Photoemission is the
ejection of
electrons from the surface of a material (metal) caused by an increase of energy, such as a
photon of light of sufficiently high frequency striking the surface. The inbound photon must have a frequency high enough such that it's energy, given by
E=hf
where h is
Planck's constant and f is the
frequency, is high enough so that an electron in the
valence band can overcome the metal's
work function and be ejected.