Velocity Map Imaging Spectroscopy of the Dipole-Bound State of CH2CN-: Implications for the Diffuse Interstellar Bands

Weakly bound anionic systems present a new domain for negative ion spectroscopy. Here we report on a multifaceted study of the CH2CN- dipole-bound state, employing high-resolution photoelectron spectroscopy from 130 different wavelengths, velocity-map imaging at threshold, and laser scanning photodetachment experiments. This uncovers a wide variety of different vibrational and rotational autodetaching resonances. By examination of both sides of the problem, absorption from the anion to the dipole- bound state and vibrational/rotational autodetachment to the neutral, a complete model of the dipolebound chemistry is formed. Precise values for the electron affinity EA = 12468.9(1) cm(-1), dipole binding energy D-BE = 40.2(3) cm(-1), and anion inversion splitting omega(5) = 115.9(2) cm(-1) are obtained. This model is then employed to study possible astronomical implications, revealing good agreement between the K = 1 <- 0 CH2CN- dipole transition and the lambda 8040 diffuse interstellar band.