This is the model from A-level, energetic electrons are used to knock weakly-bonded electrons off a molecule. One analogy is throwing bricks at the wall - the brick knocks off other bricks rather than sticking to it.
The most weakly-bound electrons are preferentially knocked off. These are typically from the lone pairs on heteroatoms, which usually represent the HOMO in MO diagrams.
It takes about 20µs to reach the detector. Many radical cations are too unstable, hence they decay before reaching it.
Chemical ionisation mass spectrometry
This uses electron impact to ionise a simple molecule such as methane, which in turn ionizes the molecule being measured by transfer of a proton.
Notice how this technique produces a cation, rather than a radical cation. Radical cations are generally less stable, so this technique produces ions which are more likely to last 20µs before decaying, enough time to reach the detector.
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