Slow electron velocity-map imaging of the cryogenically cooled HCC¯ anion reveals a strong dependence of its high-resolution photoelectron spectrum on detachment photon energy in two specific ranges, from 4000 to 4125 cm and near 5020 cm. This effect is attributed to vibrational excitation of the anion followed by autodetachment to HCC + 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 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.
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