Problems of Stark Broadening for Experimental and Astronomical Spectra Liable to Self-Absorption

Experiments are described, relevant to the problem of correlating quantitatively a Stark effect, or any alternative explanation of broadening in spectral lines, with physical state of an atmosphere. A particular difficulty is that of fitting any theoretical contour to an observed line when an unknown degree of self-absorption is suspected of distorting the broadened structure, as in solar flares or radiation from unknown depth in the expanded atmospheres of early-type stars. To study a Stark contour in the presence of self-absorption, the laboratory source chosen is a deep electrodeless plasma with unusually high charge density at low voltage. Contours of Hex are analysed by transmitting from a high-dispersion spectrometer into a photomultiplier through a slit of aperture small compared with line-width. The assumptions required in fitting theories of Stark broadening to contours are examined, to find conditions under which a degree of self-absorption in the core of the line may be estimated. Misfit in the extreme wings is related to some theories which have arisen in stellar physics.