Slow photoelectron velocity-map imaging spectroscopy of cold negative ions

High-resolution photoelectron spectra of the i-methylvinoxide anion are obtained by slow electron velocity-map imaging. The transitions between the anion ground electronic state, X ˜ 1A′, and the radical X ˜ 2A′′, and A ˜ 2A ′ states are measured. Franck-Condon simulations of the X ˜ 2A′′r X ˜ 1A ′ transition are performed to aid in assigning peaks, yielding several vibrational frequencies for the first time. Additional structure in the spectrum of the X ˜ 2A′ ′ r X ˜ 1A ′ transition is attributed to hindered rotor motion of the methyl group. Simulation of methyl torsional states allows identification of the true origin peak, yielding the experimental adiabatic electron affinity, EA) 1.747 ( 0.002 eV, and the A ˜ state term energy, T0) 1.037 ( 0.002 eV. Comparison with the fully deuterated isotopologue further supports the assignment of hindered rotor progressions. The favored methyl rotor position is eclipsed in the radical X ˜ state and staggered in the anion X ˜ and radical A ˜ states. I