Pressure Shift and Broadening of Methane Line at 3.39μStudied by Laser-Saturated Molecular Absorption

We study broadening and the anomalously small shift of a $3.39-\mu$ rotation-vibration line of methane at millitorr pressures. Saturation by the laser intracavity field allows investigation of the very sharp natural linewidth, without Doppler broadening. Two lasers were independently locked to this transition with an offset of less than ±1 kHz, a reproducibility of ±1 × ${10}^{11}$.