Coherent four-wave mixing in excited and ionized gas media: four-photon spectrochronography, ellipsometry, and nonlinear-optical imaging of atoms andions

The coherent four-photon methods of studying excited and ionized gas media are reviewed in the context of their applications and of the main lines of research and novel techniques they are involved in. Among the possibilities offered by the methods are obtaining spectrochronographical information on atoms and ions; the selective study, utilizing polarization and coherent e llipsometry techniques, of the individual components of complex, nonstation ary gas and plasma systems; and the optical imaging of the spatial distribu tion of atoms and ions in excited gases and plasmas.