Lamb dip spectroscopy with the use of frequency-modulated radiation

•Theory for Lamb dip is developed with frequency modulation of radiation.•The theory was tested for mm-wave lines of OCS and CO molecules.•Various physical mechanisms that lead to systematic errors are analyzed.•Quantitative data on collision line widths was obtained.

A theory for molecular nonlinear absorption resonance in a standing electromagnetic wave recorded with the use of frequency-modulated radiation is developed. Spectral line profiles for the second harmonic of modulation frequency derived for different actual cases have been tested by comparison with the experimental ones recorded for absorption lines of OCS and CO molecules in the mm-wave region. The derived line profiles describe all the qualitative features of the experimentally observed spectra including the apparent splitting of a nonlinear resonance at high modulation frequency and allow retrieving quantitative data on the collision line broadening coefficients.