Articles you may be interested inThe intermolecular potential energy surface for CO2-Ar: Fitting to highresolution spectroscopy of Van der Waals complexes and second virial coefficients High resolution infrared spectra of H2-Ar, HD-Ar, and D2-Ar van der Waals complexes between 160 and 8620 cm−1The structure, dissociation dynamics, and intermolecular potential energy surfaces of the He 79 Br 2 van der Waals molecule have been studied using high resolution, two color, pump-probe laser induced fluorescence spectroscopy 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. He 79 Br 2 is found to have a T-shaped average geometry with He to Br 2 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 He 79 Br 2 have been measured for three new vibrational levels and range from 0.036 cm Ϫ1 for B, vЈϭ8 to 0.062 cm Ϫ1 for B, vЈϭ12. These values are somewhat larger than was expected based on previous HeBr 2 linewidth measurements for higher vibrational levels. Forms for the potential energy surface that have previously been used to simulate the spectra of HeCl 2 have been applied to the HeBr 2 data. For the HeBr 2 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.

Excitation spectra of the He79Br2 complex have been obtained for the B state vibrational levels v′=34–48 using the pump–probe spectroscopic technique. The Δv=−1 channel for vibrational predissociation is closed for levels v′=45 and higher. The B and X state van der Waals binding energies are determined to be 13.5±1.0 and 17.0±1.5 cm−1, respectively. Lifetimes for vibrational predissociation were found to range from 5.3 ps for v′=34 to 1.8 ps for v′=44. For v′=45 the lifetime increases to 3.8 ps due to the closing of the Δv=−1 dissociation channel. The lifetimes are consistent with the momentum gap law using a Morse parameter equal to 1.9±0.1 Å−1. The trend of increasing blue shifts as v′ increases reverses at v′=40. This appears to indicate that the He–Br interaction changes dramatically when the Br–Br separation becomes larger than 4.0 Å. Based on the results presented here, we have reevaluated previous data for the HeI2 molecule, and suggest that the ground-state binding energy be revised to 17.6±1.0 cm−1 from the currently accepted value of 18.8 cm−1.