Autodetachment from Vibrationally Excited Vinylidene Anions.

Slow electron velocity-map imaging of the cryogenically cooled H2CC¯ anion reveals a strong dependence of its high-resolution photoelectron spectrum on detachment photon energy in two specific ranges, from 4000 to 4125 cm-1 and near 5020 cm-1. This effect is attributed to vibrational excitation of the anion followed by autodetachment to H2CC + e¯. In the lower energy range, the electron kinetic energy (eKE) distributions are dominated by two features that occur at constant eKEs of 114(3) and 151.9(14) cm-1 rather than constant electron binding energies, as is typically seen for direct photodetachment. These features are attributed to ΔJ = ΔK = 0 autodetachment transitions from two vibrationally excited anion states. The higher energy resonance autodetaches to neutral eigenstates with amplitude in the theoretically predicted shallow well lying along the vinylidene-acetylene isomerization coordinate. Calculations provide assignments of all autodetaching anion states and show that the observed autodetachment is facilitated by an intersection of the anion and neutral surfaces.