Thermal ionization

In thermal ionisation, also referred to as surface ionisation, chemically-purified material loaded onto a filament which is then heated to cause some of the material to be ionized as it boils off the hot filament. filaments are generally flat pieces of metal around 1-2mm wide, 0.1mm thick, bent into a U shape and welded to steel posts that supply a current.

The likelihood of ionisation 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:

${\frac {Y_{1}}{Y_{0}}}={\frac {g_{1}}{g_{0}}}\exp {\Bigg (}{\frac {\phi -IP}{kT}}{\Bigg )}$

${\frac {Y_{1}}{Y_{0}}}$ = ion to neutral ratio

${\frac {g_{1}}{g_{0}}}$ = statistical weights of ion and neutral states

$\phi$ = surface work function

IP = element ionization potential

k = Boltzmann's constant

T = surface temperature

One application of thermal ionization is thermal ionization mass spectrometry (TIMS). The ions coming of the filament are directed into a mass spectrometer to analyze the elements or isotopes present in the sample.