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Doppler broadening is the inhomogeneous broadening of the linewidth of atomic transitions caused by the random movements of atoms. For example, if the atoms have a thermal velocity distribution with temperature T, the linewidth resulting from the Doppler effect is
where ν0 is the mean optical frequency and m is the mass of the atoms. The Doppler broadening, e.g. for atoms in a gas cell, is typically much larger than the natural linewidth.
Doppler broadening can place severe constraints on precise spectroscopic measurements. However, it can be eliminated in various ways, e.g. by reducing the temperature (e.g. with laser cooling) or by employing a measurement scheme which is intrinsically insensitive to Doppler broadening, such as Doppler-free saturation spectroscopy or the use of two-photon absorption with counterpropagating beams.
Doppler broadening is the dominant line broadening mechanism in gas lasers.
See also: linewidth, inhomogeneous broadening, Doppler cooling, spectroscopy
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