Collisions of ionized hydrogen clusters with helium atoms: electron capture as a function of the cluster size

Electron capture cross sections of hydrogen cluster ions H-n(+) colliding w ith atomic helium have been measured in a large range of cluster size (5 le ss than or equal to n less than or equal to 35) for the same velocity (1.5v (0), 60 keV/u). While the electron capture cross section decreases from the H+ ion to the H-3(+) one, the cluster electron capture cross section is fo und to be independent of the cluster size and nearly equal to the capture c ross section of the H-3(-) ion. Electron capture by hydrogen clusters on a helium atom is a process involving only the H-3(+) core of the cluster wher e the positive charge is localized. It appears that this very localized ele ctron capture is not disturbed by the presence of molecules, up to sixteen, around the H-3(+) core. (C) 1999 Elsevier Science B.V. All rights reserved .