Knowledge Bank

We have investigated and 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 $15,000cm-1$ in internal energy, with $1.4cm-1$ accuracy $(1\sigma )$. This investigation has been made possible by a numerical pattern recognition analysis of our dispersed fluorescence (DF) data set for the acetylene $A~1AuX~1\Sigma g+$ system, which includes DF spectra recorded from five different vibrational levels of the $A~1Au$ state. Through this pattern recognition analysis, polyad quantum numbers have been assigned to observed transitions in the DF spectra up to $Evib=15,000cm-1$. 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, surprisingly simple, dynamics at high internal energy $(> 10,000 cm^{-1})$. Among the conclusions of this analysis is that, in many ways, the observed bending dynamics is somewhat simpler at $15,000 cm^{-1}$ than it is at $10,000 cm^{-1}$; 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.