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

Abstract

High-resolution slow photoelectron velocity-map imaging spectra of cryogenically cooled X̃ 2 B 2 H 2 CC – and D 2 CC – 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 3 B 2 (T 1 ), b̃ 3 A 2 (T 2 ), and à 1 A 2 (S 1 ) excited states of neutral vinylidene. This work provides the first experimental observation of the à singlet excited state of H 2 CC. 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 H 2 CC 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 H 2 CC are found to be T 0 ( ã 3 B 2 ) = 2.064(6), T 0 ( b̃ 3 A 2 ) = 2.738(6), and T 0 ( à 1 A 2 ) = 2.991(6) eV.