Ro-vibrational quenching calculations of C$_2^-$ in collision with H$_2$

TL;DR

This study presents new quantum scattering calculations of ro-vibrational quenching of C$_2^-$ by H$_2$, providing more accurate rate coefficients for cooling processes relevant to laser cooling applications.

Contribution

A new five-dimensional ab initio potential energy surface was developed for C$_2^-$ and H$_2$, enabling more precise quantum scattering calculations of ro-vibrational quenching.

Findings

Para-H$_2$ is more efficient than ortho-H$_2$ in cooling C$_2^-$.

Calculated rate coefficients closely match experimental data at 20 K.

Vibrational quenching rates are sensitive to the H$_2$ spin isomer and rotational excitation.

Abstract

The molecular anion C$_2^-$ has been of interest in the last few years as a candidate for laser cooling due to its electronic structure and favourable branching ratios to the ground electronic and vibrational state. Molecular hydrogen has been used by the Wester group in Innsbruck as a buffer gas to cool the molecule's internal ro-vibrational motion. In the present work, we generate a new, five dimensional (5D) interaction potential for the system by considering the H$_2$ as a rigid rotor and the C$_2^-$ as a rotating-vibrating diatomic molecule. We thereafter calculate the cross sections and rate coefficients for ro-vibrational inelastic collisions of C$_2^-$ with both para- and ortho-H$_2$ on this new 5D \textit{ab initio} potential energy surface using quantum scattering theory for the dynamics. The rates for vibrational quenching are obtained over the range of temperatures which covers the single value measured by the experiments. A comparison is also made with the earlier results using a simpler 3D interaction potential. Furthermore, para-H$_2$ is found to be more efficient than ortho-H$_2$ (with or without undergoing rotational excitation) in cooling C$_2^-$. The rate coefficients for cooling the anions has been computed by appropriately weighting the ortho- and para-H$_2$ and compared with the available experimental result at 20 K. When the vibrational de-excitation rate coefficients are taken to be the ones not causing any concurrent rotational excitations in the final C$_2^-$ anions, the properly averaged results are found to get smaller and to become very close to the experimental measurements. The implications of these new results for laser cooling of C$_2^-$ are analyzed and discussed.