The far infrared rotational spectrum of HOBr: line positions and intensities

The far infrared absorption spectrum of HOBr has been measured at high resolution between 100 and 400 cm−1 using high-resolution Fourier transform spectroscopy. It was possible to identify not only 1403 pure rotation lines within the vibrational ground state involving levels with rather high Ka quantum numbers (up to Ka=9) but also 457 pure rotation lines within the first excited vibrational state 31 up to Ka=7. The ground state lines, combined with 32 microwave transitions available in the literature, were used for a new determination of the rotational constants up to higher orders for both isotopomers HO79Br and HO81Br, by least squares fitting of the observed line positions using a Watson-type Hamiltonian for the calculation of rotational energy levels. In the same way the 31 rotational lines were fitted together with the few existing microwave transitions and the energy levels derived from the study of the ν3 band (J. Orphal, Q. Kou, F. Kwabia Tchana, O. Pirali, and J.-M. Flaud, J. Mol. Spectrosc. 221 (2003), 239-243) leading to an improved set of Hamiltonian constants. Finally relative line intensities were measured and used for the determination of rotational corrections to the b-component of the permanent dipole moment.