How does the photoelectric effect differ from photoionization in a gas?

In physics, the photoelectric effect and photoionization are fundamentally similar processes involving the interaction of electromagnetic radiation with matter to eject electrons.12

The primary distinction is contextual rather than physical:
* Photoelectric effect: This term is typically used when the process occurs on the surface of a solid or liquid, most commonly a metal. The ejection of an electron is governed by the material's photoelectric work function, which is the minimum energy required to remove an electron from the surface.2
* Photoionization: This term is more commonly applied when the interaction occurs within a gas. In this process, high-frequency photons are absorbed by neutral atoms or molecules, causing the ejection of an electron and leaving behind a positive ion.23

Despite these different labels, there is essentially little difference between the underlying physical mechanisms of the two processes. Both rely on the absorption of energy to overcome the binding forces holding an electron to an atom or material.23