The lifetime of a freely decaying hollow atom

Abstract

A highly charged ion (HCI) captures electrons resonantly from a sur- face upon impact. This capture takes place into highly excited ionic states, leaving intermediate shells empty and creating a hollow atom (HA) [1,2]. The subsequent de-excitation of these HAs can be quite complex and depends strongly on whether it decays freely in vacuum or close to a target surface [3]. In this contribution, experimental results on the de-excitation of HCIs of various mass, charge states and energies will be presented. It will be discussed how the de-excitation is different in transmission experi- ments through 2D materials [4] and in grazing incidence scattering ge- ometry from a surface [5]. In grazing incidence scattering it is possible to avoid surface-near processes such as the interatomic coulombic de- cay, which would otherwise lead to an ultra-fast (fs) de-excitation. We further present a comprehensive model calculation for highly excited HAs and their de-excitation cascade. Our code [5] takes radiative and non-radiative transition rates computed using the flexible atomic code (FAC) [6] as input and we account for screening and mirror charge ef- fects. We find that for a free decay in vacuum the total lifetime of an HA is typically in the picosecond range. Additionally, we could show that under certain conditions, HAs can survive for up to 10 ps. This would make them in principle available for further studies and use in experiments. [1] J.-P. Briand et al., Nucl. Instrum. Methods Phys. Res., B 154, 166- 173 (1999) [2] J. Burgdörfer et al., Phys. Rev. A 44, 9 (1991) [3] R.A. Wilhelm et al., Phys. Rev. Lett. 119, 103401 (2017) [4] A. Niggas et al., Commun. Phys. 4, 180 (2021). [5] M. Werl et al., Phys. Rev. Res. 7, 013176 (2025) [6] M.F. Gu, Can. J. Phys., 86, 675 (2008)