Ionization and dissociation of molecular gases by 5-45 keV helium atoms

The ionization produced by 5-45 keV helium atoms during passage through thin targets of H 2 , N 2 , O 2 , CO, CO 2 and CH 4 has 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 a high and uniform efficiency. Absolute cross sections for the production of particular fragment ions, derived from measured relative abundances, are found to be considerably higher than some previously reported values. It is shown that the possible influence of fast metastable atoms on the measured cross sections is very small. In spite of the large energy defects associated with possible formation processes for different fragment ions, at the lowest impact energies dissociative ionization is shown to be the dominant ionizing mechanism for all the gases investigated.