Intramolecular vibrational redistribution in aromatic molecules. I. Eigenstate resolved CH stretch first overtone spectra of benzene

We have used infrared-infrared double resonance spectroscopy to record a rovibrational eigenstate resolved spectrum of benzene in the region of the CH stretch first overtone. This experiment is the first of a series aimed at investigating intramolecular vibrational energy redistribution (IVR) in aromatic molecules. The experiment has been carried out in a supersonic molecular beam apparatus using bolometric detection. A tunable resonant cavity was used to enhance the on-beam intensity of the 1.5 mum color center laser used to pump the overtone, and a fixed frequency [R(30)] (CO2)-C-13 laser was used to saturate the coinciding nu (18) (r)Q(2) transition of benzene. After assigning the measured lines of the highly IVR fractionated spectrum to their respective rotational quantum number J, analysis of the data reveals that the dynamics occurs on several distinct time scales and is dominated by anharmonic coupling with little contribution from Coriolis coupling. After the fast (similar to 100 fs) redistribution of the energy among the previously observed "early time resonances" [R. H. Page, Y. R. Shen, and Y. T. Lee, J. Chem. Phys. 88, 4621 (1988) and 88, 5362 (1988)], a slower redistribution (10-20 ps) takes place, which ultimately involves most of the symmetry allowed vibrational states in the energy shell. Level spacing statistics reveal that IVR produces a highly mixed, but nonstatistical, distribution of vibrational excitation, even at infinite time. We propose that this nonintuitive phenomenon may commonly occur in large molecules when the bright state energy is localized in a high-frequency mode. (C) 2000 American Institute of Physics. [S0021-9606(00)00345-7].