Efficiency of Electron Ionization

The ionization energy (IE) defines the minimum energy required for ionization of the neutral concerned. Most molecules have lEs in the 7-15 eV range (Chap. 2.2, Tab. 2.1). If an impacting electron carrying just an amount of energy equal to IE would quantitatively transfer it when colliding with the neutral, ionization would 

Strictly speaking, every species has an ionization efficiency curve of its own. The overall efficiency of El depends on the intrinsic properties of the ionization process as well as on the ionization cross section of the analyte (Chap. 2.4). Fortunately, the curves of ionization cross section versus electron energy are all of the same type, exhibiting a maximum at electron energies around 70 eV (Fig. 5.4). This explains why El spectra are almost exclusively acquired at 70 eV. 

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