Target density dependence of molecule-molecule absolute total scattering cross sections

Abstract

The target density dependence of absolute total cross sections for LiF scattered by CO, CO₂, CHF₃, CH₄, D₂, and Ar was studied at a LiF velocity of 1.5 × 10⁵ cm/sec and a target temperature of 297°K. The experiments were performed under conditions of high angular resolution in order to observe the full quantum mechanical cross section. For the systems LiF–CO, LiF–CO₂, and LiF–CHF₃ there was a substantial variation of observed cross section with density, while for LiF–CH₄, LiF–D₂, and LiF–Ar there was no observable variation. It is shown that the target transmission is the Laplace transform of the cross section distribution function. Information was obtained about the distribution of cross sections as a function of the internal state of LiF by fitting model distributions to the data. For the lower LiF rotational states the LiF–CO and LiF–CO₃ cross sections are dominated by the dipole‐quadrupole interaction and are determined at the outermost impact parameters by processes inelastic in the rotation of LiF, and inelastic and/or elastic in the targets CO of CO₂; for the higher LiF rotational states the total cross section is determined mainly by elastic processes and is therefore dominated by induction and dispersion interactions. The dipole‐quadrupole scattering effect, so important in microwave line broadening theories, thus has been identified in experiments of this type. For LiF–CHF₃ the largest total cross sections are determined by dipole‐dipole inelastic scattering. For those targets which showed no pressure dependence (Ar, D₂, and CH₄) the standard deviation of the cross sections was less than ∼15 percent of the mean as determined by the accuracy of the experiments and the range of target densities used. This is consistent with the symmetry of the interaction and the possible degree of anistropy. No rotational inelasticity could be inferred for these targets. A theoretical analysis of the distribution of cross sections based on dipole‐quadrupole and dipole‐dipole scattering processes is presented. Various fitted models for the distribution of cross sections can be evaluated in the light of these analyses.