Effects of the spatial dispersion and instabilities in the wave electrodynamics of weakly ionized gases

The complex spectra of transverse and longitudinal modes in the conducting gas of two-level (or quasi-classical) molecules are found, taking into account the spatial dispersion due to the thermal motion of molecules. Conditions for the existence of weakly damping longitudinal waves and transverse polarization waves, as well as of electromagnetic waves with the dispersion law much different from the “vacuum” dispersion law (which is well known in spectroscopy as the Voyght one and includes Doppler broadening according to the perturbation theory) are specified. The collisionless damping modes (on the wings of the Doppler line) with the dispersion law determined by the zeroes of the complex error function are studied. The analogy with the collisionless Landau damping of the Langmuir waves in electron plasma is established. It is shown that, in the neutral gas of excited molecules, the longitudinal and transverse polarization waves possess negative energy and may lead to the dissipative and beam-type instabilities