Lamb-dip measurements of ammonia calibration lines in methylamine using a dual-mode CO 2-laser-microwave-sideband spectrometer

Abstract

This note reports a useful spin-off from an infrared study of the methylamine (CH3NH2) molecule in the 10-lm region, namely a set of accurate Lamb-dip frequencies for ammonia (NH3) present as a trace constituent. The new NH3 measurements supplement previous sub-Doppler observations for ammonia [1,2] and provide an important base of standard frequencies for calibration of the methylamine CAN stretching band. CH3NH2 is a prototype nonrigid molecule with both CH3-torsion and NH2-wagging large-amplitude motions, producing complex energy structure and dense spectra [3–11] that are challenging to analyze in detail. Furthermore, due to chemical exchange with water in the absorption cell, possible slight impurity in the original samples, and even traces in the laboratory air, ammonia lines are generally present in any CH3NH2 spectrum [8–11], leading to possible confusion in the analysis [9]. In this work, using a CO2-laser-microwave-sideband spectrometer [12] in its sub-Doppler mode, saturation dips have been resolved and measured precisely for 14 NH3 lines in the m2 symmetric bending band that appear in the region of the CAN stretch of CH3NH2. The commercial CH3NH2 sample supplied by BOC Specialty Gases has a stated purity of 99.5%, so there might be an NH3 impurity of up to 0.5% given that commercial methylamine is usually prepared by the reaction of ammonia with chloromethane. The NH3 Lamb-dip signals could still be seen in our experiment even for CH3NH2 sample pressures as low as 1 mTorr, demonstrating the high sensitivity of the spectrometer. The observations are valuable in providing precise absolute frequencies for calibration of our CH3NH2 spectra, checking our measurement accuracy against a pre-