Rovibrational, excitation of molecules by atoms

Abstract

The results of close coupling (CC) and infinite order sudden (IOS) approximation calculations of cross sections for rovibrational excitation of both para and ortho H(_2) by He are presented. Large discrepancies are found between the present CC results and those of Lin and Secrest (1979) and Lin (1979). The v = O → 1 vibrationally inelastic cross sections are found to differ from those of Lin by factors attaining four orders of magnitude close to the v = 1 excitation threshold. Also, structure in the variation of both vibrationally elastic and inelastic cross sections with energy, reported by Lin and Secrest, and Lin, is absent in the present results. The present CC results are found to be in good quantitative agreement with the coupled states calculations of Alexander and McGuire (1976). Agreement with the IOS calculations is only qualitative but improves with increasing collision energy, consistent with the progressive failure of the energy sudden component of the IOS approximation as the collision energy falls. The values of the vibrational relaxation rate coefficient calculated from the CC results fall below the experimental data of Audibert et al. (1976) at low temperature. This is most probably due to the relatively poor description of the H(_2) system employed, in particular the interaction potential of Gordon and Secrest (1970). The CC results are employed to investigate the accuracy of two energy sudden factorisation schemes. The factorisation which includes off-energy-shell effects is shown to be more accurate than that which does not. However, neither scheme produces cross sections which obey detailed balance. The present IOS results are in good agreement with the adiabatic distorted wave IOS calculations of Bieniek (1980) at low energy. However, as the collision energy increases significant discrepancies appear. For H(_2) + He it appears that at energies sufficiently high for the IOS approximation to be valid the use of adiabatic distorted wave techniques is not valid. Exploratory IOS calculations of rovibrational excitation of H(_2) by h(^+) are reported and discussed. There appears to be evidence that the comparison between theoretical and experimental values of rovibrational cross sections presented by Schinke et al. (1980) and Schinke (1980) is distorted by their restricted numerical methods and faults in their basis wavefunctions.