Probing isomer interconversion in anionic water clusters using an Ar-mediated pump- probe approach...
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Transcript of Probing isomer interconversion in anionic water clusters using an Ar-mediated pump- probe approach...
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
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!
[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'
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