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 X²B₂ H₂CC^- and D₂CC^- 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 ã ³B₂ (T₁), b ³A₂ (T₂), and à ¹A₂ (S₁) excited states of neutral vinylidene. This work provides the first experimental observation of the à singlet excited state of H₂CC. 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 b band vibrational origin. High-level ab initio calculations suggest that this anomalous structure arises from a conical intersection between the ã and b triplet states near the b state minimum, which strongly perturbs the vibrational levels in the two electronic states through nonadiabatic coupling. Using the adiabatic electron affinity of H₂CC 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 H₂CC are found to be T₀(ã ³B₂) = 2.064(6), T₀(b ³A₂) = 2.738(6), and T₀(à ¹A₂) = 2.991(6) eV.