Frequency-modulation spectroscopy has been employed to monitor the absorption and dispersion changes induced by the optical saturation of nonzero velocity groups of ${\mathrm{I}}_2$ molecules. The use of a high modulation frequency markedly increases the sensitivity of this form of saturation spectroscopy and allows convenient access to molecular velocities well above the thermal average.

• Received 9 January 1981

DOI:https://doi.org/10.1103/PhysRevA.24.166