Surface ionization
Thermal ionization, also known as surface ionization or contact ionization, is a physical process whereby the atoms are desorbed from a hot surface, and in the process are spontaneously ionized.
Thermal ionization is used to make simple ion sources, for mass spectrometry and for generating ion beams. Thermal ionization has seen extensive use in determining atomic weights, in addition to being used in many geological/nuclear applications
The likelihood of ionization is a function of the filament temperature, the work function of the filament substrate and the ionization energy of the element.
This is summarised in the Saha-Langmuir equation:
where
Negative ionization can also occur for elements with a large electron affinity against a surface of low work function.
One application of thermal ionization is thermal ionization mass spectrometry (TIMS). In thermal ionization mass spectrometry, a chemically purified material is placed onto a filament which is then heated to high temperatures to cause some of the material to be ionized as it is thermally desorbed (boiled off) the hot filament. Filaments are generally flat pieces of metal around 1-2mm wide, 0.1mm thick, bent into an upside-down U shape and attached to two contacts that supply a current.
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