Quantum dynamics of overtone relaxation in 30-mode benzene: A time-dependent local mode analysis for CH(ν=2)

We report the results of a detailed quantum dynamical investigation of energy flow in 30-mode benzene. The CH(ν= 2) overtone was examined by propagating an initial bright state in an active space of dimension N = 16000 for 9.6 ps. Two sets of initial conditions were investigated: one in which the overtone energy was distributed as in one of the E1u symmetries appropriate for dipole excitation from the ground state and the other in which the overtone energy was localized in a single CH stretching motion. The rate and extent of energy flow out of the CH chromophore is dictated by the choice of initial conditions. Conclusions from the model where energy is localized in a single CH stretch indicate that the meta in-plane CH wags and para out-of-plane CH bend are the major recipients of bursts of energy for t [less-than or equal to] 300fs; the ipso CH stretch retains only 10% of the overtone energy by 9.6 ps. For the E1u model, the initially excited CH stretches retain 30% of the overtone energy in long time; the ipso out-of-plane CH bend is activated before the ipso in-plane CH wag and with less energy, mimicing the results of previous classical investigations. For both models examined, energy is equipartitioned within the CC stretches, in-plane CH wags, and out-of-plane CH bends by 9.6 ps; the magnitude of energy taken up by the CC stretches and in-plane CH wags is the same. CCC in-plane bends and CCCC out-of-plane ring torsions are of relatively minor importance (on the time scale considered) in intramolecular vibrational energy redistribution in benzene. © 1999 American Institute of Physics.