The influence of the molecule continuum on inelastic vibrational energy transfer in atom–molecule collisions

Abstract

A study is made of the vibrational energy transfer in atom–diatomic molecule collisions for molecules with a dissociative continuum. The collision energy is kept below the dissociation limit. Comparisons are made of calculations performed using only the discrete molecular basis set, not including continuum functions, with exact calculations. A collinear model is used in these calculations in which the molecule is taken to be a Morse oscillator. It is found that for molecules with a shallow well, such as van der Waals molecules, the discrete molecular basis set never leads to converged scattering cross sections. For strongly bound molecules at low collision energies, the transitions among the lower states of the molecule are accurately described by only the discrete oscillator states, but at high collision energies or for states higher up in the well, the transition probabilities are often in error by more than a factor of 2. This is seen even when the calculation has appeared to converge with the discrete basis set. Errors due to the neglect of the continuum are not limited to transitions among states near to the continuum.