Sub-Terahertz High-Sensitivity High-Resolution Molecular Spectroscopy With a Gyrotron

We explore the possibility of a significant increase in the sensitivity of high-resolution molecular spectroscopy in the sub-THz range by the method of radio-acoustic detection of radiation absorption using the gyrotron as a source of high-power, continuous, frequency-tunable, and highly stable radiation source. We demonstrate that the sensitivity increases in direct proportion to the radiation power until the molecular line saturation is reached. For unsaturated lines corresponding to rotational transitions of methane,a sensitivity of the order of αmin≈ 10-11cm-1isachieved for a power level of about 20 W, which was limited in this work by technical reasons. Examples of spectral line records of other light molecules such as SO2, OCS, and CH3OH are given. These records permit one to examine the spectral content of gyrotron radiation. The presence of high harmonics without any external frequency multiplier is revealed and evaluated for the first time. Prospects for a further increase in the sensitivity of the method by increasing the radiation power by one or two orders of magnitude are discussed.