Precision Lamb-dip infrared spectra of the C–N stretching band of CH3NH2 with a CO2-laser/microwave-sideband spectrometer

Abstract

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).

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

“…Although the C−N stretching band center of CH 3 NH 2 overlaps well with the CO 2 laser bands, it is difficult to observe many absorption lines of CH 3 NH 2 by using just a grating CO 2 laser, because the available spectral coverage between the CO 2 laser and CH 3 NH 2 absorption lines is limited to the overlap in Doppler widths of only about 60–100 MHz for each laser line. This situation was recently improved greatly by the application of a frequency-tunable infrared source in a dual-mode sideband spectrometer to CH 3 NH 2 28 . The schematic of the experimental setup used in the present work for the Lamb-dip spectroscopy of CH 3 NH 2 is shown in Fig.…”

“…It is very desirable to carry out sub-Doppler observations on this band employing a spectrometer with very high resolution and accuracy in order to make confident and reliable measurements and line assignments. In 2010, we observed the precise Lamb-dip spectra of the C−N stretching band of CH 3 NH 2 at a spectral resolution of 0.4 MHz and determined the transition frequencies with an accuracy of ±0.1 MHz 28 , which is the first sub-Doppler observation in any spectral range of CH 3 NH 2 . However, the first Lamb-dip spectra of this band have been observed just for 43 saturation dips which are primarily in the C−N stretching Q -branch region and only in two sub-states.…”

“…Therefore, the interesting microwave 1 2 3 4 5 6 7 8 9 10 , far-infrared 11 12 13 14 15 16 17 , and infrared 18 19 20 21 22 23 spectra of CH 3 NH 2 have been extensively studied for many years to explore the splittings and the symmetry species, leading to the valuable application in the detection of interstellar methylamine 24 25 26 27 . In contrast, despite the fact that it is of great practical interest related to optically pumped far-infrared laser lines and interstellar detection, spectroscopic study by the sub-Doppler technique on the stretching of the C−N bond that connects the CH 3 and NH 2 groups has remained rare until recently 28 .…”

“…These precise Lamb-dip measurements are important not only for disentangling overlapped features in the Doppler-limited spectra but also in providing a grid of standard reference frequencies for accurate calibration of the overall FT spectrum. Accordingly, in the present work, using frequency-tunable microwave sidebands of a much larger group of CO 2 laser lines in our spectrometer 28 , the Lamb-dip spectroscopy of the C−N stretching band of CH 3 NH 2 has been systematically studied to cover a greatly expanded set of quantum states. We now report our extended experimental results.…”

Models of Geometrically “Stabilized” Laser Cavity

“…In several areas of physics [1][2][3][4], the Lamb dip characterizing the line shape of a gas laser is used as a reference for frequency and length measurements. These measurements would be more accurate if the line shape was symmetrical, in which case the center frequency of the laser would coincide with that of the Lamb dip [5].…”