thermal ionization

Atoms with low ionization potentials can be ionized by contact with the heated surface of a metal, generally a filament, having a high work function (the energy required to remove an electron from its surface) in a process called thermal, or surface, ionization.…

…calcium illustrates the phenomenon of thermal ionization. At low temperatures a calcium atom retains all of its electrons and radiates a spectrum characteristic of the neutral, or normal, atom; at higher temperatures collisions between atoms and electrons and the absorption of radiation both tend to detach electrons and to produce…