Inelastic scattering of interstellar silyl cyanide (SiH3CN) by helium atoms : cross-sections and rate coefficients for A- and E-SiH3CN

ABSTRACT Rotational inelastic scattering of silyl cyanide (SiH3CN) molecule with helium (He) atoms is investigated. Three-dimensional potential energy surface (3D-PES) for the SiH3CN–He interacting system is carried out. The ab initio 3D-PES is computed using explicitly correlated coupled cluster approach with single, double, and perturbative triple excitation CCSD(T)-F12a connected to augmented-correlation consistent-polarized valence triple zeta Gaussian basis set. A global minimum at (R = 6.35 bohr; θ = 90○; ϕ = 60○) with a well depth of 52.99 cm−1 is pointed out. Inelastic rotational cross-sections are emphasized for the 22 first rotational levels for total energy up to 500 cm−1 via close coupling (CC) approach in the case of A-SiH3CN and for the 24 first rotational levels for total energy up to 100 cm−1 via CC and from 100 to 500 cm−1 via coupled states (CS) in the case of E-SiH3CN. Rate coefficients are derived for temperature until 80 K for both A- and E-SiH3CN–He systems. Propensity rules are obtained for |ΔJ| = 2 processes with broken parity for A-SiH3CN and for |ΔJ| = 2 processes with |ΔK| = 0 and unbroken parity for E-SiH3CN.</jats:p

Explicitly correlated potential energy surface of the CO₂–CO van der Waals dimer and applications

Using ab initio methodology, we generated the 4D-PES of the CO2–CO complex for spectroscopic and dynamical computations.</p

Rigid rotor state-to-state cross-sections and rates of the PH3 + H2 collision

Accurate interpretation of observational astronomical data requires reliable collisional rate coefficients for inelasting scattering events between interstellar molecules and the abundant buffer species. A five-dimensional potential energy surface (PES) for the PH3 ( 1A1) – H2 (1Σ+g) interaction was generated using the explicitly correlated CCSD(T)-F12 method in conjunction with the correlation-consistent triple-zeta aug-cc-pVTZ basis set, and averaged over H2 orientations to yield a reduced three-dimensional surface. Inelastic rotational cross-sections for collisions between ortho and para-PH3 with para-H2 (J = 0) are calculated using the close-coupling quantum scattering method. After Boltzmann thermal averaging, the rate coefficients are evaluated for temperatures up to 100 K. Our results reveal substantial discrepancies between computed PH3-para-H2 collisional rates and scaled PH3–He values, underlining the inadequacy of scaling approaches for reliable astrophysical modelling

State-to-state inelastic rate coefficients of phosphine in collision with He at low to moderate temperature

ABSTRACTSeveral 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.</jats:p

State-to-state inelastic rate coefficients of phosphine in collision with He atlow to moderate temperature

International audienceSeveral 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 PH 3 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 PH 3-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 , } = {5.76a 0 , 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-PH 3 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-PH 3. 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 NH 3-He. Differences are observed that invalidate the use of NH 3 rates for deducing accurate abundances of phosphine in cold astrophysical media. Our results should be of great help in determining accurate PH 3 abundances and, more generally, constraining the interstellar PH 3 chemistry better