The minimum first ionization energy of many organic molecules is approximately
10 eV or less (where 1 eV, the energy obtained by an electron accelerated
by a potential of 1 volt, is about 96 kJ/mol). If the minimum first
ionization energy is transferred to a molecule, and no more, the ion
can be formed; however, the molecular ion would exist in its ground
state. In the ground state, there is no excess energy to drive the
chemical decomposition reactions required to produce an informative
mass spectrum.
The incident electrons do not transfer all of their kinetic energy
to the molecule (since this would cause a violation of the third law
of thermodynamics). Interestingly, the electron-molecule collision
process shares some similarity with the Rutherford scattering experiment:
{insert figure}
Most of the incident electrons transfer a relatively percentage
of their kinetic energy to the molecule; a distribution of energies
is transfered: {insert figure}
As this figure shows, to achieve a given nominal molecular ion internal potential energy, a considerably larger nominal incident electron energy is required.