Absolute ionisation cross sections for electron impact on H₂

Absolute electron impact cross sections for single ionisation leaving H 2 + in its electronic ground state and double ionisation leading to dissociation into all four constituents have been determined by a direct method in the energy range 0.15-2.6 keV. The values for single ionisation are in perfect agreement with, but the double-ionisation values are much larger than, the earlier determination by Edwards et al. (1988,1989). The authors' results exhibit a close relationship between the H 2 system and its atomic counterpart, helium, with respect to their response to single- and double-ionisation processes.