Fragmentation of molecular gases by 5-45 keV He+ ions

Ionization and fragmentation by 5-45 keV He + ions during passage through thin targets of H 2 , N 2 , O 2 , CO, CO 2 and CH 4 have been investigated by the mass spectrometric analysis of the secondary ionic products of single collisions. The method ensured that all secondary product ions were collected and recorded with high and uniform efficiency. Fragmentation processes are found to play a dominant role in secondary ion production for all the cases considered in the present energy range. Below about 15 keV, it is shown that charge transfer takes place mainly through dissociative processes. Dissociative ionization is also shown to be significant, particularly for targets of H 2 , N 2 and O 2 , at unexpectedly low impact energies. The energy dependence of the various cross sections for the production of fragment ions is not at all in accord with the predictions of the simple adiabatic maximum rule.