What process causes an excited atom to emit a photon without external trigger?

Spontaneous emission is the process in which an excited atom emits a photon without an external trigger. The atom drops from a higher energy state to a lower one, and the photon carries away energy equal to the difference between those states.

Unlike stimulated emission, spontaneous emission does not require a passing photon to initiate the transition. The exact emission time is probabilistic, although an excited state has a characteristic average lifetime. The emitted photon’s direction, phase, and polarization are generally not controlled, which is why ordinary lamps produce incoherent light rather than laser-like beams.

Spontaneous emission can produce visible light, ultraviolet radiation, infrared radiation, or other frequencies, depending on the energy-level spacing. Fluorescence is a related phenomenon: a material first absorbs energy and then often emits light through one or more spontaneous transitions. Phosphorescence is also related, but typically involves a longer-lived metastable state.

The process is not limited to isolated atoms. Molecules, ions, nuclei, and other quantum systems can spontaneously transition to lower-energy states. In quantum electrodynamics, interaction with the electromagnetic vacuum helps explain why these emissions occur even when no ordinary photon is present.