an excited state of atomic systems, such as atoms, molecules, or atomic nuclei, that may exist for long periods and thus are stable. Metastable states are excited states from which quantum transitions to states with lower energy, accompanied by radiation (that is, by the emission of photons), are prohibited by exact or approximate selection rules and consequently cannot take place at all or are unlikely. A measure of the metastability of a state is its lifetime τ = 1/A, where A is the total probability of a transition from a given state to all states of lower energy. As A decreases, τ increases, and the state becomes more stable. In the limiting case of strictly prohibited transitions, A = 0 and τ = ㆀ. The lifetimes of metastable states of atoms and molecules are usually fractions of a second or a few seconds.
Atoms and molecules in the metastable state play an important role in elementary processes—for example, in rarefied gases: the excitation energy may be preserved for a long period by particles in the metastable state and then may be transferred to other particles upon collision, which causes an afterglow. The processes of luminescence of complex molecules are connected with the presence of metastable molecules in triplet excited states, from which transitions to the ground singlet state are prohibited by the selection rules.
M. A. EL’IASHEVICH