state of excitation

A state of excitation is the state of a quantum system (atom, molecule, nucleus) at a higher energy level (i.e., an excited state) than the minimum (the ground state). For atoms, these are thought of as electrons in orbits further from the nucleus than they need to be (higher electron shells). Spectral lines result from numerous atoms undergoing some particular transition between states of excitation through absorption or emission of a photon (atomic excitation).

For a collection of atoms, e.g., a gas, the percentages of the gas at each state of excitation is of interest, because of its relation to temperature and to characteristics of the spectral lines (line broadening), allowing the study of spectral lines to be used to determine temperature, through the Boltzmann equation.