Evidence that irreversible intramolecular vibrational energy redistribution is slow for ring modes up to 8200 cm−1 in S0 benzene

High-resolution dispersed fluorescence spectra have been used to probe intramolecular vibrational energy redistribution (IVR) for ring vibrations in S0 benzene up to ≈ 8200 cm−1. All bands monitored show sharp structure (≈ 1 cm−1 fwhm) with resolved rotational features. An upper limit to the contribution from irreversible IVR to these linewidths is 0.5 cm−1, corresponding to rate constants < 4.7 × 1010 s−1 (τIVR > 21 ps). The ring mode linewidths appear to be narrower than CH stretch overtone linewidths at comparable energies, indicating that irreversible IVR is slower for ring modes.