IMPROVED SENSITIVITY OF TUNABLE FAR-INFRARED SPECTROSCOPY - APPLICATION TO THE DETECTION OF HBR IN THE V=1 STATE

We report the first detection of the pure rotational spectra of (HBr)- Br-79 and (HBr)-Br-81 in , the first excited vibrational state at room temperature. We recorded the spectra with a tunable far-infrared spec trometer equipped with an optimized Ge:Ga photodetector and an improve d multiple-pass gas cell to enhance the overall sensitivity. In partic ular, a minimum detectable absorption of better than 10(-7) cm(-1) was achieved. We measured a total of 60 frequencies for the two isotopome rs with an accuracy of a few parts in 10(8). The hyperfine structure w as observed at the lower J values. The derived spectroscopic parameter s, which include nuclear and quadrupole hyperfine constants, reproduce the assigned transitions within their experimental errors. (C) 1997 O ptical Society of America.