J'R' Macdonald Laboratory, Physics Department, Kansas State University, Manhattan, Kansas 66506

1
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. Suppo
rted by the Chemical Sciences, Geosciences and
Biosciences Division, Office of Basic Energy
Sciences, Office of Science, U. S. Department of
Energy.
2
Bond rearrangement
An example
Vinylidene
Acetylene
H
H
C
C
H
H
C
C
Osipov et al., PRL 90(23), 233002 (2003)
3
Ionization Dissociation of water by fast
charged particles (i.e. ionizing radiation) e-,
H, Xq, (h?) H2O ?
Our main interest
In particular
H H2O 4 MeV
Xq H2O ? H2O e- Xq
H2 O
Does the H2 actually come from water?
4
Where does the H2 come from?
Pressure dependence shows H2 from H2O is the
dominant source of H2 above 1 10-6 Torr.
5
Experiment Schematic
Target cell containing water vapor
Fragments accelerated by electric field

Bunched Beam, F7
E
Faraday Cup
_
Fragments
The buncher signal starts the clocks.
Detector
The first particle stops the first clock and so
on
6
Typical Data
7
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)
8
Slow Mechanism
Slow
tcol 1 a.u.
How would this process change upon isotopic
substitution?
As the fragments separate, the protons share the
electron
9
Fast Mechanism
tcol 1 a.u.
Sudden
The protons happened to be at the right
separation
How would this process change upon isotopic
substitution?
10
Is bond rearrangement in multiple ionization real?
Momentum conservation!
A.M. Sayler et al., AIP 680, 48 (2003)
11
Bond rearrangement in multiple ionization
19 MeV F7 H2O
12
Bond rearrangement in double and triple
ionization of H2O
F7 H2O at 1 MeV/amu
Results
H2 O
H2 O3
0.209 ? 0.006
Ill get around to it
H OH
H OH
H2 O2
0.067 ? 0.003
H OH
13
Bond rearrangement in double ionization of D2O
Results
0.116 0.006
14
Isotopic effect in double ionization of water
Double Ionization
0.555 0.033!
H2 O
0.125 0.013
H2O
0.401 0.047
Single Ionization
D2 O
0.0500 0.0028
D2O
15
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 Kolos et al., J.
Chem. Phys. 84, 3278 (1986) Werner et al., J.
Chem. Phys. 79, 905 (1983)
H-O-H angle at 104.5
16
The bond rearrangement channel is very small but
not significantly slower than other fragmentation
processes
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
17
Isotopic preference is caused by the increasing
spread of the initial nuclear wave function
18
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)
19
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

20
Motivation
IonizingRadiation

• Radiation damage
• Ionization caused mostly by secondary e-
• Ionized and dissociated water damages DNA

Cell
e-
e-
e-
H
OH
OH

• Fun along the way
• Isotopic Effect

OH