Photoexcitation is a phenomenon in physics where an excited state of a quantum system (an atom or a molecule) is created by photon absorption. The excited state originates from the interaction between a photon and the quantum system when the energy of the photon is too low to cause photoionization. A very simple example of this process is electron excitation.

A photon's energy is directly proportional to the frequency of its associated electromagnetic wave. Thus, light with lower frequencies is associated to photons with a lower energy. In contrast, light with higher frequencies is associated to photons with a higher energy. The absorption of the photon takes place in accordance with the theory of quantum mechanics.

Photoexcitation plays a role in different subjects of physics and chemistry:

Moreover, photoexcitation is exploited by many different devices, such as:

On the nuclear scale photoexcitation includes the production of nucleon and delta baryon resonances in nuclei.