Surface ionization

Surface ionization forms ions when atoms interact with a surface and leave after exchanging electrons. For positive ion production at a hot surface, the equilibrium Saha-Langmuir ratio has the form $n_{+}/n_0=(g_{+}/g_0)\exp [(W-I)/(k_{B}T)]$, where $W$ is the surface work function, $I$ the atomic ionization energy, and $g_{+}/g_0$ a statistical-weight ratio.

A high work function favors leaving an electron behind. Alkali atoms, with relatively low ionization energies, can therefore be detected efficiently by converting neutral arrivals into countable ionic signals. Surface condition and temperature matter, so the equilibrium expression is a model to check rather than a universal detector calibration.^[ref]