ROTATIONALLY RESOLVED ONE- AND TWO- PHOTON SPECTROSCOPY OF BENZENE: INVESTIGATION OF INTRAMOLECULAR COUPLINGS

/1/ E. Riedle, H.J. Neusser, E. W. Schlag, J. Chem. Phys. 75, 4231 (1981) /2/ U. Schubert, E. Riedle, H.J. Neusser, J.Chem.Phys. 84, 5326 (1986) /3/ E. Riedle, H. J. Neusser, J.Chem. Phys. 80 4686 (1984) /4/ E. Riedle, H. J. Neusser, E. W. Schlag, J. Phys. Chem. 86, 4847 (1982).Author Institution: Institut f\""{u}r Physikalische und Theoretische Chemie, TU Munchen, Lichtenbergstra{\ss}e 4In the last few years we have been able to show, that rotationally resolved electronic spectra of the prototype organic molecule benzene can be obtained by Doppler-free two-photon $spectroscopy /1/. The spectra were taken at room temperature and even lines with $J = 100$ have been identified in the meantime. As a result, the rotational constants could be determined with extreme precision and various rotational perturbations were identified. We were able to measure the decay time /2/. $linewidth /3/ and emission spectrum of single ``quasi-eigenstates''. In particular, a strong rotational dependence of the decay behaviour in the regime of high vibrational excess energy (``Channel Three'') was found /4/. The results of the two-photon experiments could, however, not be directly compared to most previous low resolution investigations on benzene, as these were performed by one-photon excitation and therefore led to different vibronic states in $S_{1}$. To obtain sub-Doppler rotationally resolved one-photon spectra, we combined our extremely narrow band pulsed laser $(\Delta v = 70 MHz$ after frequency doupling) with a collimated molecular beam. This allowed us to measure the resolved spectra of all four $e_{2g}$ fundamentals, some of their progressions and particularly some bands in the ``Channel Three'' regime. The analysis of the fundamentals bands renders for the first time a complete set of Coriolis coupling constants. The couplings found at intermediate excess energy can be compared to the ones found in the two-photon spectrum in the same energy range. The strong rotational dependence of the decay at high excess energy, as seen by the appearance of very few sharp lines in the two-photon spectrum /4/. is also seen in the one-photon spectrum of vibrations at the same excess energy. Hence it is concluded, that the relaxation behaviour of $S1$ benzene found from two-photon spectroscopy is not a sole consequence of the symmetry of the observed states, but rather depends mainly on the excess energy