Cross sections and rate constants for low-temperature 4He–H2 vibrational relaxation

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Abstract
Converged coupled‐states integral cross sections were determined for the vibrational relaxation of the v=1 j=0 level of p‐H2 in collisions with 4He. The collision energies ranged from 0.005 to 0.4 eV. The Gordon–Secrest (GS) potential was used with both a harmonic (HO) and rotating‐Morse oscillator (MO) description of the H2 molecule. Additional calculations incorporated modifications in the long‐range and spherically symmetric (V0) parts of the purely repulsive GS surface. Rotational coupling plays a major role in vibrational relaxation even at thermal energies. Although the relative importance of individual vibration–rotation transitions is a sensitive function of the choice of intramolecular potential, the HO and MO total relaxation cross sections are nearly identical. These total relaxation cross sections exhibit a power‐law dependence on the initial translational energy down to ∼0.1 eV above threshold; below which point a positive curvature appears for all the surfaces considered. Rate constants for vibrational relaxation were computed and compared with experiment for 60 °K <TV0 potential results in a modest increase in curvature. Agreement with experiment is obtained only with the use of the semiempirical V0 potential of Shafer and Gordon.