J.R. Macdonald Laboratory, Physics Department, Kansas State University, Manhattan, Kansas
66506
Mat Leonard, A. Max Sayler, Kevin D. Carnes, Brett Esry, and Itzik Ben-Itzhak
ACKNOWLEDGMENTSThanks 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.
Isotopic Effect in Bond Rearrangement Caused By Sudden Single and Multiple
Ionization of Water
H C C H H C C
H
Osipov et al., PRL 90(23), 233002 (2003)
An example:
Acetylene Vinylidene
Bond rearrangement
500 1000 1500
0
20
40
60
80
100
120
H2O+
OH+
O+
O2+
H2
+
H+
Cou
nts1/
2
Time-of-Flight (ns)
H+ + H2O4 MeVXq+ + H2O → H2O+ + e- + Xq+
H2+ + O
Does the H2+ actually come from water?
Our main interestIonization & Dissociation of water by fast charged particles (i.e. ionizing radiation)e-, H+, Xq+, (hν) + H2O
In particular:
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
1 MeV/amu
R = 0.17 %
H2+ /
H2O
+ (%
)
PIon-Gauge
[106 Torr]
F4+
F9+
Where does the H2+ come from?
Pressure dependence shows H2+ from
H2O+ is the dominant source of H2+ above
~ 1 × 10-6 Torr.
E
Detector
Fragments
Bunched Beam, F7+
The first particle stops the first clock and so on…
The buncher signal starts the clocks.
Faraday Cup
Time of flight chargemass
Target cell containing water vapor
+
_
Fragments accelerated by electric field
Experiment Schematic
Typical Data
20 40 60 80 100 120 140 160 180
20
40
60
80
100
120
H2O+
OH+O3+
Typical Data2D CoincidenceChannels
H2
++ O2+
H2
++ O+
O2+ O+
H2
++Xq+
H+ + Xq+
Time of Flight, Particle 2 (arb.)
Tim
e O
f Flig
ht, P
artic
le 1
(ar
b.)
1.000
7.023
49.32
346.4
2433
1.709E4
1.2E5
1000 1250 1500 1750 2000 2250 2500 2750
100
1000
10000
100000
1000000
N3+
O2
+
N2
+
C+
N2+
N+
O6+
D2O+
HDO+
O5+
O4+
O3+
O2+
O+
OH+
H2O+
H2
+
H+
19 MeV F7++ H2O
Typical Data - 1D Time of Flight Spectrum
Cou
nts
Time of Flight
Bond rearrangement in single ionization of water
0 5 10 15 20 250.00
0.05
0.10
0.15
0.050%
0.084%
0.125%
Open - H+
Full - e-
Half full - F9+
[H2
++ O] / H2O+
[HD++ O] / HDO+
[D2
++ O] / D2O+
pre
centa
ge o
f si
ngly
ion
ized w
ate
r
v (a.u.)
Note that H2O+ → H2+ + O
is about twice as likely as
D2O+ → D2+ + O, with
HDO+ → HD+ + O in between.
Electron impact data from: Straub et al., J. Chem. Phys. 108, 109 (1998)
A.M. Sayler et al., AIP 576, 33 (2001)
“Slow” Mechanism
tcol ~ 1 a.u.
Slow
As the fragments separate, the
protons share the electron How would this process change upon isotopic substitution?
Sudden
The protons happened to be at the right separation
“Fast” Mechanism
tcol ~ 1 a.u.
How would this process change upon isotopic substitution?
0 10 20 30 40 500
5
10
15
20
Raw Ion-Pairs Spectra
H+ + H2O+
H+ + OH+H+ + O+
TOF2 (ch.)
TO
F1
(ch
.)
50 1k 20k
0
5
10
15
20
Linear
Log
H2
+ + OH+ H2
+ + H2O+
H2
+ + O+
6 19 32
0 10 20 30 40 500
5
10
15
20
Log
H+ + H2O+
H+ + OH+H+ + O+
TOF2 (ch.)
TO
F1
(ch
.)
10 1k 10k
5
10
15
20Randomly Matched Ion-Pairs Spectra
LinearH2+ +
H2O
+
H2
+ + OH+
H2
+ + O+
3 4 6 7 8
0 10 20 30 40 500
5
10
15
20
Log
H+ + H2O+
H+ + O+
H+ + OH+
TOF2 (ch.)
