atomic excitation

The term atomic excitation (or electron excitation) refers to the state of atoms that have at least one electron currently at a higher energy level than its lowest possible level (its ground state), i.e., in a state of excitation. The term is also used for changes to a higher such state, i.e., the movement of one or more electrons to a higher energy level. Molecules undergo analogous excitation and the term electron excitation includes that case as well.

The atom's possible electron orbits (electron orbitals) are determined by quantum physics, and are quantized, i.e., only allowed at certain specific energy levels. With sufficient energy, e.g., from an incoming photon, an electron can be moved to a higher state (a higher electron shell than they need to be in), which is an excited state. Such an excited atom has a tendency to emit one or more photons at random times, moving toward its lowest possible state, i.e., its ground state, such a change termed electron relaxation. Radiated excitation is the energization of the electrons of an atom by a photon, i.e., bound-bound absorption. Gas excitation is such excitation of the atoms of a gas. The absorption of photons to excite a gas's atoms results in absorption lines and the subsequent return of atoms to less energetic states produces emission lines, e.g., hydrogen's Lyman series.

The term population inversion refers to a group of atoms in which the majority are in some excited state.