Laser-probing the rotational cooling of molecular ions by electron collisions

TL;DR

This study measures how electron collisions cool and excite rotational states of CH$^+$ ions at cryogenic temperatures, providing experimental data to benchmark theoretical models and advancing understanding of molecular ion behavior in plasmas.

Contribution

First experimental determination of inelastic electron collision rates for CH$^+$ ions using cryogenic storage and laser-dissociation techniques.

Findings

Electron-induced cooling to the ground state dominates over radiative relaxation.

Measured collision rates serve as benchmarks for theoretical calculations.

Method enables probing inelastic collisions for various plasma-relevant molecular ions.

Abstract

We present state-selected measurements of rotational cooling and excitation rates of CH$^+$ molecular ions by inelastic electron collisions. The experiments are carried out at the Cryogenic Storage Ring, making use of a monoenergetic electron beam at matched velocity in combination with state-sensitive laser-dissociation of the CH$^+$ ions for simultaneous monitoring of the rotational level populations. Employing storage times of up to 600 s, we create conditions where electron-induced cooling to the $J = 0$ ground state dominates over radiative relaxation, allowing for the experimental determination of inelastic electron collision rates to benchmark state-of-the-art theoretical calculations. On a broader scale, our experiments pave the way to probe inelastic electron collisions for a variety of molecular ions relevant in various plasma environments.