5.2.1 The Ionization Process

When a gas phase molecule is subjected to energy greater than the ionization energy, an electron can be removed. This results in the formation of a molecular ion:

$$M\;+\; energy\;\rightarrow\; M^{+.}\;+\; e^{-}$$

where energy is greater than the ionization energy of M. The energy may be provided by light, heat or collision. One easy and useful way to ionize organic molecules is by collision with energetic electrons:

$$M\;+\; e_{incident}^{-}\;\rightarrow\; M\;+\; e^{-}\;+\; e_{incident}^{-}$$

where in this equation, the incident electron energy after collision is less than the incident electron energy before the collision. Energetic electrons are easily produced via thermionic emission from a heated filament in vacuum.