What is metastable state? What is the role of metastable state in LASER?

Metastable state: An excited energy level of an atom in which the electron stays for an unusually long time, about \(10^{-3}\) s, compared with an ordinary excited state, where the lifetime is about \(10^{-8}\) s. Transitions out of it are 'forbidden' (low probability), so it decays slowly.

Role in a LASER:

• Atoms pumped to a higher short-lived level drop quickly (non-radiatively) to the metastable level and pile up there.
• Because they stay there long, the number of atoms in the metastable level becomes larger than in the lower level, which gives population inversion (\(N_2 > N_1\)).
• Population inversion is needed for stimulated emission to dominate over absorption. Without a metastable state, atoms would return spontaneously before stimulated emission could take place, and there would be no laser action.
• Example: in the He–Ne laser, the 3s and 2s levels of Ne (fed from metastable He levels) act as the upper laser levels.