SPECTROSCOPY IN THE TERAHERTZ REGION

High-resolution, broad-band scanning spectroscopy with microwave accuracy has been extended into the terahertz region by achieving worldwide for the first time frequency and phase stabilization of continuously tunable backward wave oscillators (BWOs) at frequencies up to 1 THz. This breakthrough in high-resolution scanning spectroscopy, with microwave accuracy and with hitherto unparalleled sensitivity, became possible by the opening of the borders between East and West and by the immediate start of a detailed technical collaboration between the University of Cologne, Germany, and the Institute of Applied Physics, Nizhnii Novgorod, Russia. Precision tunable frequency synthesizers, a newly constructed broad-band multiplier-mixer system, and high-frequency BWOs ensure beat signals up to 2 terahertz. This technique combined with a sensitive He-cooled InSb hot electron bolometer, extends precision broad-band spectroscopy into the entire frequency region covered by high-frequency BWOs, i.e. up to 1.5 THz. It also increases the hope that together with frequency multiplication of the BWOs' power output, spectroscopy up to about 2.5 THz may be achieved in the near future. As an example of the future perspective offered by Doppler-limited terahertz spectroscopy, new laboratory rotational spectra of the ground and torsionally excited states (upsilon(t) = 1, 2, 3) of disulphane, HSSH, and their isotopomers are presented. The (r)Q(Ka) (K-a=2, 3) branches, which contain information not accessible in other parts of the spectra, are discussed, together with new data concerning the mass dependence of the torsional problem. The new terahertz spectra are compared with the presently available highest resolution Fourier transform spectra.