Quantum-mechanical calculations of rotational-excitation cross sections for HCl, DCl, DF, and HF in slow collisions with He

Low-energy rotational-excitation cross sections have been calculated for collisions between ground-state helium atoms and the diatomic molecules HCl, DCl, DF, and HF from model-potential surfaces. Solutions of the coupled radial equations are obtained in the close-coupling coupled-channels (CC) formulation and the $j_z$ conserving coupled-states (CS) approximation, including as many channels (open and closed) as needed for convergence. Trends in the behavior of the cross sections with respect to variations in the potential surface and the spacing of rotational levels are discussed with particular emphasis on the isotope effects. In addition, cross sections calculated for various CS approximations are compared with CC results and comment is made on the influence of the closed channels.