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Stabilization and rovibronic spectra of the T-shaped and linear ground-state conformers of a weakly bound rare-gas–homonuclear dihalogen complex: He⋅⋅⋅Br2

D. S. Boucher, D. B. Strasfeld, R. A. Loomis, J. M. Herbert, S. E. Ray, and A. B. McCoy
J. Chem. Phys. 123, 104312:1–14 (2005)

Abstract

Laser-induced fluorescence spectra of Br2 entrained in a He supersonic expansion have been recorded in the Br2B-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,v″ = 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⋅⋅⋅79Br2(X,v″ = 0) in the Br2B-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⋅⋅⋅79Br2(X,v″ = 0) conformer is 17.0(8) cm–1, and that of the T-shaped He⋅⋅⋅79Br2(X,ν″ = 0) conformer is then 16.6(8) cm–1, in good agreement with the calculated values.

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