Single and Double Electron Capture by 1–16 keV Sn4+ Ions Colliding on H2

Single and double electron capture cross-sections for collisions of 118Sn4+ with molecular hydrogen have been measured in an energy range of 1 keV to 16 keV using a crossed-beam setup. The cross-sections are determined from measurements of charge-state-resolved ion currents obtained through a retarding field analyser. Remarkably, the single electron capture cross-sections for Sn4+ are more than a factor 3 smaller than the previously determined single electron capture cross-sections for Sn3+–H2 collisions and the double electron capture cross-sections are only about 20% smaller than the single electron capture cross-sections. These results are understood on the basis of potential energy curve crossings. The first active curve crossings for the Sn4+–H2 system happen at a relatively small internuclear distance of about 5.5 a.u., which should be compared to 8 a.u. for Sn3+ ions. Multi-channel Landau–Zener calculations have been performed for single electron capture and confirm these low cross-sections. The curve crossing for double electron capture by Sn4+ lies very close to the one for single electron capture, which may explain the single and double electron capture cross-sections being of similar magnitude.