Chapter 4 - Ionization and Excitation Phenomena

Abstract

This chapter provides an overview of ionization and excitation. Ionization means the removal of an electron from an atom or a molecule. The minimum energy, I, required for ionization is called the ionization potential. The minimum electron energy necessary to observe a particular ion is called its appearance potential (AP). Most ionizations brought about by charged-particle impact are very fast processes governed by the Franck–Condon principle. According to this principle, the nuclear configuration of the molecule remains unchanged during fast electronic transitions. Excitation and ionization have a common origin—raising the electronic level of an atom or a molecule from its ground state to a state of higher energy via the impact of charged particles or photons. Excited states may be formed by light absorption, direct excitation by the impact of charged particles, ion neutralization, dissociation, from ionized or superexcited states, and energy transfer. Excited states formed by the absorption of light are governed by dipole selection rules that are derived from parity and spin considerations.