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

TL;DR

This study provides the first detailed quantum mechanical analysis of collisional excitation of the large cyclic molecule c-C5H6 by helium, crucial for interpreting astronomical observations under non-LTE conditions.

Contribution

It introduces an accurate 3D interaction potential and applies the exact close coupling quantum scattering method to a large molecule-helium system, pioneering such analysis for molecules with over ten atoms.

Findings

First application of close coupling quantum scattering to large molecular collisions

Analysis of collisional propensity rules for c-C5H6 and He

Differences observed between exact and approximate scattering methods

Abstract

Recent astronomical observations revealed an increasing molecular complexity in the interstellar medium through the detection of a series of large cyclic carbon species. To correctly interpret these detections, a complex analysis is necessary that takes into account the non-local thermodynamic equilibrium (non-LTE) conditions of the emitting media (e.g. when energy level populations deviate from a Boltzman distribution). This requires proper state-to-state collisional data for the excitation and de-excitation processes of the molecular levels. Cyclopentadiene (c-C5H6), which was recently detected in multiple cold interstellar clouds, is extensively studied in many aspects due to its large importance for chemistry in general. At the same time, there are no collisional data available for this species, which are necessary for a more precise interpretation of the corresponding detections. In this work, we first provide an accurate 3D rigid-rotor interaction potential for the [c-C5H6 + He] complex from high-level of ab initio theories, which has been used to study their inelastic collision by the exact close coupling quantum scattering method. To the best of our knowledge, this is the first study where this method is systematically applied to treat the dynamics of molecular collisions involving more than ten atoms. We also analyse the collisional propensity rules and the differences in contrast to calculations, where the approximate coupled states scattering methods is used.