Isotopomer-selective overtone spectroscopy of jet-cooled benzene by ionization detected IR plus UNT double resonance:: The N=2 CH chromophore absorption of 12C6H6 and 13C12 C5H6 near 6000 cm-1

Employing our recently introduced IR + UV double resonance scheme for obtaining mass-resolved infrared spectra, we have recorded the isotopomer selected N = 2 CH chromophore absorption of (C6H6)-C-12 and (CC5H6)-C-13-C-12 near 6000 cm(-1) in a supersonic jet expansion of the benzene isotopomer mixture at natural abundance. The (CC5H6)-C-13-C-12 spectra are the first of this kind reported in the literature. For (CC5H6)-C-13-C-12, a preliminary analysis yields an approximate decay time of vibrational excitation tau greater than or equal to 130 fs, which is presumably due to strong anharmonic Fermi resonances between CH-stretching and CH-bending modes. The (C6H6)-C-12 spectrum is compatible with a proposed model of intramolecular vibrational redistribution with a distinct hierarchy of time scales: the CH-stretching state is the IR chromophore state coupled to the IR field. With a decay time of tau approximate to 100 fs, vibrational excitation is redistributed to a first tier of vibrational states, probably CH-stretching/bending combination bands coupled by strong Fermi resonances. Vibrational excitation is then further redistributed with tau approximate to 0.35 ps to a second tier of states by weaker anharmonic resonances. The observed line widths give a lower bound for the decay time into the dense background manifold, tau > 1.3 ps. Although the experimental jet spectra of (C6H6)-C-12 are in qualitative agreement with previously published calculated spectra, they disagree in finer details.