Inelastic scattering of PO+ by H2 at interstellar temperatures

Abstract

Phosphorous species are of great interest in interstellar chemistry since they are the basic blocks for building life here on Earth. Modelling the abundance and environment of recently detected PO+ under non Local Thermodynamic Equilibrium (LTE) requires rotational spectra of the molecule along with accurate collisional rates with the most abundant species, hydrogen and helium. A new 4D ab initio potential energy surface (PES) of PO+ - H2 collision is calculated using CCSD(T)/CBS(DTQ) methodology considering rigid rotor approximation. The region containing the minima of the PES is augmented using neural networks (NN) model while very high potentials (>2500 cm−1) and asymptotic region have been approximated using Slater and R−4 functions respectively. The close coupling calculations have been performed using MOLSCAT software for both ortho and para-H2. The rate coefficients have been reported for transitions j − j′ = 1 − 0, 2 − 1, 3 − 2 and 5 − 4 through which PO+ has been experimentally detected in ISM. The rate coefficients for even and odd transitions of PO+ with para-H2 are compared with that of helium and are found to be 1.1-2.0 times higher. For even transitions (Δj = 2), the ortho-H2 rates are 10% higher than para-H2 rates. However, the trend reverses in the case of odd transitions (Δj = 1) when higher J transitions are considered at low temperatures. At higher temperatures, the ortho rates cross the para-H2 rates and become larger than the latter. The new rate coefficients with both ortho and para-H2 will enable accurate modelling of the PO+ abundance in the interstellar medium under non-LTE conditions.