Theoretical and computational studies of highly vibrationally excited acetylene

Abstract

A consistent interpretation of the intramolecular vibrational dynamics of highly excited acetylene on the ground potential energy surface is proposed. Classical trajectory computations in the time domain (and their Fourier transform) and quantum-algebraic computations of the spectrum provide a picture which is in accord with the results of an analysis of the experimental stimulated emission pumping. A separation of time scales suggested by the algebraic Hamiltonian is verified in the classical simulations. There are four distinct, nested relaxation stages following the initial state preparation, each corresponding to energy exchange amongst a bigger subset of states. Initially (<1 ps) only the two CH stretch motions are effectively coupled. It is suggested that the two CH quanta states (which are primarily local mode in character) are responsible for the broad features in the spectra at ≈26000 cm −1. Next the CH stretches and the (Franck-Condon) excited CC stretch share energy. This stage is yet to be identified in the spectrum. By ≈ 10 ps, the CH trans bend excitation is also fully coupled, resulting in the “clumps” in the experimental spectrum. Finally, by about 15 ps there is complete relaxation corresponding to a concerted orbiting motion of the H atoms about the CC bond with a wide frequency band centered at ≈6 cm −1.