Nonlinear parametric resonances in ionic spectra

A study is made of two-photon resonant scattering by oscillations of excited ions in a low-temperature plasma. It is shown that an alternating electric field of frequency ω₀ accelerating these ions produces not only the usual nonlinear interference structures in the laser radiation spectra, but also–because of a parametric resonance–satellite lines at the Raman frequencies ω_μ =ω ±ω₀ (ω_μ and ω are the frequencies of laser fields). This phenomenon is used in the proposed laser spectroscopy method for detecting natural and forced oscillations of a low-temperature plasma due to high-frequency modulation of the voltage maintaining the gas discharge. Nonlinear parametric resonances corresponding to ion-acoustic and other instabilities may be used in plasma diagnostics.