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We have demonstrated a sensitive new method of Doppler-free spectroscopy, monitoring the nonlinear interaction of two monochromatic laser beams in an absorbing gas via changes in light polarization. The signal-to-background ratio can greatly surpass that of saturated absorption. Polarization spectra of the hydrogen Balmer- line, recorded with a cw dye laser, reveal the Stark splitting of single fine-structure components in a Wood discharge.
Wieman et al. (Mon,) studied this question.
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