State-selective and total single-electron capture for impact of slow He2+ (E ≤ 1 keV) on H2, O2 and CO

Projectile state-selective single-electron capture for impact of slow He2+ (impact energy less than or equal to I keV) on H2O, and CO has been studied experimentally by means of translational energy spectroscopy. Comparative measurements were made with a Ne gas target for absolute calibration of the electron capture cross-sections for the molecular targets. We compare the importance of "direct" single-electron capture (transition of one electron only without excitation or dissociation of the ionised target molecule) versus single-electron capture accompanied by target excitation (transfer excitation, possibly leading to molecular dissociation). Our measurements are extended to lower impact energies than earlier ones and are qualitatively interpreted on the basis of Landau-Zener transitions in the collisional quasimolecules. Similar as for direct single-electron capture, transfer excitation proceeds preferentially within a "reaction window" for the involved reaction energy defect. (C) 2004 Elsevier B.V. All rights reserved.