Rotational excitation of sulfur monoxide by collisions with helium at low temperature

We present two new two-dimensional potential-energy surfaces for the SO-He system calculated at SO r distance frozen at its experimental minimum-energy distance. Both are obtained at the RCCSD(T) level using two different basis sets (AVTZ and AVQZ) for the three atoms. Bond functions are placed at mid-distance between the SO center of mass and He for a better description of the van der Waals well. Close-coupling calculations of the collisional excitation cross sections of the fine-structure levels of SO by He are calculated at low energies. The exact level splitting is taken into account. It is found that the results obtained from the two surfaces are very similar, except for some small differences observed in the region of resonances at low energies. The propensity rules between fine-structure levels are studied, it is shown that F-conserving cross sections are much larger for high-N rotational levels than cross sections between F-changing levels, as expected from theoretical considerations. The use of infinite order sudden recoupling techniques from spin-free cross sections is investigated. Excitation rate coefficients among fine-structure levels are calculated at low temperatures.