TO
F1
(ch
.)
30 10k 20k
0
5
10
15
20
Raw Data with the Random Ion-Pairs Subtracted
Linear
H2
+ + H2O+
H2
+ + OH+
H2
+ + O+
5 19 32
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
F7+ + H2O at 1 MeV/amu
Bond rearrangement in double and triple ionization of H2O
Results: H2
+ + O+
= 0.209 0.006 %H+ + OH+
H2+ + O2+
= 0.067 0.003 %H+ + OH+
5 10 15 20 25 30 35 405
10
15
20
25
30
35
40H
2
++ O+
Time of Flight 2nd Particle
Tim
e of
Flig
ht 1
st P
artic
le
1.0002.6114.2225.8337.4449.05610.6712.2813.8915.5017.1118.7220.3321.9423.5625.1726.7828.3930.00
10 20 30
20
30
40
H2
++ O2+
Time of Flight 2nd ParticleT
ime
of F
light
1st P
artic
le
1.0001.3891.7782.1672.5562.9443.3333.7224.1114.5004.8895.2785.6676.0566.4446.8337.2227.6118.000
H2+ + O3+
= I’ll get around to itH+ + OH+
25 30 35 40 45 50 55 60 65
5
10
15
20
25
30
35
40
45°
D2
++ O+
2nd Particle Time of Flight (Arb.)
1st P
artic
le T
ime
of F
light
(Arb
.)
0
1.333
2.667
4.000
5.333
6.667
8.000
9.333
10.67
12.00
13.33
14.67
16.00
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+
D2+ + O+
D+ + OD+ = 0.116 ± 0.006 %
= 0.555 ± 0.033!
H2+ + O
= 0.125 ± 0.013 %H2O+
= 0.0500 ± 0.0028 %D2
+ + O
D2O+
= 0.401 ± 0.047
Double Ionization
Single Ionization
0 1 2 3 4 5 6 70
1
2
3
4
5
6
7
0.045 a.u. per div
RO
-H+
(a.u
.)
RO-H
(a.u.)
OH+ + H
H+ + OH
H+ + OHOH+ + H
≈ 67 %
• Mostly H2O+
• Dissociation to OH+ + H is energetically favored over H+ + OH
Pair PECs from:Chen PhD 1991Kołos et al., J. Chem. Phys. 84, 3278 (1986)Werner et al., J. Chem. Phys. 79, 905 (1983)
Sudden ionization – fragmentation
H-O-H angle at 104.5°
H2+ + O
OH + H+≈ 0.9 %
• Dissociation to OH + H+ is energetically favored over H2
+ + O
• In addition, the OH + H+ dissociation path is kinematically favored
The bond rearrangement channel is very small but not significantly slower than other fragmentation processes
O-H bonds held equal
0 1 2 3 4 5 6 70
1
2
3
4
5
6
7
0.05 a.u. per div
RH
-H+
(a.u
.)
RO-H
(a.u.)
O + H2+
0 1 2 3 4 5 6 70
1
2
3
4
5
6
7
0.05 a.u. per div
RH
-H+
(a.u
.)
RO-H
(a.u.)
O + H2+
0 1 2 3 4 5 6 70
1
2
3
4
5
6
7
0.05 a.u. per div
RH
-H+
(a.u
.)
RO-H
(a.u.)
O + D2+
Isotopic preference is caused by the increasing spread of the initial nuclear wave function
Note that
Ne3+
= 0.390 0.006Ne2+
H2+ + O2+
= 0.32 0.02 ~ 1/3H2
+ + O+
Ar3+
= 0.326 0.003Ar2+
CO3+
= 0.35 0.02CO2+
while
Heber et al. Phys. Rev. A 52, 4578 (1995)Ben-Itzhak et al. Phys. Rev. A 47, 2827 (1993)
BR seems to be a constant fraction of H2Oq+
An interesting observation
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
IonizingRadiation
e- e-
OH+
H+
e-
Cell
Fun along the wayIsotopic Effect
OH OH
Radiation damageIonization caused mostly by secondary e-
Ionized and dissociated water damages DNA
Top Related