Collisional excitation of interstellar PN by H₂: New interaction potential and scattering calculations

Rotational excitation of interstellar PN molecules induced by collisions with H₂ is investigated. We present the first ab initio four-dimensional potential energy surface (PES) for the PN-H₂ van der Waals system. The PES was obtained using an explicitly correlated coupled cluster approach with single, double, and perturbative triple excitations [CCSD(T)-F12b]. The method of interpolating moving least squares was used to construct an analytical PES from these data. The equilibrium structure of the complex was found to be linear, with H₂ aligned at the N end of the PN molecule, at an intermolecular separation of 4.2 Å. The corresponding well-depth is 224.3 cm^-1. The dissociation energies were found to be 40.19 cm^-1 and 75.05 cm^-1 for complexes of PN with ortho-H₂ and para-H₂, respectively. Integral cross sections for rotational excitation in PN-H₂ collisions were calculated using the new PES and were found to be strongly dependent on the rotational level of the H₂ molecule. These new collisional data will be crucial to improve the estimation of PN abundance in the interstellar medium from observational spectra.