Collisional excitation of propyne (CH₃CCH) by He atoms

Abstract

A detailed interpretation of the detected emission lines of environments where propyne (or methyl acetylene, CH$_3$CCH) is observed requires access to its collisional rate coefficients with the most abundant species in the interstellar medium, namely, helium (He) or molecular hydrogen (H$_2$). We present the first three-dimensional potential energy surface (3D PES) for the CH$_3$CCH-He molecular complex. We study the dynamics of the collision and report the first set of rate coefficients for temperatures up to 100 K for the collisional excitation of the lowest 60 ortho rotational levels and 60 para rotational levels of CH$_3$CCH by He atoms. We computed the 3D PES with the explicitly correlated coupled-cluster with a single-, double-, and perturbative triple-excitation method in conjunction with the augmented correlation-consistent triple zeta basis set (CCSD(T)-F12a/aug-cc-pVTZ). The 3D PES was then fitted to an analytical function and scattering computations of pure rotational (de-)excitation of CH$_3$CCH by collision with He atoms were performed. State-to-state cross-sections were computed using the close coupling method for total energies up to 100 cm$^ $ and with the coupled states approximation at higher energies for both the ortho- and para- symmetries of CH$_3$CCH. The PES we obtained is characterised by a large anisotropy and a potential well depth of 51.04 cm$^ $. By thermally averaging the collisional cross-sections, we determined the quenching rate coefficients for kinetic temperatures up to 100 K. A strong and even $ j$ propensity rule at almost all collision energies is present for CH$_3$CCH-He complex. To evaluate the impact of rate coefficients on the analysis of observations, we carried out non-local thermodynamic equilibrium (non-LTE) radiative transfer computations of the excitation temperatures and we demonstrate that local thermodynamic equilibrium (LTE ) conditions are not typically fulfilled for the propyne molecule.

First close-coupling study of the excitation of a large cyclic molecule: collision of c-C5H6 with He

“…At the same time, only a very few theoretical works have been devoted to studying the collisional properties of nonlinear interstellar species containing more than 4 atoms, induced by atomic or molecular projectiles. Among them, mostly symmetric top species have been studied by the exact close coupling (CC) quantum scattering method, such as the acetonitrile (CH 3 CN) and methyl isocyanide (CH 3 NC) 12 or propyne (CH 3 CCH) 13 in collision with He, as well as methanol (CH 3 OH) due to H 2 impact. 14 In the case of asymmetric top molecules, one should mention the collisional excitation of cyclic carbene (c-C 3 H 2 ) by H 2 studied by the CC method, 15 nearly simultaneously with propylene oxide (CH 3 CHCH 2 O), 16 which has been investigated in collision with He, using the approximate coupled states (CS) method (for more details regarding the differences between the CC and CS methods, see Section 3).…”