INTRAMOLECULAR VIBRATIONAL REDISTRIBUTION OF ENERGY IN THE STIMULATED-EMISSION PUMPING SPECTRUM OF ACETYLENE

Using a combination of low resolution dispersed A --> X fluorescence spectra and high resolution stimulated emission pumping, we have spectroscopically identified the first stages of vibrational energy flow in the highly vibrationally excited acetylene prepared by A --> X emission-over the energy range 5 000-18 000 cm-1. A detailed study of the stimulated emission pumping (SEP) spectrum of acetylene in the E(VIB)=7000 cm-1 region, in which we report spectroscopic constants and rovibrational term values for 12 vibrational levels, has conclusively shown that Darling-Dennison resonance between the cis and trans degenerate bending vibrations is the first step in the redistribution of vibrational energy from the initially excited Franck-Condon bright CC stretch and trans-bend vibrational combination levels. This allows an extension of our prior dispersed fluorescence (DF) assignments which suggested the crucial role of Darling-Dennison coupling between the cis and trans bends in IVR [J. Chem. Phys. 95, 6336 (1991)]. We prove that the symmetric CH stretch vibration, previously thought to play a crucial role in the redistribution of vibrational energy, is Franck-Condon inactive. We have also shown that vibrational-l-resonance among the states with excitation of both degenerate bending modes, when combined with a Fermi resonance which couples CC stretch/trans/cis-bend excited states to the antisymmetric CH stretch, determines the subsequent flow of vibrational energy after the Darling-Dennison bending resonance. These resonances all scale with vibrational excitation in nearly the simple manner expected for the lowest order anharmonic terms in the Hamiltonian, which allows the prediction of the fastest processes at high energy from a detailed study of the high resolution spectrum at lower energy. We find some interesting rules for vibrational energy flow in the short time dynamics: (i) CC stretch excitation is necessary for stretch-bend coupling; (ii) if V2'' and V4'' are the quantum numbers of the initially excited bright state, and v(b)'' = v4'' + v5'' is the total bending quantum number of a state coupled to that bright state, then V4'' greater-than-or-equal-to v(b)'' greater-than-or-equal-to (V4''-2V2'') (iii) the total stretch quantum number n(s)'' = (v1'' +v2'' +v3'') is also conserved by the short time dynamics. These are severe and well characterized restrictions on the range of quantum numbers accessible to the initial bright state during the first stages of intramolecular vibrational redistribution of energy.