Non-Adiabatic Effects on Excited States of Vinylidene Observed with Slow Photoelectron Velocity-Map Imaging
2016
DeVine, Jessalyn A. | Weichman, Marissa L. | Zhou, Xueyao | Ma, Jianyi | Jiang, Bin | Guo, Hua | Neumark, Daniel M.
High-resolution slow photoelectron velocity-map imaging spectra of cryogenically cooled X̃²B₂ 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), b̃ ³A₂ (T₂), and à¹A₂ (S₁) excited states of neutral vinylidene. This work provides the first experimental observation of the àsinglet excited state of H₂CC. While regular vibrational structure is observed for the ã and àelectronic 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 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(b̃ ³A₂) = 2.738(6), and T₀(à¹A₂) = 2.991(6) eV.
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