Electron capture in collisions of tin ions with molecular hydrogen

Electron capture is a fundamental process in which a positively charged projectile particle moves past a neutral target particle and where it takes one or more electrons from the target particle. The process plays a role in the light source of advanced lithography machines which use extreme ultraviolet (EUV) light. This light is generated by a hot plasma of tin (Sn). Hydrogen gas (H₂) is used to slow down fast Sn ions from the plasma to prevent them from possibly causing damage. 

Besides energy transfer, electron capture occurs in the collisions. In his PhD research, Klaas Bijlsma studied the electron capture by tin ions from molecular hydrogen. He used the ZERNIKELEIF facility to make a beam of energetic Sn ions which subsequently crosses a beam of H₂ gas in a custom-made experimental setup. For the first time, this allowed for determining the probability of electron capture by doubly and triply charged Sn ions, even over a wide energy range. 

Based on a simple reaction energy balance one expects the conversion of higher to lower charged Sn ions by electron capture from H₂ to stop at Sn²⁺. However, Bijlsma found two reaction pathways that efficiently produce Sn⁺ ions. He compared his experimental results to combined classical and quantum mechanical calculations by the expert group at the Universidad Autónoma de Madrid. There wa good agreement at the higher energies, while at the lower energies the values deviate. Bijlsma believes this is due to the neglect of the molecular vibrations in the calculations.

Dissertation: https://hdl.handle.net/11370/d4718086-c50a-48b2-9d1a-43957b1a992b