We report on the observation of a light-pressure-induced dispersion of laser light interacting with a Doppler-broadened atomic medium. This novel dispersion feature displays an even symmetry with respect to the optical Doppler detuning; it is due to the small modification of the atomic velocity distribution caused by the spontaneous scattering force. Our phase-sensitive measurements were performed on the $\lambda =555.65\mathrm{}$ nm ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}-{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ transition in ytterbium vapor by the use of frequency-modulation spectroscopy. The experimental results are in good agreement with theoretical predictions.

• Received 1 August 1988

DOI:https://doi.org/10.1103/PhysRevLett.61.2308