Precise measurements of line broadening and line shifts in low-pressure gases using a heterodyne CO_2 laser spectrometer: applications to C_2H_4 and CH_3OH

A kilohertz accuracy of line-shape measurements for ethylene and methanol was obtained in the 10-μm region by saturated-absorption heterodyne spectroscopy. For both molecules, precise determination of self-broadening coefficients and values of pressure-induced shifts were carried out at pressures <200 mTorr. Foreign gas effects were investigated for the combinations of collisional partners: C2H4+O2,N2, He, and CH3OH+O2. Nonlinear line narrowing and change of sign of the line shift were recorded, and some qualitative considerations are presented. Absolute frequencies of the ethylene and methanol rovibrational lines under study were measured with a total experimental uncertainty <5 kHz.