Isotopic Effect in Bond Rearrangement Caused By Sudden Single and Multiple Ionization of Water Mat Leonard, A. Max Sayler, Kevin D. Carnes, Brett Esry, and Itzik Ben-Itzhak J.R. Macdonald Laboratory, Physics Department, Kansas State University, Manhattan, Kansas 66506 ACKNOWLEDGMENTS Thanks to Mark A. Smith, Jiangfan Xia, Jack W. Maseberg, and Dag Hathiramani. Supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U. S. Department of Energy.

Bond rearrangement H H C C H H C C An example: Acetylene Vinylidene Osipov et al., PRL 90(23), 233002 (2003)

Our main interest Xq+ + H2O → H2O+ + e- + Xq+ H2+ + O Ionization & Dissociation of water by fast charged particles (i.e. ionizing radiation) e-, H+, Xq+, (hν) + H2O  In particular: H+ + H2O 4 MeV Xq+ + H2O → H2O+ + e- + Xq+ H2+ + O Does the H2+ actually come from water?

Where does the H2+ come from? Pressure dependence shows H2+ from H2O+ is the dominant source of H2+ above ~ 1 × 10-6 Torr.

Experiment Schematic Target cell containing Fragments accelerated water vapor Fragments accelerated by electric field + Bunched Beam, F7+ E Faraday Cup _ Time of flight  Fragments The buncher signal starts the clocks. Detector The first particle stops the first clock and so on…

Typical Data

Bond rearrangement in single ionization of water Note that H2O+ → H2+ + O is about twice as likely as D2O+ → D2+ + O, with HDO+ → HD+ + O in between. A.M. Sayler et al., AIP 576, 33 (2001) Electron impact data from: Straub et al., J. Chem. Phys. 108, 109 (1998)

“Slow” Mechanism Slow How would this process change tcol ~ 1 a.u. How would this process change upon isotopic substitution? As the fragments separate, the protons share the electron

“Fast” Mechanism Sudden How would this process change tcol ~ 1 a.u. Sudden The protons happened to be at the right separation How would this process change upon isotopic substitution?

Is bond rearrangement in multiple ionization real? Momentum conservation! A.M. Sayler et al., AIP 680, 48 (2003)

Bond rearrangement in multiple ionization 19 MeV F7+ + H2O

Bond rearrangement in double and triple ionization of H2O F7+ + H2O at 1 MeV/amu Results: H2+ + O+ H2+ + O3+ = 0.209  0.006 % = I’ll get around to it H+ + OH+ H+ + OH+ H2+ + O2+ = 0.067  0.003 % H+ + OH+

Bond rearrangement in double ionization of D2O Results: D2+ + O+ D+ + OD+ = 0.116 ± 0.006 %

Isotopic effect in double ionization of water H2+ + O+ = 0.209 ± 0.006 % H+ + OH+ = 0.555 ± 0.033! Double Ionization D2+ + O+ D+ + OD+ = 0.116 ± 0.006 % H2+ + O = 0.125 ± 0.013 % H2O+ = 0.401 ± 0.047 Single Ionization D2+ + O = 0.0500 ± 0.0028 % D2O+

Sudden ionization – fragmentation H+ + OH H+ + OH ≈ 67 % OH+ + H Mostly H2O+ Dissociation to OH+ + H is energetically favored over H+ + OH OH+ + H Pair PECs from: Chen PhD 1991 Kołos et al., J. Chem. Phys. 84, 3278 (1986) Werner et al., J. Chem. Phys. 79, 905 (1983) H-O-H angle at 104.5°

The bond rearrangement channel is very small but not significantly slower than other fragmentation processes O + H2+ H2+ + O ≈ 0.9 % OH + H+ Dissociation to OH + H+ is energetically favored over H2+ + O In addition, the OH + H+ dissociation path is kinematically favored O-H bonds held equal

Isotopic preference is caused by the increasing spread of the initial nuclear wave function O + H2+ O + D2+

An interesting observation H2+ + O2+ Note that = 0.32  0.02 ~ 1/3 H2+ + O+ while Ne3+ = 0.390  0.006 Ne2+ Ar3+ = 0.326  0.003 Ar2+ CO3+ = 0.35  0.02 CO2+ BR seems to be a constant fraction of H2Oq+ Heber et al. Phys. Rev. A 52, 4578 (1995) Ben-Itzhak et al. Phys. Rev. A 47, 2827 (1993)

Summary Bond rearrangement – forming H-H+ bond upon dissociation of transient water molecular ion Bond rearrangement occurs in single and multiple ionization An isotopic effect has been observed in single and double ionization, bond rearrangement is more probable for less massive isotopes. The data suggests that bond-rearrangement is approximately a constant fraction of each ionization level A sudden mechanism is suggested in which the matching of the initial and final states are the key factor Future work Theory –time evolution of the nuclear wave functions on the PES of H2Oq+

Motivation Fun along the way Isotopic Effect Radiation damage Ionizing Radiation Radiation damage Ionization caused mostly by secondary e- Ionized and dissociated water damages DNA Cell e- e- e- H+ OH+ OH OH Fun along the way Isotopic Effect