Electron capture induced fragmentation of CO$_2^{3+}$: Influence of projectile charge on sequential and concerted break-up pathways

Abstract

We investigate the $\text{O}^+:\text{C}^+:\text{O}^+$ fragmentation channel of CO$_2^{3+}$ produced in slow collisions with Ar$^{q+}$ projectiles ($4 \le q \le 16$, velocities $\approx 0.3$ a.u). Using the native-frames method, we disentangle the sequential and concerted break-up processes and their corresponding kinetic energy release (KER) distributions. \emph{Ab initio} potential energy curves of CO$_2^{3+}$ are calculated and mapped to the KER spectra to identify the underlying electronic states involved in the fragmentation. While the sequential KER distributions remain nearly unchanged for across the projectile charge range, the concerted KER distributions exhibit pronounced but non-systematic variations with projectile charge. In addition, a low KER feature around 15.5 eV -- previously associated with sequential break-up in electron and proton impact -- is observed for Ar$^{4+}$ impact and, to a lesser extent, for Ar$^{6+}$ impact. It originates predominantly from concerted break-up of the low-lying $^2\Pi_\text{g}$ and $^{2,4}\Pi_\text{u}$ states. Branching ratios of the two break-up pathways deviate from simple monotonic trends for certain projectiles, but barring these exceptions, the fraction of concerted break-up decreases with increasing $q$, while that for sequential break-up increases. These findings underscore the necessity of accounting for the detailed electronic structure of the projectile, rather than its charge alone, to achieve a comprehensive understanding of collisional dynamics in slow, highly charged ion collisions.