15min:

C.-L. HUANG, Department of Applied Chemistry, National Chiayi University, Chiayi, Taiwan; C.-L. LIU, C.-K. NI, Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, P.O. Box 23-166, Taiwan; J. T. HOUGEN, Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8441.

The laser-induced fluorescence excitation spectrum of the biacetyl A ¹\!A_u(S₁)-X ¹\!A_g(S₀) transition in the region from 22 182 to 28 000 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 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, (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 500 cm^-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.