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Lamb-dip spectroscopy of the C−N stretching band of methylamine has been systematically extended to  P -,  Q -, and  R -branch by using microwave sidebands of a large number of CO 2  laser lines as frequency-tunable infrared sources in a sub-Doppler spectrometer. Lamb-dip signals of more than 150 spectral lines have been observed with a resolution of 0.4 MHz and their frequencies have been precisely measured with an accuracy of ±0.1 MHz. More than 30 closed combination loops have been formed, which unambiguously confirm the assignments. For over 150 vibrational excited levels in 27 substates, refined term values have been determined and expanded in  J ( J  + 1) power-series to determine the substate origins and the effective rotational constants. For transitions with  Aa  torsion-inversion symmetry in torsional state  υ  t  = 0, 57  K -doublet lines displaying asymmetry splittings have been observed and the splitting constants for levels with  K  = 1, 2, and 3 in the excited states have been determined. Our results provide accurate experimental information for spectroscopic studies of the interesting vibrational perturbations and intermode interactions related to the C−N stretching mode, directly support astronomical surveys, and are very relevant in practice to identification and frequency determination of the CO 2 -laser-pumped far-infrared laser lines of methylamine.

Lamb-dip spectroscopy of the C−N stretching band of methylamine by using frequency-tunable microwave sidebands of CO2 laser lines