- Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for
energy levelwhich is often associated with an atom being excited to an excited state.
quantum mechanicsan excited state of a system (such as an atom, moleculeor nucleus) is any quantum stateof the system that has a higher energythan the ground state(that is, more energy than the absolute minimum). The temperatureof a group of particles is indicative of the level of excitation.
The lifetime (see
resonance) of a system in an excited state is usually short: spontaneous or induced emission of a quantum of energy (such as a photonor 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 oxygenare two examples of this.
A simple example of this concept comes by considering the
The ground state of the hydrogen atom corresponds to having the atom's single
electronin 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 photonof 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 spectrumshowing a series of characteristic emission lines (including, in the case of the hydrogen atom, the Lyman series, the Balmer series, the Paschen series, and the 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 Maxwell-Boltzmann distribution. This phenomenon has been studied in the case of a
two-dimensional gasin some detail, analyzing the time taken to relax to equilibrium.
* [http://www.klimaforschung.net/kernreaktion/Orbital01.gifPicture of a hydrogen atom changing from ground state to an excited state]
* [http://imagine.gsfc.nasa.gov/docs/teachers/lessons/xray_spectra/background-atoms.html NASA background information on ground and excited states]
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