Sub-Doppler spectroscopy of the C—O stretching fundamental band of methanol by use of microwave sidebands of CO_2 laser lines

Microwave sidebands of CO2 laser lines were used in a sub-Doppler spectrometer to observe sub-Doppler spectrum of the C—O stretching fundamental band (vCO=1←0) of methanol. Frequencies of more than 200 vibration-rotation lines were measured with an accuracy of better than 0.20 MHz. Sixty-one blended spectral lines in the Fourier-transform spectrum were resolved with a resolution of 0.2 MHz. For transitions involving A-species levels with K=2, 3, and 4 in the vt=0 state and K=2 in the vt=1 state, 64 doublet lines arising from asymmetry splittings were observed. From these observed asymmetry splittings and calculated ground-state splittings the asymmetry splittings and asymmetry-splitting constants for the vCO=1 state were determined. The R- and Q-branch transitions for the (vt, E, K)=(1, E, 2) and (1, E, 5) sequences were assigned by observation of their Stark effects and by use of the Ritz's combination principle. Term values of the levels in the vCO=1 state for these two sequences were given, and their Taylor-series expansion coefficients were determined.