metastable state

Excited state (see excitation) of an atom, nucleus, or other system that has a longer lifetime than the ordinary excited states and generally has a shorter lifetime than the ground state. It can be considered a temporary energy trap or a somewhat stable intermediate stage of a system of which the energy may be lost in discrete amounts. The many photochemical reactions of mercury are a result of the metastable state of mercury atoms, and radiation from metastable oxygen atoms accounts for the characteristic green colour of the aurora borealis and aurora australis.

metastable state [¦med·ə′stā·bəl ¦stāt]

(quantum mechanics)

An excited stationary energy state whose lifetime is unusually long.

Metastable state

In quantum mechanics, a state that is not truly stationary but is almost stationary.

In practice, especially in atomic and nuclear physics applications, the designation metastable state usually is reserved for states whose lifetimes are unusually long. For example, the excited states of atoms usually decay with the emission of a single photon, in a time of the order of 10^-8 s. However, the necessity for angular momentum and parity conservation forces the second excited state (2S_1/2) of atomic hydrogen to decay by simultaneous emission of two photons; consequently, the lifetime is increased to an estimated value of 0.15 s. Thus, the 2S_1/2 state of atomic hydrogen is usually termed metastable, but most other hydrogenic states are not. Similarly, emission of a gamma-ray photon by an excited nucleus usually occurs in 10^-13 s or less; however, the lifetime of one excited state of the ¹¹³In nucleus, the state that customarily is termed metastable, is about 100 min. Since radiative transition probabilities for emission of photons generally decrease rapidly with decreasing frequency, a low-lying atomic or nuclear excited state may have a lifetime longer than most excited states of atoms and nuclei and yet not be metastable in the practical sense just described, because photon emission from the state may not be hindered by any general requirement or selection rule, such as is invoked for the 2S_1/2 state of hydrogen. See Excited state, Nuclear isomerism, Radioactivity