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

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