High-Resolution UV Laser Spectroscopy of Jet-Cooled Benzene Molecules: Complete Rotational Analysis of the S1 <-- S0 6(1)0 (l = +/-1) Band.

The UV spectrum of the 6(1)0 S1 <-- S0 vibronic transition of benzene, located at 259 nm, was recorded by means of a combination of a cw laser doubling unit and a molecular beam apparatus. A typical linewidth of 27 MHz was observed, limited only by residual Doppler broadening in the supersonic beam. Together with an estimated rotational temperature of 25 K, the rotational band structure could be completely resolved, and nearly 900 transitions were assigned in the spectrum. A set of 13 molecular parameters of the vibronic transition was simultaneously adjusted to the measured data by means of a weighted nonlinear least-squares fit procedure. The weights themselves were determined by a careful analysis of the uncertainties in the measured line frequencies. The overall rms deviation of the fit was 3.1 MHz or about 10% of the observed linewidth. Monte Carlo simulations were applied to the best-fit parameter set in order to find error estimates for the single constants as realistic as possible. The interdependencies of the parameters were analyzed by means of correlation coefficients obtained from the fit output. These statistical findings were incorporated into comparisons with results from other works. As far as ground state-related data are concerned general agreement with IR-band investigations was found. In the electronically excited state, however, data are scarce and both centrifugal distortion and higher order Coriolis coupling parameters were determined for the first time. Copyright 1999 Academic Press.