Mode selectivity in the classical power spectra of highly vibrationally excited molecules

Classical trajectory computations are used to generate power spectra for polyatomic molecules (C₂H₄, CH₃ONO, and SiF₄) at energies both below and above the dissociation threshold. Realistic force fields are employed for each of the three molecules that were studied. A variety of initial conditions were employed, and the trajectories were integrated for 10 ps. The two primary observations are that, on that time scale, spectral mode identity is essentially retained even well above the threshold for dissociation and that mode mixing (or "intramolecular vibrational energy redistribution") is not global. Molecular rotation increases the degree of mode mixing. For ensembles of localized initial conditions the trajectories fail to representatively sample the available phase space, even at energies where significant numbers of trajectories dissociate within the 10-ps time interval.