Knowledge Bank

The laser-induced fluorescence excitation spectrum of the biacetyl $A1Au(S1)-X1Ag(S0)$ transition in the region from 22 182 to $28000cm-1$ 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 talk we discuss three topics: (i) a numerical calculation of these energy levels using a kinetic and potential energy formalism and preliminary constants from the $literature,a$ (ii) a qualitative description of the calculated energy level pattern using local mode ideas applied to the two equivalent methyl rotors as well as $G36$ permutation-inversion group symmetry species, and (iii) a comparison of the calculated energy levels with the low-resolution spectrum taken in the region from 0 to $500cm-1$above the A-X band origin at IAMS in Taipei. The levels with one quantum of torsional excitation are best described by a normal mode formulation, but levels with more than one quantum of torsional excitation are better described by a local mode formulation. Good agreement with the low resolution spectrum is obtained, but full confirmation of the interpretation will require high resolution studies of some additional bands.