15min:

MATTHEW P. JACOBSON, JONATHAN P. O'BRIEN, ROBERT J. SILBEY AND ROBERT W. FIELD, Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139.

We have investigated the large-amplitude bending dynamics of acetylene, in its ground electronic state, using an effective Hamiltonian model that reproduces all relevant experimental data (84 vibrational levels), up to 15,000 cm^-1 in internal energy, with 1.4 cm^-1 accuracy (1). This investigation has been made possible by a numerical pattern recognition analysis of our dispersed fluorescence (DF) data set for the acetylene A¹ A_u -> X¹ _g⁺ system, which includes DF spectra recorded from five different vibrational levels of the A¹ A_u state. Through this pattern recognition analysis, polyad quantum numbers have been assigned to observed transitions in the DF spectra up to E_vib = 15,000 cm^-1. Our analysis of the ``pure bending'' polyads, which involve excitation exclusively in the trans\/ and cis\/ bending modes, has revealed a rich, but in many ways, suprisingly simple, dynamics at high internal energy (> 10,000 cm<sup>-1</sup>). Among the conclusions of this analysis is that, in many ways, the observed bending dynamics is somewhat simpler at 15,000 cm<sup>-1</sup> than it is at 10,000 cm<sup>-1</sup>; this rather surprising result can be explained in terms of qualitative changes in the structures of the pure bending polyads as a function of increasing internal energy. In addition, the eigenfunctions of the effective Hamiltonian at high internal energy are classifiable in terms of ``local bending'' and ``librational'' motions; this observation implies that a localized model of the bending dynamics is more appropriate at high internal energy than a normal model model, and we have derived analytical expressions for converting between these two representations of the dynamics.