Fragmentation dynamics of CO23+ investigated by multiple electron capture in collisions with slow highly charged ions

Fragmentation of highly charged molecular ions or clusters consisting of more than two atoms can proceed in an onestep synchronous manner where all bonds break simultaneously or sequentially by emitting one ion after the other. We separated these decay channels for the fragmentation of CO23+ ions by measuring the momenta of the ionic fragments. We show that the total energy deposited in the molecular ion is a control parameter which switches between three distinct fragmentation pathways: the sequential fragmentation in which the emission of an O+ ion leaves a rotating CO2+ ion behind that fragments after a time delay, the Coulomb explosion and an in-between fragmentation - the asynchronous dissociation. These mechanisms are directly distinguishable in Dalitz plots and Newton diagrams of the fragment momenta. The CO23+ ions are produced by multiple electron capture in collisions with 3.2 keV/u Ar8+ ions.

💡 Research Summary

In this work the authors investigate the fragmentation dynamics of the trication CO₂³⁺ that is produced by multiple electron capture in collisions with slow, highly charged Ar⁸⁺ ions (3.2 keV/u). The experiment employs a Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) apparatus, which records the three‑dimensional momenta of all ionic fragments (C⁺, O⁺, O⁺) in coincidence. By reconstructing the full momentum vectors the authors are able to analyse the correlations among the three fragments and to distinguish different breakup pathways.

The data are visualised using Dalitz plots, which map the relative kinetic‑energy sharing among the three fragments onto a triangular coordinate system, and Newton diagrams, which display the momentum vectors of two fragments with respect to the third. These representations reveal three distinct fragmentation mechanisms that dominate under different internal energy conditions.

1. Coulomb explosion (simultaneous breakup). When the total energy deposited in the molecular ion is high, the three bonds break essentially at the same instant. The Dalitz plot shows a uniform distribution near the centre of the triangle, indicating equal energy sharing, while the Newton diagram displays an almost equilateral triangle of momentum vectors, characteristic of a prompt three‑body Coulomb repulsion.

2. Sequential fragmentation. At lower deposited energies the first step is the emission of a single O⁺ ion, leaving behind a rotating CO₂⁺ fragment. After a measurable time delay the remaining CO₂⁺ dissociates into C⁺ and a second O⁺. In the Dalitz plot this pathway appears as a linear band displaced from the centre, reflecting an asymmetric energy partition. The Newton diagram shows a dominant momentum vector for the first O⁺ and a much smaller, correlated pair for the C⁺–O⁺ pair, evidencing the delayed second step.

3. Asynchronous (intermediate) dissociation. Between the two extremes a third pathway is observed, in which the two bonds break almost simultaneously but not perfectly symmetrically. The Dalitz plot exhibits curved structures offset from the centre, and the Newton diagram shows a slightly distorted triangular arrangement. This “asynchronous” breakup carries a kinetic‑energy release (KER) intermediate between the pure Coulomb explosion and the sequential case, and the energy sharing among fragments is modestly asymmetric.

A key finding is that the total internal energy deposited by the electron‑capture process acts as a control parameter that switches the system from sequential → asynchronous → Coulomb‑explosion regimes. By varying the collision energy and the charge state of the projectile, the authors map out how the probability of each pathway changes. The KER distributions corroborate this picture: low‑energy events show a small KER peak (sequential), intermediate energies produce a broader distribution (asynchronous), and high‑energy collisions yield a large‑KER peak (Coulomb explosion).

The study demonstrates that momentum‑correlation techniques can unambiguously separate complex three‑body breakup mechanisms in polyatomic ions. It also provides quantitative insight into how multiple electron capture redistributes charge and internal energy, thereby dictating the subsequent nuclear dynamics. The methodology and conclusions are directly relevant to other fields where highly charged ions interact with molecules, such as ion‑beam cancer therapy, plasma processing, and ultrafast laser‑matter interaction studies. By establishing the deposited energy as a tunable knob for controlling fragmentation pathways, the work opens avenues for steering chemical reactions at the most fundamental, few‑body level.