Electronic excitations by keV ions in matter - what can we learn from exit charge states, photon emission and their correlation?

KeV ions penetrating matter induce a series of different electronic excitations in the material and in the projectile itself. We investigate charge state distributions and the photon emission of He, F and Ne projectiles transmitted through C, Si and SiC. Measurements were conducted utilizing pulsed ion beams of about 1 ns pulse length at the Time-of-Flight Medium Energy Ion Scattering (ToF-MEIS) setup at Uppsala University [1]. By measuring absolute photon yields, we show that several 10% of transmitted projectiles exit the sample in an excited state and that the contribution of the sample to the overall EUV photon emission is negligible (≤0.03 photons/projectile) [2]. The dominance of photons originating from the freely decaying projectile is beneficial for studying fundamental processes.

By restricting the incident beam to <1 ion per pulse, we can link an individual photon to the specific projectile emitting it. We furthermore measure the charge state of the exiting projectile, obtaining charge resolved photon yields. Despite the very different charge state distributions, the obtained charge state resolved photon yields are largely independent of target material and trajectory. Different from He, for F and Ne, the results were largely independent of the exit velocity. The photon yields increase strongly with final charge state and we find close to 100% of the F and Ne particles exiting in an excited state for charges q≥2. Our observations for F and Ne contradict common models of the charge formation (e.g. electron stripping) and we propose a new model (see Fig. 1) inspired by the interaction of highly charged ions with solids. The model is verified by measuring electrons emitted by autoionization upon transmission of Al, P and Ar through C and Si.  Implications for the energy deposition of keV ions in matter and it’s relevance for particle range will be discussed.

Fig.1 New model of the charge formation: a) Multiple electrons are excited in close collisions within the sample resulting in the formation of b) hollow atoms. The finally detected charge state is defined by c) Interatomic Coulombic Decay (ICD) and d) subsequent autoionization processes. The deexcitation ends with e) the emission of a photon.

References.

[1] M.A. Sortica et al. Nucl. Instrum. Methods Phys. Res. B 463 (2020)

[2] K. Vomschee et al. Phys Rev A 113 (2026)