Rotational excitation of AlCl induced by its collision with helium: cross sections and collisional rate coefficients

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In this work, inelastic rotational collision of AlCl with helium was studied. The CCSD(T) method was used for the computation of an accurate two dimensional potential energy surface (PES). In the calculation of the PES, Al-Cl bond was frozen at the experimental value $4.02678~\mbox{a}_{0}$ . The aug-cc-pVQZ basis sets of Dunning was used throughout the computational process. This basis was completed with a set of 3s3p2d2f1g bond functions placed at mid-distance between the center of mass of AlCl and He atom for a better description of the van der Waals interaction energy. The PES of AlCl-He was found to have a global minimum at ( $R=8.65~\mbox{a}_{0}$ , $\theta=0$ degree), a local minimum at ( $R=7.45~\mbox{a}_{0}$ , $\theta=82$ degree) and a saddle point at ( $R=7.9~\mbox{a}_{0}$ , $\theta=56~\mbox{degree}$ ). The depths of the minima were $20.2~\mbox{cm}^{-1}$ and $19.8~\mbox{cm}^{-1}$ respectively for $\theta=0$ and 84 degrees. The height of the saddle point with respect to the global minimum was $1.3~\mbox{cm}^{-1}$ . The PES, the result of an analytical fit, was expanded in terms of Legendre polynomials, then used for the evaluation of state-to-state rotational integral cross sections for the collision of AlCl with He in the close coupling approach. The collisional cross sections for the transitions occurring among the 17 first rotational levels of AlCl were calculated for kinetic energies up to $4000~\mbox{cm}^{-1}$ . Collisional rate coefficients between these rotational levels were computed for low and moderate kinetic temperatures ranging from 30 to 500 K. A propensity rule that favors odd $\Delta j$ transitions was found.