Doppler-Free Two-Photon Polarization-Spectroscopic Measurement of the Stark-Broadened Profile of the Hydrogen ${\mathrm{L}}_{\alpha }$ Line in a Dense Plasma

It is demonstrated that Doppler-free two-photon polarization spectroscopy makes possible Stark profile measurements without Doppler-broadened background under high-density and high-temperature conditions, e.g., in dense arc plasmas, where other sub-Doppler laser-spectroscopic techniques introduced so far fail. This method, based on the polarization rotation connected with a two-photon transition, is not limited by collisions changing the velocity or the excitation of the atoms or by photoionization of the upper level.