metastable state

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.

This entry comes from Encyclopædia Britannica Concise.
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