Probing isomer interconversion in anionic water clusters using an Ar-mediated pump-

probe approach

T. L. Guasco, G. H. Gardenier,

L. R. McCunn, B. M. Elliott, and

M. A. Johnson

How do water clusters bind an excess electron?

Multiple Isomers for Water Cluster Anions

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Ph

oto

elec

tro

n Y

ield

Electron Binding Energy (eV)

II I

How do water clusters bind an excess electron?

Multiple Isomers for Water Cluster Anions

How do water clusters bind an excess electron?

Multiple Isomers for Water Cluster Anions

What are the structural characteristics of the different isomer classes?

Isomer I shows a single bound water molecule with

AA binding motif

What are the barriers for interconversion?

?

?

I' + m Ar

[I, I', II ·Arm] [I']‡ ·Arm

I

II

Isomer Selective Vibrational Excitation

PES Probes Isomer distribution of

quenched ensemble

Rapid quenching by Ar evaporation

Photoelectron Imager

Nd:YAG Laser (1064 nm) Nd:YAG Laser (1064 nm)

e- Gun

OPO/OPA Laser (tunable 600-4500 cm-1)

Time-of-flight

Mass Spectrometer

Reflectron

H2O / Ar Expansion

tandem time-of-flight mass spectrometer

vibrational predissociation spectroscopy

photoelectron spectroscopy

(H2O)6-·Ar7 + h → (H2O)6

+ 7 Ar

(H2O)6 + h → (H2O)6 + e

Infrared excitation followed by photoelectron velocity-map imaging

1064 nm3350 cm-1

[I·Ar7] [I]‡ ·Ar7

3200 3400 3600 3800Photon Energy

(cm -1)

Ar evaporation

I

II

PES Probe

First study of photoisomerization

(H2O)6·Ar7

Only has Isomer I

+ 7 Ar

Two-laser experiment:

(H2O)6·Ar7 + 3350 cm -1 (Isomer I)

→ (H2O)6 + 7 Ar

photoelectron spectrum of daughter fragment (H2O)6

at 1064 nm

Infrared excitation of the cluster does not induce isomerization

bare (H2O)6-

(H2O)6-Ar7 parent

(H2O)6- daughter

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Ph

oto

elec

tro

n Y

ield

Electron Binding Energy (eV)

(H2O)6·Ar7

II I

[I·Ar7] [I]‡ ·Ar7

3200 3400 3600 3800Photon Energy

(cm -1)

Ar evaporation

I

II

PES Probe

First study of photoisomerization

(H2O)6·Ar7

Only has Isomer I

+ 7 Ar

FAILED!

3308 cm -1

(H2O)7·Arm

[I, I', II ·Ar8] [I']‡ ·Ar8

Ar evaporation

3308 cm -1

I·Ar

II·Ar

I'·Ar + 7 Ar

Isomer I' Vibrational Excitation

PES Probe

Photoisomerization in (H2O)7·Ar8

(H2O)7-Ar8 parent

(H2O)7-Ar

(H2O)7-Ar daughter

0.0 0.2 0.4 0.6 0.8 1.0

Ph

oto

elec

tro

n Y

ield

Electron Binding Energy (eV)

Conversion from Isomer I’ to I occurs!!

(H2O)7·Ar8

II I I’

Two-laser experiment:

(H2O)7·Ar8 + 3308 cm -1 (Isomer I’)

→ (H2O)7·Ar + 7 Ar

photoelectron spectrum of daughter fragment (H2O)7

·Ar at 1064 nm

I'·Ar + 7 Ar

II·Ar

[I, I', II ·Ar8] [I']‡ ·Ar8

Ar evaporation

3308 cm -1

Isomer I' Vibrational Excitation

PES Probe

Photoisomerization in (H2O)7·Ar8

SUCCESS!

I·Ar

bII (H2O)7- Parent

(H2O)7- Isomer I

(H2O)7- Isomer II

I

(H2O)7·Arm

m = 4

m = 0

1592 cm -1

0.0 0.2 0.4 0.6 0.8 1.0

Ph

oto

elec

tro

n Y

ield

Electron Binding Energy

(H2O)7- Isomer II

(H2O)7- Daughter Two-laser experiment:

(H2O)7·Ar3 + 1592 cm -1 (Isomer II)

→ (H2O)7 + 3 Ar

photoelectron spectrum of daughter fragment (H2O)8

at 1064 nm

Conversion from Isomer II to I occurs!!

(H2O)7·Ar3

0.0 0.2 0.4 0.6 0.8 1.00

2000

4000

6000

8000

Ph

oto

elec

tro

n C

ou

nts

Electron Binding Energy (eV)

II I I'

[II]‡

Argon evaporation traps

geometry

I

II

I'1592 cm -1

[I, I', II ·Ar3] ?

SUCCESS!

Conclusions

• New technique for monitoring isomer conversion in anions

• Conversion from Isomer I’ to I does occur in (H2O)7·Ar8 when

symmetric OH stretch of I’ is excited, thus setting a barrier maximum at 3308 cm -1

• Conversion from Isomer II to I does occur in (H2O)7·Ar3 when

II’s HOH bend is excited, thus setting a barrier maximum at 1592 cm -1

Isomer I

Isomer II

Acknowledgements

• Department of Energy• National Science Foundation• Prof. Mark Johnson• Prof. Gary Weddle• Joe Bopp• Rob Roscioli• Rachael Relph• Kristin Breen• Helen Gerardi• Michael Kamrath• Jennifer Laaser