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Description
The evolution of charge-state-resolved kinetic energy spectra of Sn ions ejected from a laser-produced plasma (LPP) of Sn as a function of the density of the H$_2$ buffer gas surrounding the LPP is investigated. Without a H$_2$ buffer gas, energetic 1 - 5 keV Sn ions in charge states of 4+ up to 8+ are detected. In this keV regime, lower Sn charge states, i.e., below 4+ are absent. When H$_2$ is introduced into the system, low-charged energetic Sn ions can be produced by a series of consecutive electron capture processes. However, as electron capture by Sn$^{2+}$ ions from H$_2$ is endothermic, no significant population of singly charged Sn ions is expected in the keV regime. At H$_2$ pressures of 6x10$^{-4}$ mbar and higher, however, we only detect Sn$^{2+}$ and Sn$^+$ ions.
To explain the production of keV Sn$^+$ ions, electron capture by metastable Sn$^{2+*}$ ions has been proposed [1]. Semi-classical calculations on Sn$^{3+}$- H$_2$ collisions [2] indicate that one-electron capture by Sn$^{3+}$ ions populates Sn$^{2+}$ ions in metastable states. Model simulations (using theoretical 2-state Landau-Zener cross sections to account for capture by each of the three metastable $^3$P$_J$ levels) to track the charge states of Sn ions while traversing the H$_2$ gas agree with our measured data. This underpins the key role of metastable Sn$^{2+*}$ ions as a gateway to the production of Sn$^+$ ions. From an LPP-based EUV source perspective, the production of energetic Sn$^+$ ions in the buffer gas is of high relevance, as it shifts the charge state balance from Sn$^{2+}$ towards Sn$^+$ ions, which have a larger stopping cross section than Sn$^{2+}$ ions [3].
[1] Rai et al., 2023 to appear in Plasma Sources Sci. Techn.
[2] Rai et al., 2022, Phys. Rev. A. 106, 012804
[3] Abramenko et al., 2018, Appl. Phys. Lett. 112, 164102
| Presenting Author | Luc Assink |
|---|---|
| Presenting Author Email Address | l.assink@rug.nl |
| Presenting Author Affiliation | University of Groningen |
| Country | Netherlands |
| Presenting Author Gender | Male |
