Unimolecular processes in diatomic carbon anions at high rotational excitation

TL;DR

This study models the high rotational and vibrational excitation of diatomic carbon anions to explain their unimolecular decay, identifying quasi-stable quartet states as the decay source, with implications for understanding ion stability.

Contribution

It provides a comprehensive theoretical model of high-excitation states in C$_2{}^-$, linking these states to observed decay rates and identifying the decay origin.

Findings

Quasi-stable quartet states cause unimolecular decay.

High excitation levels significantly influence decay rates.

Model aligns well with experimental decay measurements.

Abstract

On the millisecond to second time scale, stored beams of diatomic carbon anions C$_2{}^-$ from a sputter ion source feature unimolecular decay of yet unexplained origin by electron emission and fragmentation. To account for the magnitude and time dependence of the experimental rates, levels with high rotational and vibrational excitation are modeled for the lowest electronic states of C$_2{}^-$, also including the lowest quartet potential. Energies, spontaneous radiative decay rates (including spin-forbidden quartet-level decay), and tunneling dissociation rates are determined for a large number of highly excited C$_2{}^-$ levels and their population in sputter-type ion sources is considered. For the quartet levels, the stability against autodetachment is addressed and recently calculated rates of rotationally assisted autodetachment are applied. Non-adiabatic vibrational autodetachment rates of high vibrational levels in the doublet C$_2{}^-$ ground potential are also calculated. The results are combined to model the experimental unimolecular decay signals. Comparison of the modeled to the experimental rates measured at the Croygenic Storage Ring (CSR) gives strong evidence that C$_2{}^-$ ions in quasi-stable levels of the quartet electronic states are the so far unidentified source of unimolecular decay.