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Associative ionisation (AI), a fundamental atomic collision process, plays a crucial role in atomic physics, plasma physics and astrophysics, contributing to the ionisation of atoms and the formation of molecules. In this work we study AI in low-energy hydrogen atom collisions, in particular the reaction $\mathrm{H}(1s) + \mathrm{H}(ns) \to \mathrm{H}_2^++e^-$ with $2 \leq n \leq 10$, using a semi-classical approach. We extend the Duman-Shmatov-Mihajlov-Janev (DSMJ) model 1 to include short-range interactions and multistate transitions, thereby improving the predictive capabilities for low-energy AI processes. While the approach in 2 provided valuable insights, it relied on highly sophisticated quantum mechanical calculations and is therefore limited to collisions with $n \leq 4$. In contrast, our semi-classical model reproduces the results of 2 very well at significantly lower computational cost and gives a better intuition of the underlying physical processes, making it a more efficient alternative while remaining in good agreement with the experimental data [3] (see figure). This work not only aims at reproducing these results, but has also been used to explore the regime with $n \gg 1$.
1 R. K. Janev and A. A. Mihajlov, Excitation and de-excitation processes in slow collisions of Rydberg atoms with ground-state parent atoms, Physical Review A 20, 1890–1904 (1979).
2 J. Hörnquist et al, Associative ionization in collisions of $\text{H}^+ + \text{H}^-$ and $\text{H}(1s) + \text{H}(ns)$, Physical Review A 108, 052811 (2023).
[3] F. Brouillard and X. Urbain, Associative Ionisation in Low Energy Collisions, Phys. Scr. T{\bf 96}, 86 (2002).
