Quantal calculations on the rovibrational excitation of H(_2) and HD induced by H

Abstract

Cross sections have been computed for non-reactive rovibrational transitions of H(_2) and HD, induced by collisions with H atoms, using the H+H(_2) interaction potential calculated recently by Mielke et al. [39] and an earlier potential of Boothroyd et d. [38]. The calculations relate explicitly to non-reactive scattering, excluding the proton-exchange channels. Cross sections derived using a simple harmonic oscillator approximation to the vibrational motion are compared with results obtained using numerically 'exact' solutions of the rovibrational eigenvalue equation. The convergence of the cross sections with respect to the size of the rovibrational basis set is investigated. Convergence is found to be slow, owing to the strength of the collisional coupling between vibrational manifolds. The cross sections are used to derive the rate coefficients for the rovibrational excitation of H(_2) and HD by H. For vibrationally inelastic transitions, the new rate coefficients at T ~ 1000 K are larger than the results of earlier calculations in which the vibrational motion was treated approximately, by means of a simple harmonic oscillator model. As a result, much better agreement is obtained with the empirical estimates by Allers et al. [59] of the rate coefficients for vibrational relaxation of the levels (v,j) = (1, 3) and (2, 3) of H(_2) . However, the effects of the new data on the resists of illustrative astrophysical models are less pronounced than the changes to the rate coefficients for vibrationally inelastic transitions might suggest.