Rotational excitation of methylidynium (CH+) by helium atom at low temperature

A two-dimensional (2D) potential energy surface (PES) for the CH+(X1Σ+)-He(1S) system is calculated at the Coupled Cluster with Single and Double and perturbative Triple excitations (CCSD(T)) level of theory with the augmented correlation consistent valence quadruple zeta (aVQZ) gaussian basis set for a fixed value of the CH+ bond length (2.1371 bohr). Our computations account for basis set superposition errors (BSSE). Bond functions, which have been proven very efficient in reproducing the intersystem correlation interaction energy are placed at mid-distance between the CH+ center of mass and He. The PES is found to have a minimum about 537 cm−1 below the CH+ + He dissociation limit. This well depth is enough to give rise to a cluster-like, bound structure with a considerable number of rotational levels in the electronic ground state. The PES is fitted on a basis of Legendre polynomials functions. This allows to perform the calculation of state to state rotational integral cross sections of the CH+ collision with He in the close-coupling (CC) approach. By averaging the cross sections over a Maxwell-Boltzmann velocity distribution, collisional rates are computed at low temperature (T ⩽ 200 K). It is shown that there is, except for energies E < 500 cm−1, a propensity towards ΔJ even parity transitions. The present study may be of great practical interest for astrophysical observations and laboratory experiments.