Abstract
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 CO2 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 υt = 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 CO2-laser-pumped far-infrared laser lines of methylamine.
Highlights
Methylamine (CH3NH2) is the simplest primary amine in chemistry
To date there are still many overlapped lines and unresolved asymmetry doublet lines in the FT spectrum for this band of CH3NH2 due to limit on spectral resolution imposed by the Doppler width
The schematic of the experimental setup used in the present work for the Lamb-dip spectroscopy of CH3NH2 is shown in Fig. 1, and has been described in detail previously[28]
Summary
Methylamine (CH3NH2) is the simplest primary amine in chemistry. Its CH3 group is connected to the NH2 group by the C−Nbond that exhibits a vibrational stretching motion. The C−N stretching band of CH3NH2, earlier studied and reported sixty years ago at the relatively low resolution of 1 cm−1 and an accuracy of about 0.1 cm−1 by Gray and Lord[19], displays strong characteristic absorptions in the infrared region around 1044 cm−1 It is characterized by a parallel structure with vibrational P, Q, and R branches, which overlaps well with the CO2 laser bands. There is a clear need to observe many more Lamb-dip signals over a wider range of transitions in P-, Q-, and R-branch to obtain accurate experimental information and parameters for detailed spectroscopic analysis These precise Lamb-dip measurements are important for disentangling overlapped features in the Doppler-limited spectra and in providing a grid of standard reference frequencies for accurate calibration of the overall FT spectrum.
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