Electron Impact Ionization. Generation of Positive Ions

1 Electron Impact Ionization. Generation of Positive Ions 

No systematic quantum-mechanical calculations of the ionization cross section of many-electron atoms have hitherto been made. Particular difficulties arise in the calculation of the ionization cross section near the threshold. For this reason approximate and semi-empirical expressions are used for the computation of absolute ionization cross sections over a wide range of electron kinetic energies. Usually, the ionization cross section increases monotonically from zero (for an electron energy equal to the ionization potential) to maximum at (5 — 10) I, after which it gradually decreases with higher energies. 

Partial ionization cross sections are of fundamental interest to radiation chemistry. The calculation of such a cross section for atomic hydrogen ionization has revealed that an atom ionized by electron impact does not release very fast electrons (with an energy of the ionization potential order). This seems to hold with sufficient accuracy also for other processes involving more complex atoms and molecules. 

The information available makes possible to establish in detail the ionization mechanism. Ionization appears to occur by two paths. The first is the collision of the incident electron with one of the electrons in the atom. The core plays here the part of a spectator and receives no additional momentum upon escape of two electrons. The second is the elementary event of collision involving two electrons and a core to which the momentum is imparted in the direction of the ionizing electron velocity. 

The approximate theories which are fairly valid for the total ionization cross section cannot describe the differential cross sections with the same accuracy. 

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