Electron impact excitation and ionization in liquids

The paper deals with evaluation of impact ionization coefficients in noble liquids over a wide range of elevated reduced electric fields up to ~10³ Td. Two electron assisted mechanisms are assumed to contribute to the ionization process. The first one is due to electron impact immediately, while the second mechanism primarily involves forming an exciton followed by an ionizing collision with a free unexcited particle. As a result, the upper and lower limits for ionization coefficients in a liquid can be assessed depending on whether or not account is taken of associative ionization. Using the free electron energy distribution function, the ionization coefficients are determined by analytically solving the electron kinetic Boltzmann equation allowing for liquid structure and density effects. Simple analytical expressions are derived for the impact excitation and ionization rates at high electric fields. Also presented is the relation between the density of impact-produced electrons and the specific energy released in the bulk of the liquid