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.