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Excited state

After absorbing energy, an electron may jump from the ground state to a higher energy excited state.
Excitations of copper 3d orbitals on the CuO2 plane of a high-Tc superconductor. The ground state (blue) is x2–y2 orbitals; the excited orbitals are in green; the arrows illustrate inelastic x-ray spectroscopy.

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 refers to an increase in energy level above a chosen starting point, usually the ground state, but sometimes an already 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).[citation needed]

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.

Long-lived excited states are often called metastable. Long-lived nuclear isomers and singlet oxygen are two examples of this.[citation needed]

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