The excited state describes an atom, ion or molecule with an electron in a higher than normal energy level than its ground state.
The length of time a particle spends in the excited state before falling to a lower energy state varies. Short duration excitation usually results in release of a quantum of energy, in the form of a photon or phonon. The return to a lower energy state is called decay. Fluorescence is a fast decay process, while phosphorescence occurs over a much longer time frame. Decay is the inverse process of excitation.
An excited state that lasts a long time is called a metastable state. Examples of metastable states are single oxygen and nuclear isomers.
Sometimes the transition to an excited state enables an atom to participate in a chemical reaction. This is the basis for the field of photochemistry.
Non-Electron Excited States
Although excited states in chemistry and physics almost always refer to the behavior of electrons, other types of particles also experience energy level transitions. For example, the particles in the atomic nucleus may be excited from the ground state, forming nuclear isomers.