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

We report the first detection of the pure rotational spectra of H79Br and H81Br 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.