Non-Adiabatic Effects on Excited States of Vinylidene Observed with Slow Photoelectron Velocity-Map Imaging.

High-resolution slow photoelectron velocity-map imaging spectra of cryogenically cooled X̃B HCC and DCC in the region of the vinylidene triplet excited states are reported. Three electronic bands are observed and, with the assistance of electronic structure calculations and quantum dynamics on ab initio-based near-equilibrium potential energy surfaces, are assigned as detachment to the [Formula: see text] B (T), b̃ A (T), and à A (S) excited states of neutral vinylidene. This work provides the first experimental observation of the à singlet excited state of HCC. While regular vibrational structure is observed for the ã and à electronic bands, a number of irregular features are resolved in the vicinity of the b̃ band vibrational origin. High-level ab initio calculations suggest that this anomalous structure arises from a conical intersection between the ã and b̃ triplet states near the b̃ state minimum, which strongly perturbs the vibrational levels in the two electronic states through nonadiabatic coupling. Using the adiabatic electron affinity of HCC previously measured to be 0.490(6) eV by Ervin and co-workers [J. Chem. Phys. 1989, 91, 5974], term energies for the excited neutral states of HCC are found to be T(ã B) = 2.064(6), T(b̃ A) = 2.738(6), and T(à A) = 2.991(6) eV.