photoelectrons

Photoelectrons are electrons ejected from a material, typically from a solid surface, as a consequence of photoemission caused by incident electromagnetic radiation. The phenomenon, known as the photoelectric effect, is observed when photons have enough energy to overcome the material's work function.

According to Einstein's photoelectric equation, the maximum kinetic energy of emitted electrons is KE_max = hν - φ, where hν is photon energy and φ is the work function of the material. If hν ≤ φ, no emission occurs. For frequencies above threshold, increasing light frequency raises kinetic energy; increasing light intensity increases the number of emitted electrons up to a limit set by other factors.

The emitted electrons can have a range of kinetic energies up to KE_max and are typically emitted from the surface with a distribution in angle, influenced by surface structure, polarization of light, and electronic states. The rate of emission and resulting photoelectric current depend on photon flux, quantum efficiency, and the material's surface cleanliness and temperature.

Photoelectrons are central to many techniques and devices. Photoelectron spectroscopy uses KE measurements to determine electronic binding energies and band structure; angle-resolved variants map momentum. Photocathodes in photomultiplier tubes and electron guns rely on photoemission to convert light into an electron current. In solids and semiconductors, materials are engineered to optimize quantum yield for specific wavelengths.

Historically, the effect was observed in the late 19th century and explained by Einstein in 1905, helping establish quantum theory. Typical work functions for metals range roughly 2–5 eV, implying ultraviolet to visible photons can drive emission depending on the material.