Anion photoelectron spectroscopy of B_2N~-

摘要

Vibrationally resolved 355 and 266 nm anion photoelectron spectra of B_2N are presented. Photodetachment to two electronic states of linear B-N-B is observed and, aided by electronic structure calculations, assigned to the X-tilde~1#SIGMA#_g~+ -> X-tilde~2#SIGMA#_u~+ + e~- and X-tilde~1#SIGMA#_g~+ -> A-tilde~2#SIGMA#_g~+ + e~- transitions. The electron affinity of B_2N is 3.098+-0.005 eV and the A-tilde~2#SIGMA#_g~+ term energy T_0 is 0.785+-0.005 eV. Observation of excitations involving uneven quanta of the antisymmetric stretching mode (v_3) indicates a breakdown of the Franck-Condon (FC) approximation and results from Herzberg-Teller vibronic coupling between the X-tilde~2#SIGMA#_u~+ and A-tilde~2#SIGMA#_g~+ states involving the v_3 mode. Measurement of the angular dependence of the photodetached electrons serves as a sensitive probe for the identification of these FC forbidden transitions. A linear vibronic coupling model qualitatively reproduces the perturbed v_3 potentials of the X-tilde and A-tilde states. Artifactual symmetry breaking along the v_3 coordinate is observed in the ab initio wave functions for the neutral ground state up to the coupled-cluster level of theory, even when Brueckner orbitals are used. No evidence is found for an energetically low-lying cyclic state of B_2N, which has been invoked in the assignment of the matrix infrared spectrum of B_2N. However, the matrix infrared data agrees well with the peak spacing observed in the photoelectron spectra and reassigned to the linear X-tilde~2#SIGMA#_u~+ ground state.