Precision Lamb-dip infrared spectra of the C-N stretching band of CH _3 NH_2 with a CO_2 -laser/microwave-sideband spectrometer

摘要

We report saturation dip spectroscopy in the C-N stretching band of CH _3 NH_2 with a resolution of 0.4 MHz and an accuracy of 0.1 MHz by use of a CO_2 -laser/microwave-sideband spectrometer. The wide tunability, Lamb-dip resolution, absolute frequency accuracy, and high sensitivity of our dual-mode instrument were all key features in making precise measurements for a range of lines in the densely crowded spectrum with its complex pattern of splittings arising from the large-amplitude CH_3 torsion and NH_2 inversion. We focused on achieving resolution and assignment of transitions within the highly blended Q branch of the C-N stretch and on observations of the important K=0 sequences of Aa and Ea torsion-inversion symmetry. Term values of the latter were fitted to J (J+1) power-series expansions to obtain the K=0 C-N stretching effective B values and substate origins, from which calculated ground-state substate energies were subtracted to yield values of 1044.7061 and 1044.8011 cm~(-1) for the Aa and Ea subband origins, respectively. We thereby estimate a mean value of 1044.75 (5) cm~(-1) for the vibrational band origin and 0.7323 (5) cm~(-1) for the effective upper-state B value for the C-N stretching fundamental of CH_3 NH_2.