Electronic spectra of molecules with two C3v internal rotors: Torsional analysis of the A˜1Au-X˜1Ag LIF spectrum of biacetyl

The laser-induced fluorescence excitation spectrum of the biacetyl A1Au (S1)-X1Ag (S0) transition in the region from 22 182 to 22 800 cm−1 shows a complicated absorption line spectrum, which is believed to arise from a long progression in the torsional vibrations of the two equivalent methyl tops in this molecule. In this paper, we discuss three topics: (i) a numerical calculation of these energy levels using a kinetic and potential energy formalism and constants from the literature [M.L. Senent, D.C. Moule, Y.G. Smeyers, A. Toro-Labbé, F.J. Peqalver, J. Mol. Spectrosc. 164 (1994) 66], (ii) a qualitative description of the calculated energy level pattern using local-mode ideas applied to the two equivalent methyl rotors together with G36 permutation-inversion group symmetry species, and (iii) a least-squares refinement of the torsional potential parameters based on results of some of our high-resolution rotational analyses, followed by a comparison of energy levels calculated from the refined parameters with a low-resolution spectrum taken in the region from 0 to 500 cm−1 above the A-X band origin. Concerning (ii), we find that the two vibrational levels with one quantum of torsional excitation are best described by a normal mode formulation, but that many levels with more than one quantum of torsional excitation are better described by a local-mode formulation. Concerning (iii), we obtain low-order molecular constants quite similar to those reported by Senent et al., but higher-order constants which are quite different. Our calculated spectrum agrees well with the low-resolution spectrum, but full confirmation of the present interpretation for levels with three or more quanta of torsion excited will require high-resolution studies of additional bands.