title: Rotational excitation of CN(X (2)Sigma(+)) by He: Theory and comparison with experiments.
abstract: 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 Deltaj=DeltaN transitions are much larger than those for Deltaj not equalDeltaN transitions, as expected from theoretical considerations. Our calculated rate coefficients are compared to experimental results at 295 K of Fei et al. [J. Chem. Phys. 100, 1190 (1994)]. The excellent agreement confirms the accuracy of the PESs and of the scattering calculations.
authors.affiliation: LOMC-FRE 3102-CNRS-Université du Havre, 25 rue Philippe Lebon, BP 540, 76058 Le Havre, France. francois.lique@univ-lehavre.fr
authors.forename: François
authors.initials: F
authors.lastname: Lique
authors.forename: Annie
authors.initials: A
authors.lastname: Spielfiedel
authors.forename: Nicole
authors.initials: N
authors.lastname: Feautrier
authors.forename: Ioan F
authors.initials: IF
authors.lastname: Schneider
authors.forename: Jacek
authors.initials: J
authors.lastname: Kłos
authors.forename: Millard H
authors.initials: MH
authors.lastname: Alexander