2020MNRAS.499.1578B

Several phosphorus-bearing molecules, such as the phosphine of interest here, have been detected in astrophysical media. With the aim of satisfying the precision required by the astrophysical community, we report the rate coefficients of PH₃ in collision with helium from low to moderate temperature. To this end, we constructed the first three-dimensional potential energy surface (3D-PES) of the PH₃-He van der Waals complex, which governs the nuclear motions. The 3D-PES was worked out by means of the standard coupled cluster with single, double and non-iterative triple excitation approach, in conjunction with the aug-cc-pVQZ basis set and complemented by mid-bond functions. This 3D-PES presents a well of 34.92 cm^–1 at {R, θ, Φ} = {5.76 a₀, 90°, 60°}. Afterwards, we incorporated this 3D-PES into time-independent close-coupling quantum dynamical computations to derive the inelastic cross-sections of rotational excitation of (ortho-) para-PH₃ after collision with He up to (1000) 500 cm^–1. Subsequently, we evaluated the rate coefficients for temperatures up to (100 K) 50 K populating the (41) 42 low-lying rotational levels of (ortho-) para-PH₃. These data were derived by averaging the cross-sections thermally over the Maxwell-Boltzmann velocity distribution. No general propensity rules are found. We also performed a comparison with the rates for NH₃-He. Differences are observed that invalidate the use of NH₃ rates for deducing accurate abundances of phosphine in cold astrophysical media. Our results should be of great help in determining accurate PH₃ abundances and, more generally, constraining the interstellar PH₃ chemistry better.