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 Photoelectron Yield 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? ? ?

What are the structural characteristics of the different isomer classes? How do water clusters bind an excess electron? Isomer I shows a single bound water molecule with AA binding motif Multiple Isomers for Water Cluster Anions What are the barriers for interconversion? ? ?

I ' + m Ar [I, I ', II ·Ar m ] [I ' ] ‡ ·Ar m I II Isomer Selective Vibrational Excitation PES Probes Isomer distribution of quenched ensemble Rapid quenching by Ar evaporation

I'I' I, I ', II [ I']‡[ I']‡ I II Isomer Selective Vibrational Excitation PES Probe (H 2 O) n  ·Ar m [(H 2 O) n  ] ‡ ·Ar m (H 2 O) n  + m Ar Rapid quenching by Ar evaporation

Starting point: (H 2 O) 6  photoelectron spectrum at 1064 nm Compare to: (H 2 O) 6  ·Ar 7 photoelectron spectrum at 1064 nm Adding argon atoms kills isomer II Photoelectron Yield Electron Binding Energy (eV) II I

Photoelectron Imager Nd:YAG Laser (1064 nm) e - Gun OPO/OPA Laser (tunable cm -1 ) Time-of-flight Mass Spectrometer Reflectron H 2 O / Ar Expansion tandem time-of-flight mass spectrometer vibrational predissociation spectroscopy photoelectron spectroscopy

(H 2 O) 6 - ·Ar 7 + h → (H 2 O) 6  + 7 Ar (H 2 O) 6  + h → (H 2 O) 6 + e  Infrared excitation followed by photoelectron velocity-map imaging 1064 nm 3350 cm -1

[I ·Ar 7 ] [I] ‡ ·Ar Photon Energy (cm -1 ) Ar evaporation I II PES Probe First study of photoisomerization (H 2 O) 6  ·Ar 7 Only has Isomer I + 7 Ar

(H 2 O) 6  ·Ar Photon Energy (cm -1 ) 3350 cm -1

Two-laser experiment: (H 2 O) 6  ·Ar cm -1 (Isomer I ) → (H 2 O) 6  + 7 Ar photoelectron spectrum of daughter fragment (H 2 O) 6  at 1064 nm Infrared excitation of the cluster does not induce isomerization bare (H 2 O) 6 - (H 2 O) 6 - Ar 7 parent (H 2 O) 6 - daughter Photoelectron Yield Electron Binding Energy (eV) (H 2 O) 6  ·Ar 7 II I

? ?

[I ·Ar 7 ] [I] ‡ ·Ar Photon Energy (cm -1 ) Ar evaporation I II PES Probe First study of photoisomerization (H 2 O) 6  ·Ar 7 Only has Isomer I + 7 Ar FAILED!

3308 cm -1 (H 2 O) 7  ·Ar m

[I, I ', II ·Ar 8 ] [I ' ] ‡ ·Ar 8 Ar evaporation 3308 cm -1 I ·Ar II ·Ar I ' ·Ar + 7 Ar Isomer I ' Vibrational Excitation PES Probe Photoisomerization in (H 2 O) 7  ·Ar 8

(H 2 O) 7 - Ar 8 parent (H 2 O) 7 - Ar (H 2 O) 7 - Ar daughter Photoelectron Yield Electron Binding Energy (eV) Conversion from Isomer I’ to I occurs!! (H 2 O) 7  ·Ar 8 II I I’ Two-laser experiment: (H 2 O) 7  ·Ar cm -1 (Isomer I ’) → (H 2 O) 7  ·Ar + 7 Ar photoelectron spectrum of daughter fragment (H 2 O) 7  ·Ar at 1064 nm

I ' ·Ar + 7 Ar II ·Ar [I, I ', II ·Ar 8 ] [I ' ] ‡ ·Ar 8 Ar evaporation 3308 cm -1 Isomer I ' Vibrational Excitation PES Probe Photoisomerization in (H 2 O) 7  ·Ar 8 SUCCESS! I ·Ar

b II (H 2 O) 7 - Parent (H 2 O) 7 - Isomer I (H 2 O) 7 - Isomer II I (H 2 O) 7  ·Ar m m = 4 m = cm Photoelectron Yield Electron Binding Energy

(H 2 O) 7 - Isomer II (H 2 O) 7 - Daughter Two-laser experiment: (H 2 O) 7  ·Ar cm -1 (Isomer II ) → (H 2 O) 7  + 3 Ar photoelectron spectrum of daughter fragment (H 2 O) 8  at 1064 nm Conversion from Isomer II to I occurs!! (H 2 O) 7  ·Ar Photoelectron Counts Electron Binding Energy (eV) II I I'I'

[II] ‡ Argon evaporation traps geometry I II I'I' 1592 cm -1 [I, I ', II ·Ar 3 ] ? SUCCESS!

Conclusions New technique for monitoring isomer conversion in anions Conversion from Isomer I’ to I does occur in (H 2 O) 7  ·Ar 8 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 (H 2 O) 7  ·Ar 3 when II ’s HOH bend is excited, thus setting a barrier maximum at 1592 cm -1 Isomer I Isomer II

P451 cragnavy

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

3308 cm -1