Rotationally resolved spectrum of the A˜(060)-X˜(000) band of HCBr

The rotationally resolved spectrum of the (0 6 0)0-(0 0 0) band in the A˜1A″-X˜1A′ system of HCBr has been recorded in absorption using a frequency-modulated c.w. tunable dye laser. Both 79Br and 81Br isotopomer spectra were rotationally assigned and effective rotational constants determined. The spectrum contains both perpendicular and parallel rotational sub-band structures. The latter are partially due to axis-switching in the transition and Coriolis coupling in the excited state, but are relatively more intense and show a different J-dependence to that expected on the basis of these mechanisms. Intensity borrowing from a predicted but unobserved shorter wavelength B˜-X˜ transition explains the observations. We have calculated the energy and structure of HCBr in the B˜ state at the MRCI/cc-pVTZ level and find Te(B˜-X˜)=20908cm-1. The B˜-X˜ band system should be experimentally observable.