Doppler broadening

The term Doppler broadening refers to a type of line broadening (mechanism that creates width to spectral lines so they are not infinitely narrow) stemming from the motion of the molecules, which results in a variety of Doppler shifts affecting the wavelengths emitted or absorbed. Though the identical molecules produce photons of the same wavelength, those from molecules moving toward the observer have slightly shorter wavelengths, and from molecules moving away have slightly longer, according to the amount of movement, so a the observer sees a range of wavelengths, a broadened line. This can be thermal broadening (thermal Doppler broadening), i.e., from random motions associated with the temperature of the material, or may be due to turbulence, or due to an ordered (non-random) motion. The shape stemming from thermal broadening is a Gaussian profile and I think that sufficiently-random turbulence can produce the same profile, making it harder to work out temperature from the line profile. Any non-random motion produces its own specific shape, for example, two peaks if from a shell-shape structure that is expanding such that some of the gas is moving toward us and other moving away from us.