q2 (see text, and [13]).
Fig. 9. The contour plot of the OH+–OD+ cut through the HDO2+ PES. Note that we use scaled coordinates in this plot in contrast to the previous ones in order to visualize the isotopic preference for breaking the O–H bond over the O–D bond (see text).
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