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

Rotational excitation of interstellar PN molecules induced by collisions with H is investigated. We present the first ab initio four-dimensional potential energy surface (PES) for the PN-H 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 H 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. The dissociation energies were found to be 40.19 cm and 75.05 cm for complexes of PN with ortho-H and para-H, respectively. Integral cross sections for rotational excitation in PN-H collisions were calculated using the new PES and were found to be strongly dependent on the rotational level of the H molecule. These new collisional data will be crucial to improve the estimation of PN abundance in the interstellar medium from observational spectra.