Rotational excitation of AlCl induced by H2 collisions

We present the first study of the rotational excitation of AlCl(X1Σ+) induced by collisions with H2. We have calculated a four dimensional potential energy surface (PES) to describe the AlCl-H2 interaction, with AlCl and H2 being treated as rigid rotors. This PES has been computed using the explicitly correlated coupled-cluster method with single, double, and perturbative triple excitation in conjunction with the augmented-correlation consistent-polarized valence triple zeta basis set. Then, we have calculated the rotational excitation cross sections between the first 41 rotational levels of AlCl induced by H2 collisions using the time-independent quantum mechanical close-coupling and coupled-states formalisms. Convergence tests revealed that the inclusion of excited para-H2 energy levels in the rotational basis has a minor effect on the magnitude of the excitation cross sections. We also found that AlCl excitation cross sections induced by ortho-H2 collisions are in good agreement with those induced by para-H2 collisions. Hence, we limited the calculations to the excitation cross sections induced by H2 in its ground rotational state. To derive excitation rate coefficients for temperatures up to 250 K, the cross sections computed for energies up to 1500 cm-1 were averaged over a Maxwell-Boltzmann velocity distribution. The new rate coefficients were compared to those available for the AlCl-He collisional system, and major differences were found at high temperature, showing that actual AlCl-H2 rate coefficients should be used for accurate astrophysical models. The new data are expected to play a crucial role in the modeling of AlCl observational spectra and will help in better constraining its abundance in space.