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

Abstract

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.

Full-dimensional multi configuration time dependent Hartree calculations of the ground and vibrationally excited states of He2,3Br2 clusters

“…This is most likely due to the spectral congestion when exciting the linear conformer to the highly delocalized intermolecular vibrational levels in the excited state, as they are very close in energy. 9 Further, in these studies, the analysis of the pressure and temperature dependence indicates the higher stability of the (1,1) isomer with respect to the (2,0), as predicted by the 5D variational calculations 29 and confirmed in this work by the 6D MCTDH results (see Table V). The binding energy of the different isomers is 32.21, 31.22, and 30.29 cm −1 , for the (0,2), (1,1), and (2,0) isomers, respectively.…”

“…However, based on a sum of the experimental values of the binding energy of the linear (0,1) and T-shape (1,0) triatomic conformers, Boucher et al proposed binding energies of 33.6(1.1) and 33.2(1.1) cm −1 for the (1,1) and (2,0) He 2 Br 2 complexes, respectively. 9 According to our calculations, the assumption of neglecting the He-He interaction does not affect the relative stability of the different conformers, with differences between the 6D result and the sum of the theoretical triatomic binding energies (see Table V) for the (0,2), (1,1), and (2,0) isomers, respectively. However, it has been shown that this assumption fails for systems such as He 2 I 2 , where the competition of the smaller differences between the linear and T-shape triatomic binding energies and the rotational effects results in a change of the relative stability of the isomers.…”

“…1) support the experimental observation of three different conformers with (1,2), (2,1), and (3,0) geometries. 9 The one dimensional distributions in θ 1,2,3 and in R 1,2,3 , and the two dimensional distributions in ϕ 1 and ϕ 2 , presented in Figs. 4-6 allow the assignment of the calculated bound states with binding energies of 47.35, 46.78, and 45.76 cm −1 to the (1,2), (2,1), and (3,0) isomers, respectively (see Table V).…”

“…In spite of the theoretical efforts to develop new methods for handling such systems, in applications for He N Br 2 only simple pairwise PESs have been employed. Although, as it has been shown 9,14,29,34 multibody potential terms are very important for an accurate representation of the PESs of the triatomic and tetra-atomic clusters of this series. Therefore, in order to achieve a satisfactory understanding of the spectroscopy and dynamics of these systems accurate quantum-dynamics simulations are needed employing reasonable accurate potential surfaces.…”

“…The first experiments on van der Waals (vdW) clusters consisting of a dihalogen molecule and rare-gas atoms have been performed by Levy et al, 1,2 followed by a large number of studies on systems such as He n I 2 (n=1-3), 3,4 Ne n I 2 (n=1-6), 2,5,6 Ne n Br 2 , 7 Rg 1,2 X 2 (Rg=He, Ne, Ar and X=Cl, Br), [8][9][10][11] Ne n ICl (n=1-5), 12,13 He n ICl ((n=1-3). 14,15 More recently the interest in studying such species has been intensified due to additional experiments, rotational, and infrared spectroscopy on doped helium nanodroplets, [16][17][18] and on small/intermediate-sized doped helium clusters.…”