Rotational excitation and de-excitation of CP -(X 1 Σ + ) in collisions with He ( 1 S): cross sections and rate coefficients

Abstract

Collisional energy transfer at low temperatures is crucial for astrophysical modeling. As an analogue to the CN -, the simplest interstellar anion detected to date, the phosphorus-bearing CP -is a plausible candidate for astronomical searches. This paper investigates state-to-state rotational (de)-excitation of CP -(X 1 Σ + ) in collisions with He ( 1 S) over temperatures relevant to the cold interstellar medium and circumstellar envelopes. A new two-dimensional potential energy surface (PES) for the CP --He system was constructed within the rigid-rotor approximation using CCSD(T) method extrapolated to the complete basis set (CBS) limit. The PES features a global minimum of -31.39 cm -1 . Ab initio points were fitted analytically with Legendre polynomials for quantum scattering calculations. Quantum close-coupling calculations were performed to obtain rotational inelastic cross sections for rotational levels with j ≤ 10 of CP -at collision energies up to 1000 cm -1 . Resonances from quasibound states of the CP --He complex were present at low energies. By thermally averaging the cross sections, we derived state-to-state rate coefficients (k j→ j ′ ) over the temperatures range 3-200 K for ∆ j = ±1, ±2, ±3. The results indicate that dominant collision-induced transitions shift from ∆ j = 1 to ∆ j = 2 with increasing temperature. This work provides critical state-specific rotational rate coefficients for modeling CP -abundance in the interstellar medium and for interpreting future astronomical observations seeking CP -.