Highly sensitive millimetre-wave spectrometer based on an orotron

The design and operation of a gas millimetre-wave spectrometer based on a tunable generator orotron are presented. The absorption cell occupies a part of the high quality (Q approximately=104) orotron Fabry-Perot cavity. The limit of sensitivity gamma min approximately=(3-5)*10-10 cm-1 has been achieved with source frequency modulation at the output band of a receiver of 1 Hz. The narrow spectral width of the orotron radiation (10-15 kHz without locking) provides the Doppler resolution of spectral lines. An absorption signal of gas under investigation is detected by variations of the collector current of the orotron. New results on rotational transition of SiH4 in a vibrational dyad v2/v4 with the absorption coefficient gamma approximately=10-8 cm-1 are briefly given as an illustration of the sensitivity of this spectrometer.