Editing Work function (section)

=== Methods based on photoemission ===

[[File:Photoelectric diode forward bias.svg|thumb|300 px|Photoelectric diode in ''forward bias'' configuration, used for measuring the work function ''W''<sub>e</sub> of the illuminated emitter.]]

The photoelectric work function is the minimum [[photon]] energy required to liberate an electron from a substance, in the [[photoelectric effect]].
If the photon's energy is greater than the substance's work function, [[photoelectric effect|photoelectric emission]] occurs and the electron is liberated from the surface.
Similar to the thermionic case described above, the liberated electrons can be extracted into a collector and produce a detectable current, if an electric field is applied into the surface of the emitter.
Excess photon energy results in a liberated electron with non-zero kinetic energy.
It is expected that the minimum [[photon energy]] <math> \hbar \omega </math> required to liberate an electron (and generate a current) is 
:<math>\hbar \omega = W_{\rm e} </math>
where ''W''<sub>e</sub> is the work function of the emitter.

Photoelectric measurements require a great deal of care, as an incorrectly designed experimental geometry can result in an erroneous measurement of work function.<ref name="pitfalls"/> This may be responsible for the large variation in work function values in scientific literature.
Moreover, the minimum energy can be misleading in materials where there are no actual electron states at the Fermi level that are available for excitation. For example, in a semiconductor the minimum photon energy would actually correspond to the [[valence band]] edge rather than work function.<ref>{{cite web|url=http://www.virginia.edu/ep/SurfaceScience/PEE.html|title=Photoelectron Emission|website=www.virginia.edu|access-date=11 April 2018}}</ref>

Of course, the photoelectric effect may be used in the retarding mode, as with the thermionic apparatus described above. In the retarding case, the dark collector's work function is measured instead.