15min:

NOBUKIMI OHASHI, Department of Physics, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan; JON T. HOUGEN, Optical Technology Division, NIST, Gaithersburg, MD 20899-8441, USA; CHENG-LIANG HUANG, Department of Applied Chemistry, National Chiayi University, Chiayi, Taiwan; CHEN-LIN LIU, CHI-KUNG NI, Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, P.O. Box 23-166, Taiwan.

The A ¹\!A_u(S₁)-X ¹\!A_g(S₀) LIF spectrum of biacetyl (CH₃C(=O)C(=O)CH₃) shows a long progression in the torsional vibrations of the two methyl tops. The S₁ torsional level pattern can be qualitatively understood using local mode ideas applied to the two equivalent methyl rotors and symmetry ideas from the PI group G₃₆. For the present rotational analysis, we assigned a G₃₆ symmetry species, two local-mode torsional quantum numbers, and rotational quantum numbers J_Ka,Kc to each observed torsion-rotation level. Transitions were globally fit with a two-equivalent-top computer program, which uses the molecular principal axis system, a free-rotor basis set for each top, a symmetric-top basis set for the rotational functions, and a single-step diagonalization procedure. The program is similar to our two-inequivalent-top program for N-methylacetamide. Last year, we refit 179 previously published rotational lines involving 7 torsional tunneling sublevels with zero or one quantum of torsional excitation in the S₁ state of biacetyl, using 14 parameters (5 for S₀, 9 for S₁) to obtain a standard deviation of 0.0041 cm^-1. We have now extended that fit by adding 131 unpublished lines involving 5 tunneling sublevels with two quanta of torsional excitation, using 19 parameters to obtain a standard deviation of 0.0040 cm^-1. Including transitions with still higher torsional excitation leads to a degraded standard deviation. We thus suspect that a number of higher-order terms describing top-top-overall-rotation interactions are missing from the present model, but the possibility of misassignments in the higher-energy regions must also be considered.