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Improved sensitivity of tunable far-infrared spectroscopy: application to the detection of HBr in the ? = 1 state.

We report the first detection of the pure rotational spectra of H79 Br and H81 Br in the first excited vibrational state at room temperature. We recorded the spectra with a tunable far-infrared spectrometer equipped with an optimized Ge:Ga photodetector and an improved multiple-pass gas cell to enhance the overall sensitivity. In particular, a minimum detectable absorption of better than 10-7 cm-1 was achieved. We measured a total of 60 frequencies for the two isotopomers with an accuracy of a few parts in 108 . The hyperfine structure was observed at the lower J values. The derived spectroscopic parameters, which include nuclear and quadrupole hyperfine constants, reproduce the assigned transitions within their experimental errors.