Electron ionization and photoionization under vacuum

These reactions occur under high vacuum. Thus, no ion-molecule reaction occurs. The species formed during the ionization process is a radical cation. Ionization efficiency depends on the ionization energy of the molecule. The presence or not of the molecular ions also depends on how easy it fragments. 

Fragmentation often produces both a radical and a cation. This can be represented by the following equation  

The factor that determines which of the fragments is a radical or a cation can be emphasized as a competition between two cations to capture the electron  

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