Photoionization

Photoionization occurs when an atom or molecule absorbs a photon energetic enough to eject an electron, leaving behind an ion.

Ionization energy: The minimum energy required to remove an electron from an isolated atom or molecule in a specified state. A photon must supply at least this energy for direct photoionization.
Photoionization mass spectrometry: A mass-spectrometric method that uses photons to ionize molecules before measuring their mass-to-charge ratios. It turns photon-driven ion formation into a tool for identifying molecules.
Photon: A quantum of electromagnetic radiation that carries energy and momentum. Absorbing a photon provides the energy that can remove a bound electron.
Multiphoton ionization: Ionization caused by the absorption of multiple photons, each individually below the ionization threshold. It reaches the continuum through accumulated photon energy rather than one sufficiently energetic photon.
Photoelectric effect: The emission of electrons from matter after it absorbs electromagnetic radiation. It is the closely related surface process, governed by photon energy and electron binding.
Photoelectron spectroscopy: A family of methods that measures electrons emitted when matter absorbs photons. Electron energies reveal binding energies and the states involved in photoionization.
Atomic orbital: A quantum-mechanical description of an electron's spatial state in an atom. The initial orbital helps determine the electron's binding energy and emission pattern.
Field ionization: The removal of an electron when a strong electric field lowers or eliminates its binding barrier. It can ionize matter without requiring absorption of a photon above the threshold.
Electronvolt: A unit of energy equal to the energy gained by an electron moving through a potential difference of one volt. Photon energies and ionization thresholds are commonly compared in electronvolts.
Atmospheric photoionization: The production of atmospheric ions when atoms or molecules absorb solar or other high-energy radiation. It initiates ion chemistry in the upper atmosphere.

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