Sub-Doppler frequency-modulation spectroscopy of I2

Frequency-modulation spectroscopy has been employed to monitor the absorption and dispersion changes induced by the optical saturation of nonzero velocity groups of I2 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. © 1981 The American Physical Society.