Rotational state-changing collisions of C$_2$H$^-$ and C$_2$N$^-$ anions with He under interstellar and cold ion trap conditions: a computational comparison

TL;DR

This study uses quantum calculations to analyze rotational state-changing collisions of C$_2$H$^-$ and C$_2$N$^-$ anions with helium, providing new rate data relevant for interstellar chemistry and cold ion trap experiments.

Contribution

It provides the first comprehensive ab initio calculations of inelastic collision rates for C$_2$N$^-$, comparing them with C$_2$H$^-$ to enhance understanding of molecular ion dynamics in space and laboratory conditions.

Findings

Rates differ significantly between the two anions.

Collision rates increase with temperature up to 100 K.

The study offers detailed quantum cross sections for both systems.

Abstract

We present an extensive range of quantum calculations for the state-changing rotational dynamics involving two simple molecular anions which are expected to play some role in evolutionary analysis of chemical networks in the Interstellar environments, C$_2$H$^-$($X^1\Sigma^+$) and C$_2$N$^-$ ($X^3 \Sigma^-$) but for which inelastic rates are only known for C$_2$H$^-$. The same systems are also of direct interest in modelling selective photo-detachment (PD) experiments in cold ion traps where the He atoms function as the chief buffer gas at the low trap temperatures. This study employs accurate, \textit{ab initio} calculations of the interaction potential energy surfaces (PESs) for these anions, treated as Rigid Rotors (RR) and the He atom to obtain a wide range of state-changing quantum cross sections and rates at temperatures up to about 100 K. The results are analysed and compared for the two systems, to show differences and similarities between their rates of state-changing dynamics.