Exited state formation in low energy ion-atom, ion-molecule interactions With particular reference to the methodology of E.P. Sanders

Abstract

The technique of Translational Energy Spectrometry (TES) has been used to study electron capture processes for a number of ion-atom and ion-molecule collision systems at energies < 1 keV/amu. The systems studied were: He"2"+ ions in H_2 and CO; helium-like C"4"+, N"5"+ and O"6"+ ions in H and H_2; and hydrogen-like C"5"+, N"6"+, and O"7"+ ions in H and H_2. For each system, the main reaction channels have been identified, and in most cases, their relative importance assessed. For He"2"+ ions in H_2 and CO, dissociative one-electron capture processes have been found to account for almost 100% of the total one-electron capture cross at low energies. Dissociative one-electron capture processes were also observed in the C"5"++H_2 and O"7"++H_2 systems. However, in the case of N"5"++H_2, two electron capture into autoionising states of N"3"+ with subsequent N"4"+(n=2) formation was found to be an important process at low energies, accounting for approximately 50% of the total electron capture cross section at 214 eV/amu. Dissociative capture processes were not found to be important for C"4"+, N"6"+, and O"6"+ systems. In the case of electron capture from an atomic hydrogen target, capture was found to occur predominantly into n=4 levels for the N"5"+, N"6"+, C"5"+, and O"6"++H systems. For the C"4"++H system, C"3"+(n=3) formation was found to be the dominant reaction channel, with O"6"+(n=5) formation the favoured reaction channel in the O"7"++H system. Total charge-changing cross sections have also measured for the N"5"++H_2 system using a simple beam attenuation method. While agreeing with previous PES measurements in the region of overlap, the measurements are in poor agreement with OPIG measurements and theoretical calculations. For some of the systems studied, Landau-Zener and classical over-barrier reaction-windows have been calculated. For N"5"+, O"5"+, and C"3"+ ions colliding with H and H_2, agreement with the present results was found to be goodSIGLEAvailable from British Library Document Supply Centre- DSC:DXN060082 / BLDSC - British Library Document Supply CentreGBUnited Kingdo