SPECTROSCOPY IN THE TERAHERTZ REGION

High-resolution, broad-band scanning spectroscopy with microwave accur acy 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. T his breakthrough in high-resolution scanning spectroscopy, with microw ave accuracy and with hitherto unparalleled sensitivity, became possib le by the opening of the borders between East and West and by the imme diate start of a detailed technical collaboration between the Universi ty of Cologne, Germany, and the Institute of Applied Physics, Nizhnii Novgorod, Russia. Precision tunable frequency synthesizers, a newly co nstructed 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 bro ad-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 to gether with frequency multiplication of the BWOs' power output, spectr oscopy up to about 2.5 THz may be achieved in the near future. As an e xample of the future perspective offered by Doppler-limited terahertz spectroscopy, new laboratory rotational spectra of the ground and tors ionally excited states (upsilon(t) = 1, 2, 3) of disulphane, HSSH, and their isotopomers are presented. The (r)Q(Ka) (K-a=2, 3) branches, wh ich contain information not accessible in other parts of the spectra, are discussed, together with new data concerning the mass dependence o f the torsional problem. The new terahertz spectra are compared with t he presently available highest resolution Fourier transform spectra.