Evidence of production of keV Sn+ ions in the H2 buffer gas surrounding an Sn-plasma EUV source

Charge-state-resolved kinetic energy spectra of Sn ions ejected from a laser-produced plasma (LPP) of Sn have been measured at different densities of the H2 buffer gas surrounding a micro-droplet LPP. In the absence of H-2 , energetic keV Sn ions with charge states ranging from 4+ to 8+ are measured. For the H-2 densities used in the experiments no appreciable stopping or energy loss of the ions is observed. However, electron capture by Sn ions from H-2 results in a rapid shift toward lower charge states. At the highest H-2 pressure of 6 x 10(-4) mbar, only Sn2+ and Sn+ ions are measured. The occurrence of Sn+ ions is remarkable due to the endothermic nature of electron capture by Sn2+ ions from H-2. To explain the production of keV Sn+ ions, it is proposed that their generation is due to electron capture by metastable Sn2+* ions. The gateway role of metastable Sn2+* is underpinned by model simulations using atomic collision cross sections to track the charge states of Sn ions while traversing the H-2 buffer gas.