High-sensitivity detection of the rotation spectrum of HCl in the v = 1 state by tunable FIR spectroscopy

We have observed in absorption 14 roatational transitions in the first excited vibrational state of H35Cl and H37Cl. A tunableB FIR spectrometer was used with a mulmtiple-pass cell to improve the sensitivity. Transitions with J″ ranging from 2 to 8 were recorded, with Doppler resolution, in the region between 1.8 and 5.5 THz and their frequencies were measured with an average accuracy of several parts in 108. These transition were analysed together with high J rotational transitions previously measured in emissiion in the infrared on the basis of an effective Hamiltonian, which includes terms up to the eight power in the angular momentum.