Which phenomenon involves energy being released as photons while transitioning from a higher to a lower energy state?

The phenomenon that involves energy being released as photons while transitioning from a higher to a lower energy state is spontaneous emission. In this process, an atom or molecule that has absorbed energy and reached an excited state will return to its ground state by emitting energy in the form of a photon. This transition happens naturally and occurs without any external influence, meaning that it is not prompted by incoming radiation.

Spontaneous emission is a fundamental concept in quantum mechanics and plays a significant role in various applications, including lasers and light sources. It demonstrates how energy levels in atoms affect photon emission and is vital in understanding the behavior of light and matter interactions.

The other concepts relate to different mechanisms of photon interactions with matter: stimulated emission involves the stimulated release of photons in a coherent manner, absorption refers to the process where photons are taken in by matter and promote electrons to higher energy states, and photon excitation generally describes the process of energy absorption without specifying the subsequent emission mechanism.