The ejection of electrons from a solid (or less commonly, a liquid) by incident electromagnetic radiation. Photoemission is also referred to as the external photoelectric effect. The visible and ultraviolet regions of the electromagnetic spectrum are most often involved, although the infrared and x-ray regions are also of interest. Photoemission has several distinguishing experimental features: (1) the process of photon absorption and photoelectron generation is instantaneous, that is, there is no detectable time lag; (2) at a given frequency the number of photoelectrons ejected per second is proportional to the intensity of the incident radiation; and (3) the kinetic energies of the photoelectrons depend on the incident photon frequency and the work function of the surface, but are independent of the incident intensity.