Energy loss by an excited atom can be performed in two basic ways:

• Non-radiative decay - by transferring the energy to mechanical vibrations of neighboring atoms. The rate for this type of decay is g_nr. The macroscopic effect of these vibrations is what we call heat (or rise in temperature).
• Radiative decay - by spontaneous emission at a rate g_rad.

Examples from everyday life of radiative decay:

• In a phosphorescent material, the atoms are excited by radiation at one wavelength, and after the excitation stops, we can still see (under proper illumination) a glow which is the photons emitted by excited atoms when they return to their ground state.
• Spontaneous Emission in Neon signs - Neon gas atoms inside a tube are excited by an electrical discharge through the gas. When these atoms return to their ground state, they emit photons in the visible spectrum.

Explanation: Neon gas is transparent to radiation in the visible spectrum, but the spectrum emitted out of a neon sign is in the Orange-Red spectrum. The natural absorption spectrum of Neon atoms is in the Ultra-Violet (UV) spectrum, but in a Neon sign gas tube, the Neon gas is ionized because of the electrical discharge. The ionized Neon atoms emit the visible radiation.