Plasma radiation and spectral line profiles

Plasma spectroscopy is a highly specialized field of physics, even though it is also a highly interdisciplinary science. Its origins lie in astronomy, where the study of the spectra of the Sun and stars led, among other things, to the first results of what was to become quantum physics. In 1906, Lorentz laid the foundations of spectral line theory by proposing a profile that now bears his name. However, it was not until the early 1960s that the main concepts of plasma spectroscopy and spectral line profile theory emerged, with Baranger (1962) and Griem (1964). Over the years, both the theories and the mass of experimental data have grown, both in the field of laboratory plasmas and (back to basics) in the field of astrophysics, where the subject is still of great importance.

Despite the quantum origin of the electromagnetic radiation that originates in plasmas, the basic concepts of spectral line profile theory can, to a certain extent, be introduced using classical tools such as statistical mechanics and semi-classical theory. This is the point of view we'll be adopting here in order to lay the foundations of this physics. The spectral line profile is the result of a two-fold problem: on the one hand, a collisional aspect that modifies, among other things, the phase of the wave train emitted by the atom, and on the other, a statistical aspect linked to the nature of the gas or plasma in which the radiating atom is immersed.

The experimental study of plasma spectra is a remarkable tool for diagnosing these environments. The abundant scientific literature in this field is proof of the method's potential. Recent references in this field can be found not only in Griem's book, but also in the exhaustive NIST (National Institute of Standards and Technology) compilation. We won't go into laser spectroscopy, which has been undergoing considerable development for several years now: Light Induce Fluorescence (LIF) or four-wave mixing spectroscopy are increasingly used in laboratories; these methods are very powerful, because they are local and sensitive. Nevertheless, the study carried out in this article remains a preamble to these techniques. However, it should not be assumed that the study of spectral line profiles can completely dispense with quantum methods, which remain the starting point for in-depth study, especially in the field of collisions. Readers will find references to treatises that go into greater depth on the concepts outlined in this article.