<P> In quantum mechanics, an excited state of a system (such as an atom, molecule or nucleus) is any quantum state of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum). Excitation is an elevation in energy level above an arbitrary baseline energy state . In physics there is a specific technical definition for energy level which is often associated with an atom being raised to an excited state . The temperature of a group of particles is indicative of the level of excitation (with the notable exception of systems that exhibit negative temperature). </P> <P> The lifetime of a system in an excited state is usually short: spontaneous or induced emission of a quantum of energy (such as a photon or a phonon) usually occurs shortly after the system is promoted to the excited state, returning the system to a state with lower energy (a less excited state or the ground state). This return to a lower energy level is often loosely described as decay and is the inverse of excitation . </P> <P> Long - lived excited states are often called metastable . Long - lived nuclear isomers and singlet oxygen are two examples of this . </P>

What happens when a hydrogen atom changes from the excited state to the ground state