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 ro vibrational 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 aro matic molecules. The experiment has been carried out in a supersonic molecu lar beam apparatus using bolometric detection. A tunable resonant cavity wa s used to enhance the on-beam intensity of the 1.5 mum color center laser u sed to pump the overtone, and a fixed frequency [R(30)] (CO2)-C-13 laser wa s used to saturate the coinciding nu (18) (r)Q(2) transition of benzene. Af ter 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. Aft er the fast (similar to 100 fs) redistribution of the energy among the prev iously 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 redistri bution (10-20 ps) takes place, which ultimately involves most of the symmet ry 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 noni ntuitive phenomenon may commonly occur in large molecules when the bright s tate energy is localized in a high-frequency mode. (C) 2000 American Instit ute of Physics. [S0021-9606(00)00345-7].