fotoelektron

A fotoelektron, or photoelectron, is an electron ejected from a material after the absorption of a photon. It is a central observable in the study of the electronic structure of atoms, molecules, and solids. In metals and semiconductors, photoelectrons can escape from the surface into vacuum when illuminated with light of sufficient energy; in gas-phase species, photons can remove electrons from neutral molecules.

The process is governed by the photoelectric effect. An electron is emitted if the incident photon energy hν exceeds the material’s work function φ, leaving kinetic energy KE = hν − φ. The emission yield depends on photon energy, light polarization, intensity, and surface condition, and the angular distribution of emitted electrons provides information about the initial electronic state.

Historically, the effect helped establish quantum theory. Einstein proposed the photon concept and the relation hν = φ + KE in 1905, for which he received the Nobel Prize. The photoelectric effect demonstrated that light consists of quanta and that energy transfer in light-m matter can be quantized.

Applications include photoelectron spectroscopy techniques such as X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS), which measure binding energies and chemical states of surfaces. Angle-resolved photoemission spectroscopy (ARPES) maps electronic band structures. Photoemission is also used to create electron sources for microscopes and vacuum devices; at high light intensities, multiphoton photoemission can occur.

Experiments typically require ultra-high vacuum and clean surfaces to minimize scattering and energy loss. The concept applies to metals, semiconductors, and molecular systems, and remains a fundamental tool in surface science and condensed matter physics.