Anion photoelectron spectroscopy of B2N-

Vibrationally resolved 355 and 266 nm anion photoelectron spectra of B2N are presented. Photodetachment to two electronic states of linear B-N-B is observed and, aided by electronic structure calculations, assigned to the (X) over tilde (1)Sigma(g)(+)-->(X) over tilde (2)Sigma(u)(+)+e(-) and (X) over tilde (1)Sigma(g)(+)-->(A) over tilde (2)Sigma(g)(+)+e(-) transitions. The electron affinity of B2N is 3.098 +/- 0.005 eV and the (A) over 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 (A) over tilde (2)Sigma(u)(+) and (A) over 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) over tilde and (A) over 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 B2N, which has been invoked in the assignment of the matrix infrared spectrum of B2N. However, the matrix infrared data agrees well with the peak spacing observed in the photoelectron spectra and reassigned to the linear (X) over tilde (2)Sigma(u)(+) ground state. (C) 1999 American Institute of Physics. [S0021-9606(99)00743-6].