Doppler broadening

In atomic physics, Doppler broadening is broadening of spectral lines due to the Doppler effect caused by a distribution of velocities of atoms or molecules.Different velocities of the emitting (or absorbing) particles result in different Doppler shifts, the cumulative effect of which is the emission (absorption) line broadening.Then, the broadening depends only on the frequency of the spectral line, the mass of the emitting particles, and their temperature, and therefore can be used for inferring the temperature of an emitting (or absorbing) body being spectroscopically investigated.When a particle moves (e.g., due to the thermal motion) towards the observer, the emitted radiation is shifted to a higher frequency.Likewise, when the emitter moves away, the frequency is lowered.In the non-relativistic limit, the Doppler shift is whereSince there is a distribution of speeds both toward and away from the observer in any volume element of the radiating body, the net effect will be to broaden the observed line.is the fraction of particles with velocity componentalong a line of sight, then the corresponding distribution of the frequencies is whereis the velocity towards the observer corresponding to the shift of the rest frequencyWe can also express the broadening in terms of the wavelengthThen We can simplify this expression as which we immediately recognize as a Gaussian profile with the standard deviation and full width at half maximum (FWHM)In astronomy and plasma physics, the thermal Doppler broadening is one of the explanations for the broadening of spectral lines, and as such gives an indication for the temperature of observed material.Other causes of velocity distributions may exist, though, for example, due to turbulent motion.For a fully developed turbulence, the resulting line profile is generally very difficult to distinguish from the thermal one.[2] Another cause could be a large range of macroscopic velocities resulting, e.g., from the receding and approaching portions of a rapidly spinning accretion disk.For example, a sufficiently high particle number density may lead to significant Stark broadening.Doppler broadening can also be used to determine the velocity distribution of a gas given its absorption spectrum.In particular, this has been used to determine the velocity distribution of interstellar gas clouds.[3] Doppler broadening, the physical phenomenon driving the fuel temperature coefficient of reactivity also been used as a design consideration in high-temperature nuclear reactors.In principle, as the reactor fuel heats up, the neutron absorption spectrum will broaden due to the relative thermal motion of the fuel nuclei with respect to the neutrons.The end result is that reactors designed to take advantage of Doppler broadening will decrease their reactivity as temperature increases, creating a passive safety measure.Saturated absorption spectroscopy, also known as Doppler-free spectroscopy, can be used to find the true frequency of an atomic transition without cooling a sample down to temperatures at which the Doppler broadening is negligible.