Answer to Question #143805 in Quantum Mechanics for Sneha

Question

Explain doppler broadening.

Answer 1

Doppler broadening is caused by the Doppler effect from the nonzero velocities of atoms or molecules. Due to the different velocities of emitting particles the Doppler shifts of the wavelengths with respect to the center of the spectral line are also different. The total effect looks like a broadening of the spectral line.

One of the important cases is the thermal broadening. If we assume that the velocity distribution is given by the Maxwellian distribution, then the profile of the line will have a Gaussian form (see https://en.wikipedia.org/wiki/Doppler_broadening). The parameters of the spectral line, therefore, may provide us the information of the temperature.

But there are a plenty of other causes for the Doppler broadening, namely the turbulence or the macroscopic motions as in the disc of accretion.