THE (R)Q(3) BRANCH OF HSSD AT 656 GHZ - EXAMPLE OF A FREQUENCY-TRIPLED BACKWARD-WAVE OSCILLATOR

The (r)Q(3) branch of HSSD with its band head located in the submillimetre-wave region at about 656.5 GHz has been measured up to J = 28 for the ground and for the first excited stretching state. Commencing with medium J quantum numbers (J > 11) the inertial asymmetry splitting is observed, whereas for J > 27 each K-split line separates further into equally strong b- and c-type transitions. For the first time it has been possible to assign Q-branch rotational lines of HSSD to an excited vibrational state, the first excited stretching state upsilon(s) = 1, which yield differences in the rotational constants: (i) A - B = 93 731.204 (71) MHz and (ii) B - C = 149.69 (14) MHz. The rotational spectra have been recorded with a newly designed wave-guide multiplier, optimized for providing local oscillator power at 660 GHz for astrophysical applications when pumped at 220 GHz. The input radiation was provided by a carcinotron (backward wave oscillator).