Application of Gyrotrons for Molecular Gas Spectroscopy

We give a brief review of the results of using gyrotrons to study molecular spectra by means of a radioacoustic absorption detection (RAD) spectrometer. Examples of recording molecular spectral lines of sulfur dioxide (SO 2 ), carbon sulfide (OCS), and methane (CH 4 ) are presented. The achievement of a record-breaking sensitivity for the sub-terahertz frequency range in terms of the absorption coefficient is demonstrated when observing weak methane lines. The possibility of spectroscopy at harmonics of the gyrotron radiation with a frequency of up to 1 THz is shown. Spectroscopic estimates of the radiation power at the second and third harmonics with respect to the power at the fundamental frequency are given. The possibilities of using the methods of nonlinear spectroscopy, namely, two-photon spectroscopy, which extend the range of potential spectroscopic studies using intense gyrotron radiation, are demonstrated. The results of observations with a RAD spectrometer and a direct absorption spectrometer for the lines of two-photon rotational transitions of OCS and SO 2 molecules, which have well-studied spectra and a large dipole moment, are presented. Promising opportunities for using gyrotrons to solve fundamental and practical problems are discussed.