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

Laser-induced fluorescence spectra of Br 2 entrained in a He supersonic expansion have been recorded in the Br 2 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 ⋯ Br 2 ( 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 + Br 2 ( 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 ⋯ Br 2 79 ( X , ν ″ = 0 ) in the Br 2 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 ⋯ Br 2 79 ( X , ν ″ = 0 ) conformer is 17.0 ( 8 ) cm − 1 , and that of the T -shaped He ⋯ Br 2 79 ( X , ν ″ = 0 ) conformer is then 16.6 ( 8 ) cm − 1 , in good agreement with the calculated values.