A photochemical process that produces a positively charged ion. For atoms A and B, the process of Penning ionization is written:A*+B → A+B++e−, where A* denotes that the atom A has absorbed a photon, thus acquiring enough energy for the process to take place, and e− is an electron. An example of Penning ionization is the ionization of mercury by argon:Ar*+Hg → Ar+Hg++e−, which occurs because the energy of the metastable state of argon is higher than the ionization energy of mercury. The process of Penning ionization was discovered by F. M. Penning in 1927.
A*+B → A+B++e−
Ar*+Hg → Ar+Hg++e−