Effect of electronic excitation in reactant ions

The usual electron bombardment method, which utilizes 50—100 eV electrons, produces ions in various electronically excited states, as well as in vibrationally and rotationally excited states. The experimental fact that the collision properties of a given ionic species are dependent on the mode of ion formation is evidence that the fraction of these excited states is quite different from mode to mode and that the reaction channels and cross-sections are largely dependent on these states. 

Recently, the development of devices in which electron energies can be controlled to a reasonable extent has enabled one to discriminate between reactions of these different electronic states. In some favourable cases, the abundances of excited species in primary ion beams are determined fairly accurately as a function of the bombarding electron energy. 

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