Spectral Line Broadening

Spectral Line Broadening

As pointed out before (Laser Diode Absorption), the profile of a spectral line (width and shape) can also reveal information about the plasma. The profile is determined by several processes. The most important ones are Doppler Broadening and Stark Broadening. From the (Gaussian) Doppler profile, the temperature of radiating atoms can be determined (c.f. the width of a Thomson profile). The (Lorenzian) Stark profile can provide information about the electron density in the plasma. In general, the line profile will thus be a Voigt profile, which is a convolution of a Gaussian and Lorenzian profile. The Gaussian and Lorenzian halfwidths can be determined from a fit, which can then yield the heavy particle temperature and the electron density. The experimental setup for the measurement of the spectral line profiles is the same as that used for ALI.

The Gaussian and Lorenzian contributions to a measured profile of a spectral line.

Other contributions to the line profile are Van der Waals (pressure) broadening, a broadening due to the finite lifetimes of excited states of the atom, and an additional broadening by the detection system itself. This latter one has to be corrected for, by deconvolution of the measured profile with the instrumental profile of the spectrograph. The first two, and possible other broadening mechanisms, are usually neglected.

A commonly used spectral line for these experiments is the Hb Balmer line. This spectral line has been studied extensively and therefore the relation between line width and the plasma parameters is tabulated reasonably well. In addition, usually there is always enough hydrogen in a plasma to be able to detect its spectral lines. But of course, also other spectral lines can be used.