Calculation of collisional line-broadening and shifting of acetylene using Complex Robert–Bonamy–Ma approach

Abstract

A comprehensive semi-classical study of the collisional line broadening and shift coefficients of C${}_2$H${}_2$ by several key perturbers (H${}_2$, He, N${}_2$, C${}_2$H${}_2$, CO, and CO₂) for astronomical applications using the Complex Robert–Bonamy–Ma (CRBM) framework is presented. Following the CRBM computational protocol, the intermolecular interaction potentials are constructed from atom–atom and electrostatic interactions, and then fitted to reproduce experimental room-temperature line-broadening parameters taken from the literature. In total, 657 experimental values are used in the fitting. The empirical potentials are then used to predict line broadening coefficients over a wide temperature range. Reference collisional line widths ${\gamma }_0$ and temperature exponents $n$ for the commonly used single-power law are produced, as well as a set of parameters for the double-power law, which better reproduces the temperature dependence of theoretical predictions. The vibrational dependence of the line widths is studied using a new ab initio isotropic polarizability surface of C${}_2$H${}_2$ and is found to be negligible. The computed line broadening parameters are found to agree well with the experimental data, while the modelling of line shifts of HCCH is not satisfactory when compared to the experiment. The new line broadening data of C${}_2$H${}_2$ with the $J$ (or $m$) dependence have been used to populate the ExoMol database www.exomol.com as part of the ExoMol pressure-broadening diet and can be used to model opacities of atmosphere of (extrasolar) planets. The CRBM methodology tested here on C${}_2$H${}_2$ can be used for other similar (closed-shell) systems in ExoMol that are important for exoplanetary atmospheric studies.