Photoelectron Imaging of Vibrational Autodetachment from Nitromethane Anions

Photoelectron Imaging of Vibrational Autodetachment from Nitromethane Anions Chris L. Adams, Holger Schneider, J. Mathias Weber JILA, University of Colorado, Boulder, CO 80309 -0440 OSU International Symposium on Molecular Spectroscopy June 23, 2009

Motivation Novel Approach to studying intramolecular vibrational relaxation (IVR).

Motivation What happens when a photon of hn interacts with an anion with Ee. BE < hn ?

Motivation What happens when a photon of hn interacts with an anion with Ee. BE < hn ? 1. Direct photoemission of the excess electron. A- + h n → A + e -

Motivation What happens when a photon of hn interacts with an anion with Ee. BE < hn ? 1. Direct photoemission of the excess electron. A- + hn → A + e- 2. Vibrational excitation followed by vibrational autodetachment (VAD) of the excess electron. A- + hn → [A-]* → A + e. First example: NH- (Lineberger and coworkers, 1985)

Nitroalkane Anions: A Model System • The excess electron is largely localized on the nitro group.

Nitroalkane Anions: A Model System • The excess electron is largely localized on the nitro group. • The fundamental CH vibrational transitions have energies in excess of the adiabatic electronic affinity (AEA) ~200 me. V (1600 cm-1).

Intramolecular Vibrational Relaxation (IVR) Dark States ZOBS e-

Experimental Setup MCP detector

IR Spectrum of Me. NO 2 Autodetachment spectrum CH 3 NO 2 - + hn CH 3 NO 2 + e-

Experimental Setup MCP detector

Velocity Map Imaging Photoelectron Spectroscopy (VMIPES) Ion Beam Laser Beam Direction

Example: VMIPES of S- (532 nm) Transformed Raw Image Integration BASEX over emission angles V. Dribinski et al. , RSI 73, 2634 2002. Photoelectron Spectrum Transformed Image

IR Spectrum of Me. NO 2 Autodetachment spectrum CH 3 NO 2 - + hn CH 3 NO 2 + e-

What do we expect from the direct photodetachment PES?

What is the Geometry of the Anion and the Neutral? Ө = 14° Ө = 0° Anion Neutral

Dominant FCF Active Modes • The wagging vibration of the neutral should give the most prominent vibrational progression in the PES.

Dominant FCF Active Modes • The wagging vibration of the neutral should give the most prominent vibrational progression in the PES. NO 2 Wag ~ 655 cm-1 (81 me. V )

Dominant FCF Active Modes • The wagging vibration of the neutral should give the most prominent vibrational progression in the PES. NO 2 Wag ~ 655 cm-1 (81 me. V ) • Upon emission the methyl rotor goes from being hindered to a free rotor.

- at 3200 cm-1 1 Me. NO 2

Peak Assignments – AEA determination Me. NO 23200 cm-1 Me. NO 2 -·Ar 3200 cm-1 • Peaks are spaced by ~ 645 cm-1 (80 me. V), corresponding to the wagging motion of the neutral.

Peak Assignments – AEA determination Me. NO 23200 cm-1 Me. NO 2 -·Ar 3200 cm-1 • Peaks are spaced by ~ 645 cm-1 (80 me. V), corresponding to the wagging motion of the neutral. • The first prominent peak, located at (172± 6) me. V, is identified as the origin of the vibrational progression (vanion=0, vneutral=0).

Peak Assignments – AEA determination Me. NO 23200 cm-1 Me. NO 2 -·Ar 3200 cm-1 • Peaks are spaced by ~ 645 cm-1 (80 me. V), corresponding to the wagging motion of the neutral. • The first prominent peak, located at (172± 6) me. V, is identified as the origin of the vibrational progression (vanion=0, vneutral=0). • Argon solvation shifts the vibrational progression by ~63 me. V (508 cm-1).

Peak Assignments – AEA determination Hot band Me. NO 23200 cm-1 Me. NO 2 -·Ar 3200 cm-1 • Peaks observed at binding energies less than 172 me. V are identified as hot bands.

