Vibrational Autodetachment in Nitroalkane Anions Chris L. Adams, J. Mathias Weber JILA, University of Colorado, Boulder, CO OSU International Symposium on Molecular Spectroscopy June 24, 2010

Nitroalkane Anions Novel Approach to studying intramolecular vibrational relaxation (IVR).

Motivation: What happens when a photon of h = E vib > E eBE ? Conventional PES (off-resonance) + e - Direct photoemission governed by Franck-Condon factors Vibrational Autodetachment (VAD) PES (on-resonance) + e - VAD governed by Intramolecular Vibrational Relaxation (IVR) prior to photoemission Nitroalkane Anions

Model System The excess electron is largely localized on the nitro group. The fundamental CH vibrational transitions (>2750 cm -1 ) have energies in excess of the adiabatic electronic affinity (AEA) <200 meV (1600 cm -1 ). Nitroalkane Anions

Ion Optics Tunable IR ( cm -1 ), 1064nm, or 532 nm light Experimental Set-Up e - beam

Deflection and Focusing Optics Microchannel plate assembly Phosphor Screen CCD Camera Imaging Optics and photoelectron flight tube Laser Beam Neutral Experimental Set-Up

Raw ImageTransformed Image BASEX Transformed Image Integration over emission angles Photoelectron Spectrum Experimental Set-Up

Nitromethane Anion Autodetachment spectrum CH 3 NO h  CH 3 NO 2 + e -

Ө ≈ 30° Ө ≈ 0° AnionNeutral Comparing geometry of neutral and anion Nitroalkane Anions Expect wagging vibration of the neutral should give the most prominent vibrational progression in the PES. NO 2 Wag ~ 603 cm -1 (74 meV ) Upon emission of excess electron  hindered to free rotor

Nitromethane Anion Off-Resonance cm - 1 AEA = 172± 6 meV 74 meV Adams et al., J. Chem. Phys. 130, (2009)

ZOBS Dark States Intramolecular Vibrational Relaxation (IVR) e-e- Nitroalkane Anions J. M. Weber et al., JCP 115 (2001) 10718

Nitromethane Anion Autodetachment spectrum CH 3 NO h  CH 3 NO 2 + e -

Nitromethane Anion Off-Resonance cm - 1 On-Resonance cm - 1

Nitromethane Anion To extract the VAD photoelectron yield we subtract the off-resonance photoelectron spectrum from the VAD photoelectron spectrum. The baseline is then shifted by the AEA leaving us with the amount of energy remaining in the neutral molecule.

Nitromethane Anion Where do we go from here? Start with vibrational state (0,0,1,0,0,0,...,0,0,0) System evolves to (0,0,0,n 4,n 5,...,n 13,n 14,n 15 ) When enough energies is pooled in NO 2 wag we expect electron emission to occur. Based on this simple idea, we expect the population of vibrational states in the neutral will provide a rough map of how energy was distributed in the anion just prior to electron loss.  Model the VAD spectrum with final states of the neutral

Nitromethane Anion Fourteen of the fifteen vibrational modes of the neutral have been experimentally determined. The last degree of freedom corresponds to the free internal rotor. These internal rotor states were described using a particle-on-a-ring model where Counting States where J is the quantum number of the free internal rotor and I is the reduced moment of inertia for the torsional motion 390 vibrational and torsional states of neutral CH 3 NO 2 within the first 200 meV of the ground state

First Model: CH stretching modes couple to other vibrations, but not to torsional motion, because of large energy mismatch → all a v, J are zero for |J| > 0, adjust the a v, 0 for best fit Nitromethane Anion Modeling Intensity distribution, I VAD (E) written as v  and J are the vibrational and free internal rotor quantum states f thr (E) is an energy dependent emission probability a v,J gives the intensities of the states at energies E v,J I 0 (E - E v,J ) is the experimental response functions, represented by a gauss function corresponding to the experimental resolution

Nitromethane Anion First Model: The high-energy states are well represented. The low-energy region is not recovered at all Peak at 100 meV is completely missing The width of the peaks is too narrow

Nitromethane Anion Intensity distribution, I VAD (E) written as Second Model (DOS): Energy is completely randomized before VAD occurs, so the density of states describes the population of final states → all coefficients a v, J are given equal weight. v  and J are the vibrational and free internal rotor quantum states f thr (E) is an energy dependent emission probability a v,J gives the intensities of the states at energies E v,J I 0 (E - E v,J ) is the experimental response functions, represented by a gauss function corresponding to the experimental resolution Modeling

Nitromethane Anion Second Model: The DOS closely resembles the spectrum at low energies At high energies, the two curves deviate quickly Peak at 100 meV is missing

Nitromethane Anion Second Model: The feature at low energy is due to free internal rotor excitations without contributions from vibrational modes The contribution of ΙJΙ = 7 and ΙJΙ = 8 overestimate the experimental curve.  Ignore all states with ΙJΙ > 8 and keep the weight of ΙJΙ < 8 constant

Nitromethane Anion Intensity distribution, I VAD (E) written as Third Model (Partial Randomization Model): Energy is only partially randomized among the vibrations before VAD occurs. Model Two indicates randomization holds for the low-energy internal rotor states. → keep internal rotor contour for vibrations constant, adjust vibrational intensities v  and J are the vibrational and free internal rotor quantum states f thr (E) is an energy dependent emission probability a v,J gives the intensities of the states at energies E v,J I 0 (E - E v,J ) is the experimental response functions, represented by a gauss function corresponding to the experimental resolution Modeling

Nitromethane Anion Third Model: Agreement is excellent with the exception of the peak at 100 meV There are two potential candidates: w(NO 2 ) + |J|=6  (NO 2 ) + |J|=5

Nitromethane Anion Higher Energy CH Stretches

Higher Energy Stretches Nitromethane Anion

Conclusions and Future Directions Methyl torsion plays important role in IVR Modeling recovers PES remarkably well, with the exception of the feature at 100 meV. Determine AEA of nitroethane and larger nitroalkanes and extend analysis to larger molecular systems.

Acknowledgements Mathias Weber Holger Schneider Jesse Marcum and the rest of the cast Carl Lineberger and the Lineberger Lab