Thermal Doppler Broadening

Spreading of a spectral line due to the temperature of the emitting medium. In a gas, the individual atoms, elements or molecules are continuously moving in random directions, with an average speed proportional to the temperature of the gas:

The term on the left-hand side is the mean kinetic energy. The individual gas particles (with mass m) follow a Maxwellian velocity distribution, resulting in a spread of velocities (v) about the average value. On the right hand side of the equation, T is the temperature of the gas and σ is the Stefan-Boltzmann constant.

At a given time, any particular atom, element or molecule could be moving along the line-of-sight, perpendicular to the line-of-sight, or some combination of both. As a consequence, every spectral line emitted is Doppler shifted slightly higher or lower relative to the observer, broadening the spectral line. Thermal Doppler broadening is also possible for absorption lines, where particles in the absorbing medium have random motions, so Doppler shifts in the absorbed wavelengths can occur.

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