Heterodyne radiometry measurements of the 557 GHz H 2 O rotational line

A high-resolution heterodyne radiometer that utilizes a crossed-guide harmonic mixer pumped by a 300 GHz carcinotron as a local oscillator has been developed for submillimeter wavelengths. Previous investigations of the radiation transfer properties of spectral lines in supersonic gas flow have been extended to longer wavelengths using the tunability of this system. With less than 2 mW of fundamental local oscillator power, traces of the 557 GHz ortho-H 2 O rotational transition ( 1_{10} \leftarrow 1_{01} ), Doppler-broadened to only a few MHz, were observed in absorption against a continuum background.