Quasi-Optical Sub-Doppler Lamb-Dip Spectrometer

We describe a new sub-Doppler spectrometer with an enlarged gas cell, which was created at the IAP RAS for high-precision laboratory measurements of molecular transitions at millimeter and submillimeter wavelengths in the interests of radio astronomy. By using a larger diameter with a shortened cell length, a calibrated attenuator for radiation power adjustment, and synthesizers with lower phase noise, it was possible to eliminate a number of shortcomings of the previous spectrometer and not only to measure with high accuracy the transition frequencies of a number of molecules taking into account hyperfine splitting, but also to study their shifts due to both pressure and radiation power. In particular, information about precise frequencies will be used to examine the inner dynamics in the star-forming regions, and also to search for variations of fundamental constants. The principle of frequency-independent cell-aperture irradiation was employed when the optical scheme of the spectrometer was designed. The examples show Lamb-dip measurements of the hyperfine structure in the CH 3 CN and HNCO molecular lines.