Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for
energy level which is often associated with an atom being excited to an excited state.
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).
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.
Atomic excitation
A simple example of this concept comes by considering the
hydrogen atom.
The ground state of the hydrogen atom corresponds to having the atom's single
electron in the lowest possible
orbit (that is, the spherically symmetric "1s"
wavefunction, which has the lowest possible
quantum numbers). By giving the atom additional energy (for example, by the absorption of a
photon of an appropriate energy), the electron is able to move into an excited state (one with one or more quantum numbers greater than the minimum possible). If the photon has too much energy, the electron will cease to be
bound to the atom, and the atom will become
ionised.
After excitation the atom may return to a lower excited state, or the ground state, by emitting a photon with a characteristic energy. Emission of photons from atoms in various excited states leads to an
electromagnetic spectrum showing a series of characteristic
emission lines (including, in the case of the hydrogen atom, the
Lyman, Balmer, Paschen and Brackett series.)
An atom in a high excited state is termed
Rydberg atom. A system of highly excited atoms can form a long-lived condensed excited state e.g. a condensed phase made completely of excited atoms:
Rydberg matter. Hydrogen can also be excited by heat or electricity.
Perturbed gas excitation
A collection of molecules forming a gas can be considered in an excited state if one or more molecules are elevated to kinetic energy levels such that the resulting velocity distribution departs from the equilibrium
Boltzmann distribution. This phenomenon has been studied in the case of a
two-dimensional gas in some detail, analyzing the time taken to relax to equilibrium.
See also
External links
