Answer: An excited state with an unusually long lifetime (about \(10^{-3}\) s against \(10^{-8}\) s); it lets atoms pile up so that population inversion, and hence laser action, becomes possible.
Metastable state: an excited energy level in which an atom stays for a comparatively long time, about \(10^{-3}\) s, compared with the usual excited-state lifetime of about \(10^{-8}\) s. Transitions from it by spontaneous emission are much less likely.
Role in a laser:
- Pumping lifts atoms to a higher short-lived level; they quickly drop to the metastable level and stay there.
- Because they stay long, the number of atoms in the metastable level becomes greater than in the lower level: this is population inversion.
- A photon of the right energy then triggers stimulated emission from the many atoms in the metastable state, giving amplification of coherent light. Without a metastable state the atoms would decay spontaneously before inversion could build up.
Example: in the He–Ne laser the upper Ne levels (fed by long-lived metastable He levels) act this way.