Peak Assignments – AEA determination • Peaks observed at binding energies less than 172 me. V are identified as hot bands. Me. NO 23200 cm-1 Me. NO 2 -·Ar 3200 cm-1 • The difference in binding energies of the hot band origin of the vibrational progression matches the energy of the anionic wag.

Peak Assignments – AEA determination • Peaks observed at binding energies less than 172 me. V are identified as hot bands. Me. NO 23200 cm-1 Me. NO 2 -·Ar 3200 cm-1 • The difference in binding energies of the hot band origin of the vibrational progression matches the energy of the anionic wag. • The hot bands are suppressed upon Ar solvation.

Comparison of Experiment and Theory Franck-Condon Simulation (PESCAL) by Kent M. Ervin • B 3 LYP/6 -311++G(2 df, 2 p) for anion and neutral geometries • Independent Harmonic Oscillator Approximation with Duschinsky rotation • 14 vibrational modes treated in simulation • CH 3 torsion treated separately

Contribution of Torsion to the PES • There exists a pronounced shoulder on all of the dominant features of the PES regardless of Ar solvation.

Contribution of Torsion to the PES • There exists a pronounced shoulder on all of the dominant features of the PES regardless of Ar solvation. • The direct photodetachment involves a transition from hindered-to-free methyl rotor.

Contribution of Torsion to the PES • There exists a pronounced shoulder on all of the dominant features of the PES regardless of Ar solvation. • The direct photodetachment involves a transition from hindered-to-free methyl rotor. • This leads to progressions of the free internal rotor states superimposed on all transitions

IR Spectrum of Me. NO 2 Autodetachment spectrum CH 3 NO 2 - + hn CH 3 NO 2 + e- CH Stretching Vibrations ν 13 = 2775 cm-1 n 13 ν 14= 2922 cm-1 ν 15= 2965 cm-1 n 14 n 15

Comparison of Off and On Resonance Images Vibrational Autodetachement Direct Photoelectron Emission

Comparison of Off and On Resonance Images Direct Photoelectron emission Vibrational Autodetachement

On-Resonance Interpretation Both on-resonant and direct detachment contributions subtract contribution of direct photodetachment

On-Resonance Interpretation Compare with vibrational states of the neutral, neglecting torsion Without Torsion

On-Resonance Interpretation Compare with vibrational states of the neutral, including torsion With Torsion

On-Resonance Interpretation Inconsistencies with purely statistical argument. • Some states preferentially occupied • Nonstatistical population With Torsion

Summary • Considerable differences between direct detachment and vibrational autodetachment

Summary • Considerable differences between direct detachment and vibrational autodetachment • Redistribution of vibrational energy before electron emission

Summary • Considerable differences between direct detachment and vibrational autodetachment • Redistribution of vibrational energy before electron emission • Retention of vibrational energy in the molecule, leading to emission of low-energy electrons.

Summary • Considerable differences between direct detachment and vibrational autodetachment • Redistribution of vibrational energy before electron emission • Retention of vibrational energy in the molecule, leading to emission of low-energy electrons. • Methyl torsion very important for IVR

Summary Continue the study with the larger nitroalkane chains:

Summary Continue the study with the larger nitroalkane chains: • Determine AEA and assign the vibrational features in the direct photodetachment spectra

Summary Continue the study with the larger nitroalkane chains: • Determine AEA and assign the vibrational features in the direct photodetachment spectra • Monitor the evolution of the VAD PES as the site of initial excitation is moved further away from the nitro group

Acknowledgements Mathias Weber Holger Schneider Jesse Marcum Kent Ervin (UN Reno) Carl Lineberger and the Lineberger Lab

On-Resonance Interpretation CN stretch (918 cm-1) 2 quanta NO 2 rocking (475 X 2 cm-1) NO 2 Scissor (657 cm-1) NO 2 Wag (603 cm-1) NO 2 rocking (475 cm-1) 1 quanta NO 2 rocking (475 cm -1) and 1 quanta NO Scissor (657 2 cm-1) 1 quanta NO 2 rocking (475 cm-1) and 1 quanta NO 2 Wag (603 cm-1) CH 3 rocking (1096 cm-1)

Averaging in the Lab Frame along the Transition Dipole of the CH Stretch Vibration (2775 cm-1)