Anion photoelectron spectroscopy of B2N-

Vibrationally resolved 355 and 266 nm anion photoelectron spectra of B2N ar e presented. Photodetachment to two electronic states of linear B-N-B is ob served and, aided by electronic structure calculations, assigned to the (X) over tilde (1)Sigma(g)(+)-->(X) over tilde (2)Sigma(u)(+)+e(-) and (X) ove r tilde (1)Sigma(g)(+)-->(A) over tilde (2)Sigma(g)(+)+e(-) transitions. Th e 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 excitatio ns involving uneven quanta of the antisymmetric stretching mode (v(3)) indi cates 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. Measurem ent of the angular dependence of the photodetached electrons serves as a se nsitive probe for the identification of these FC forbidden transitions. A l inear vibronic coupling model qualitatively reproduces the perturbed v(3) p otentials of the (X) over tilde and (A) over tilde states. Artifactual symm etry breaking along the v(3) coordinate is observed in the ab initio wave f unctions for the neutral ground state up to the coupled-cluster level of th eory, even when Brueckner orbitals are used. No evidence is found for an en ergetically low-lying cyclic state of B2N, which has been invoked in the as signment of the matrix infrared spectrum of B2N. However, the matrix infrar ed data agrees well with the peak spacing observed in the photoelectron spe ctra and reassigned to the linear (X) over tilde (2)Sigma(u)(+) ground stat e. (C) 1999 American Institute of Physics. [S0021-9606(99)00743-6].