Rotational excitation and de-excitation of CP− (X1Σ+) in collisions with He (1S): cross-sections and rate coefficients

Abstract

Collisional energy transfer at low temperatures is crucial for astrophysical modeling. As an analogue to 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⁻ (X¹Σ⁺) in collisions with He (¹S) 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 the 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 quasi-bound 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 Δj = ±1, ±2, ±3. The results indicate that dominant collision-induced transitions shift from Δj = 1 to Δj = 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⁻.