Rotational excitation of CN(X 2Σ+) by He: Theory and comparison with experiments

Rotational excitation of the CN(X (2)Sigma(+)) molecule with He is investigated. We present a new two-dimensional potential energy surface (PES) for the He-CN system, calculated at an internuclear CN distance frozen at its experimental equilibrium distance. This PES was obtained using an open-shell, coupled-cluster method including all single and double excitations, as well as the perturbative contributions of connected triple excitations [RCCSD(T)]. Bond functions were placed at mid-distance between the center of mass of the CN molecule and He atom for a better description of the van der Waals interaction. State-to-state collisional excitation cross sections of the fine-structure levels of CN by He are calculated for energies up to 2500 cm(-1), which yield after thermal averaging, rate coefficients up to 350 K. The exact spin splitting of the energy levels is taken into account. The propensity rules between fine-structure levels are studied, and it is shown that the rate constants for Delta j=Delta N transitions are much larger than those for Delta j not equal Delta N transitions, as expected from theoretical considerations. Our calculated rate coefficients are compared to experimental results at 295 K of Fei [J. Chem. Phys. 100, 1190 (1994)]. The excellent agreement confirms the accuracy of the PESs and of the scattering calculations.