Ionization and fragmentation of water molecules caused by fast proton impact; an isotopic effect in bond rearrangement

Max Sayler^*
(J.R. Macdonald Laboratory, Department of Physics, Kansas State University)

Fast proton impact on water molecules results mainly in single ionization, which, about a third of the time, is followed by rapid dissociation. A few percent of the molecules are doubly ionized and fragment rapidly into ion pairs. This production of ion pairs, and of OH radicals in particular, is believed to be the key to understanding DNA damage by ionizing radiation. Also of interest is a better understanding of bond rearrangement processes. One such process is the dissociation of the transient H₂O⁺ ion into H₂⁺ + O, which requires the formation of a H-H bond during dissociation. This process exhibits an isotopic dependence. Specifically, the production rate of the hydrogen molecular ion strongly depends on its mass when the H₂O, HDO, and D₂O isotopes are ionized. This measured dependence suggests that the rearrangement does not happen during the slow dissociation process, but rather during the very fast ionization process. Calculations are underway to determine the relative production rates for the different isotopes from the overlap of the initial and final vibrational wave functions. This model also predicts that the dissociation of the transient H₂O²⁺ ion into H₂⁺ + O⁺ will show a similar isotopic effect. We are now trying to measure such an isotopic effect, which to the best of our knowledge has never been seen before in double ionization.

^* in collaboration with: Jack W. Maseberg, Dag Hathiramani, Kevin D. Carnes, Brett D. Esry, and Itzik Ben-Itzhak.

This work was supported by the Chemical Sciences, Geosciences and Biosciences Division,
Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy.

Submitted to DAMOP 2002, May 2002 in Williamsburg, VA.

 

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