Molecular beam scattering studies of orbiting resonances and the determination of van der Waals potentials for H–Ne, Ar, Kr, and Xe and for H2–Ar, Kr, and Xe

Abstract

The velocity dependence of the integral cross sections for the scattering of H atoms and normal-H2 molecules from the rare gases has been measured for primary beam velocities from 300 m/sec (Ec.m.=0.5 meV) to about 3000 m/sec (Ec.m.=100 meV). Three distinct resonance maxima were observed in each of the systems H–Xe, H–Kr, H2–Kr, and H2–Ar. At least one resonance maximum was found in H2–Xe and H–Ar, while H–Ne showed no maxima. The H2–Ar cross sections were also measured with a cold beam of pure para-H2 consisting entirely of spherically symmetric j=0 molecules. No noticeable difference was found when compared to the n-H2 cross sections indicating that the observed resonances depend only on the spherical symmetric potential. Extensive comparisons with spherical symmetric model potentials showed that all the observed resonances are due to temporary capture in the well of the effective potential. This phenomenon is usually referred to as ’’orbiting.’’ The energetic location of the observed orbiting resonances and glory maxima were found to be very sensitive to the potential shapes and parameters. Only models which included at least two terms in the long range dispersion potential C6/R6 and C8/C8 could describe the data in a satisfactory way. For H–Ar the data are in very good agreement with the experimental potential of Bassi et al. and in reasonable agreement with the ab initio MCSF potential of Wagner et al. For H2–Ar, –Kr, –Xe good agreement is found with the potentials recently determined from infrared spectra by Le Roy and co-workers. For all the other systems new best fit potentials have been determined. The best fit values of ε and Rm are for H–Ne: 1.90 meV and 3.15 Å; H–Ar: 4.16 meV and 3.62 Å; H–Kr: 5.90 meV and 3.57 Å; H–Xe: 7.08 meV and 3.82 Å; H2–Ar: 6.30 meV and 3.57 Å; H2–Kr: 7.19 meV and 3.72 Å, and H2–Xe: 8.10 meV and 3.92 Å.