Editing Metastability (section)

===Atomic and molecular physics===
Some atomic energy levels are metastable. [[Rydberg atom]]s are an example of metastable excited atomic states. Transitions from metastable excited levels are typically those forbidden by electric dipole [[selection rule]]s. This means that any transitions from this level are relatively unlikely to occur. In a sense, an electron that happens to find itself in a metastable configuration is trapped there. Since transitions from a metastable state are not impossible (merely less likely), the electron will eventually decay to a less energetic state, typically by an electric quadrupole transition, or often by non-radiative de-excitation (e.g., collisional de-excitation).

This slow-decay property of a metastable state is apparent in [[phosphorescence]], the kind of [[photoluminescence]] seen in glow-in-the-dark toys that can be charged by first being exposed to bright light. Whereas spontaneous emission in atoms has a typical timescale on the order of 10<sup>−8</sup> seconds, the decay of metastable states can typically take milliseconds to minutes, and so light emitted in phosphorescence is usually both weak and long-lasting.