Boucher David S, Strasfeld David B, Loomis Richard A, Herbert John M, Ray Sara E, McCoy Anne B
Department of Chemistry, Washington University in St. Louis, CB 1134, Missouri, 63130, USA.
J Chem Phys. 2005 Sep 8;123(10):104312. doi: 10.1063/1.2006675.
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,nu(") = 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,nu(") = 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...(79)Br(2)(X,nu(") = 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...(79)Br(2)(X,nu(") = 0) conformer is 17.0(8) cm(-1), and that of the T-shaped He...(79)Br(2)(X,nu(") = 0) conformer is then 16.6(8) cm(-1), in good agreement with the calculated values.
在不同的下游距离，从而在不同的温度条件下，记录了在氦超声速膨胀中夹带的Br₂在Br₂的B - X、8 - 0、12 - 0和21 - 0光谱区域的激光诱导荧光光谱。识别出了与T形和线性He...Br₂(X, ν″ = 0)络合物跃迁相关的特征。膨胀过程中随着冷却，T形和线性特征的相对强度变化表明线性构象异构体在能量上比T形构象异构体更稳定。使用在耦合簇理论水平上用大的灵活基组计算的He + Br₂(X, ν″ = 0)从头算势能面，来计算两种构象异构体的结合能，T形和线性络合物的结合能分别为15.8和16.5 cm⁻¹。该势能和一个激发态势能[M. P. de Lara - Castells, A. A. Buchachenko, G. Delgado - Barrio, and P. Villareal, J. Chem. Phys. 120, 2182 (2004)]被用于计算He...(⁷⁹)Br₂(X, ν″ = 0)在Br₂的B - X、12 - 0区域的激发光谱。计算得到的光谱用于进行光谱归属，并确定在观测跃迁中所涉及的激发态分子间振动能级的能量。温度依赖的激光诱导荧光光谱和一个简单的热力学模型[D. S. Boucher, J. P. Darr, M. D. Bradke, R. A. Loomis, and A. B. McCoy, Phys. Chem. Chem. Phys. 6, 5275 (2004)]被用于估计线性构象异构体比T形构象异构体的结合更强0.4(2) cm⁻¹。双激光作用光谱实验表明线性He...(⁷⁹)Br₂(X, ν″ = 0)构象异构体的结合能为17.0(8) cm⁻¹，T形He...(⁷⁹)Br₂(X, ν″ = 0)构象异构体的结合能为16.6(8) cm⁻¹，与计算值吻合良好。
