ELECTRON-CAPTURE IN THE COLLISION OF MASS-SELECTED HYDROGEN-CLUSTER IONS WITH HELIUM-ATOMS

The electron capture cross sections of hydrogen-cluster ions H-n(+) co lliding with atomic helium have been measured in a large range of clus ter size (5 less 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 se ction decreases from the H+ ion to the H-3(+) one, the cluster electro n-capture cross section is found to be independent of the cluster size and nearly equal to the capture cross section of the H-3(+) ion. The electron capture by hydrogen clusters on a helium atom is a process in volving only the H-3(+) core of the cluster when the positive charge i s localized. It appears that this very localized electron capture is n ot disturbed by the presence of molecules, up to 16, around the H-3(+) core. [S1050-2947(98)09511-0].