Vibrational Autodetachment in Nitroalkane Anions Chris L Adams

Vibrational Autodetachment in Nitroalkane Anions Chris L. Adams, J. Mathias Weber JILA, University of Colorado, Boulder, CO 80309 -0440 OSU International Symposium on Molecular Spectroscopy June 24, 2010

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

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

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 me. V (1600 cm-1).

Experimental Set-Up e- beam Ion Optics Tunable IR (2000 -4000 cm-1), 1064 nm, or 532 nm light

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

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

Nitromethane Anion Autodetachment spectrum CH 3 NO 2 - + hn CH 3 NO 2 + e-

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

Nitromethane Anion Off-Resonance - 2740 cm-1 74 me. V AEA = 172± 6 me. V Adams et al. , J. Chem. Phys. 130, 074307 (2009)

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

Nitromethane Anion Autodetachment spectrum CH 3 NO 2 - + hn CH 3 NO 2 + e-

Nitromethane Anion Off-Resonance - 2740 cm-1 On-Resonance - 2775 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 Counting States 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 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 me. V of the ground state

Nitromethane Anion Modeling Intensity distribution, IVAD(E) written as • v and J are the vibrational and free internal rotor quantum states • fthr(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 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 av, 0 for best fit

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

Nitromethane Anion Modeling Intensity distribution, IVAD(E) written as • v and J are the vibrational and free internal rotor quantum states • fthr(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 Second Model (DOS): Energy is completely randomized before VAD occurs, so the density of states describes the population of final states → all coefficients av, J are given equal weight.

Nitromethane Anion Second Model: • The DOS closely resembles the spectrum at low energies • At high energies, the two curves deviate quickly • Peak at 100 me. V 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 Modeling Intensity distribution, IVAD(E) written as • v and J are the vibrational and free internal rotor quantum states • fthr(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 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

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

Nitromethane Anion Higher Energy CH Stretches

Nitromethane Anion Higher Energy Stretches

Conclusions and Future Directions • Methyl torsion plays important role in IVR • Modeling recovers PES remarkably well, with the exception of the feature at 100 me. V. • 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