Stabilization and rovibronic spectra of the T -shaped and linear ground-state conformers of a weakly bound rare-gas-homonuclear dihalogen complex: He⋯Br ₂

Laser-induced fluorescence spectra of Br2 entrained in a He supersonic expansion have been recorded in the Br2 B-X, 8-0, 12-0, and 21-0 spectral regions at varying downstream distances, and thus different temperature regimes. Features associated with transitions of the T -shaped and linear He⋯ Br2 (X, ν″ =0) complexes are identified. The changes in the relative intensities of the T -shaped and linear features with cooling in the expansion indicate that the linear conformer is energetically more stable than the T -shaped conformer. A He+ Br2 (X, ν″ =0) ab initio potential-energy surface, computed at the coupled cluster level of theory with a large, flexible basis set, is used to calculate the binding energies of the two conformers, 15.8 and 16.5 cm-1 for the T -shaped and linear complexes, respectively. This potential and an excited-state potential [M. P. de Lara-Castells, A. A. Buchachenko, G. Delgado-Barrio, and P. Villareal, J. Chem. Phys. 120, 2182 (2004)] are used to calculate the excitation spectra of He⋯ Br279 (X, ν″ =0) in the Br2 B-X, 12-0 region. The calculated spectra are used to make spectral assignments and to determine the energies of the excited-state intermolecular vibrational levels accessed in the observed transitions. Temperature-dependent laser-induced fluorescence spectra and a simple thermodynamic model [D. S. Boucher, J. P. Darr, M. D. Bradke, R. A. Loomis, and A. B. McCoy, Phys. Chem. Chem. Phys. 6, 5275 (2004)] are used to estimate that the linear conformer is 0.4 (2) cm-1 more strongly bound than the T -shaped conformer. Two-laser action spectroscopy experiments reveal that the binding energy of the linear He⋯ Br279 (X, ν″ =0) conformer is 17.0 (8) cm-1, and that of the T -shaped He⋯ Br279 (X, ν″ =0) conformer is then 16.6 (8) cm-1, in good agreement with the calculated values.