Excited State

CORE DEFINITION

Excitation is the elevation of energy to any level. In physics, there is a specific definition for this level: it is often associated with an atom being excited to an excited state. In quantum mechanics, an excited state of a system (e.g., an atom, molecule, or nucleus) is any quantum state of the system that has a higher energy than the ground state (i.e., it has an energy higher than the lowest energy the system can have). Generally, systems in an excited state are unstable and can only last for a short time: a quantum (e.g., a photon or a phonon) exists only for the moment of energy elevation, then returns to a lower energy state (a lower excited state or the ground state) after spontaneous emission or stimulated emission. This energy decay is generally called 'decay', which is the inverse process of 'excitation'. Excited states with longer durations are called metastable states. Isomers and singlet oxygen are two examples.