photonelectron

A photoelectron, or photonelectron, is an electron ejected from an atom, molecule, or solid after absorption of a photon. The process, known as the photoelectric effect in solids and atoms, occurs when the photon energy exceeds the binding energy that confines the electron to its source.

In atoms and ions, the relevant quantity is the ionization energy. An electron can be ejected if the photon energy hν is greater than the ionization energy I. The excess energy becomes the kinetic energy of the emitted electron: KEmax = hν − I. In solids, such as metals or semiconductors, electrons bound by a work function φ can be emitted when hν > φ, with KEmax = hν − φ. The emitted electrons often escape into vacuum or into an analysis chamber, where their kinetic energy and angular distribution carry information about the electronic structure.

Photoelectron spectroscopy (PES) is a family of techniques that uses photoemission to study electronic structure. X-ray photoelectron spectroscopy (XPS) probes core levels and chemical states, while ultraviolet photoelectron spectroscopy (UPS) examines valence band structure. Angle-resolved PES (ARPES) adds momentum information. Metal work functions typical range from about 2 to 5 eV, influencing emission thresholds.

Factors affecting emission include photon flux (intensity), photon energy, surface cleanliness and roughness, temperature, and material electronic structure. Practical limits arise from quantum efficiency and the finite escape depth of electrons in solids.

Historically, the photoelectric effect provided pivotal evidence for quantum theory, with Einstein describing the linear relation between kinetic energy and photon energy. Today, photoelectrons are central to surface science, materials analysis, and solid-state physics.