High resolution spectroscopy of the He79Br2 van der Waals molecule: An experimental and theoretical study

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Abstract
The structure, dissociationdynamics, and intermolecular potential energy surfaces of the He79Br2 van der Waals molecule have been studied using high resolution, two color, pump–probe laser induced fluorescencespectroscopy and three dimensional quantum mechanical calculations. A conical nozzle produces higher centerline cluster densities than a standard nozzle, and allows data collection further downstream from the nozzle. This yields improved signal to noise ratios and lower Doppler widths. He79Br2 is found to have a T‐shaped average geometry with He to Br2 center‐of‐mass distances of 3.98 Å and 4.11 Å for the X and B states, respectively, somewhat longer than previously reported. Spectra were also obtained for excitation to excited bending levels of the van der Waals coordinate. However, these spectra have yet to be rotationally assigned. Vibrational predissociation line widths for the B state of He79Br2 have been measured for three new vibrational levels and range from 0.036 cm−1 for B, v′=8 to 0.062 cm−1 for B, v′=12. These values are somewhat larger than was expected based on previous HeBr2 linewidth measurements for higher vibrational levels. Forms for the potential energy surface that have previously been used to simulate the spectra of HeCl2 have been applied to the HeBr2 data. For the HeBr2 X state, two potentials are tested. First, a slightly anisotropic, one center Morse–Spline–van der Waals potential with angle dependent parameters is used. Second a much more anisotropic potential obtained from ab initio calculations is tested. The more anisotropic potential produces a significantly better fit to the data. The B state potential is constructed using Morse atom–atom potentials for the short range part of the He–Br interaction. This simple potential is sufficient to fit the main excitation band, but does not yield a fit to spectra involving vibrationally excited van der Waals modes.

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