Subnatural coherence effects in saturation spectroscopy using a single traveling wave

We have performed zero-field level-crossing resonance measurements (nonlinear Hanle effect) in a metastable Ca atomic beam. By saturating the optical transitions 3P1,2 → 3S1, for which the radiative linewidth is 10.5 MHz, we have recorded resonances corresponding to widths of about 31 kHz. The effect has been investigated as a function of the saturating-radiation intensity. The frequency jitter of the laser used in the measurements is about 40 times wider than the recorded widths.