STATISTICAL SPECTROSCOPY OF VIBRATIONALLY EXCITED ACETYLENE

Stimulated Emission Pumping spectra of HCCH above $14,000 cm^{-1}$ indicate that the rotation-vibration separation has partially broken down. In addition, the large amplitude isomerization of acetylene to vinylidene has been demonstrated to occur near $15,600 cm^{-1}$. Throughout this region, the density of states observed by SEP exceeds the total calculated by an anharmonic direct count by up to a factor of two. To test for the quantum analog of classical chaos, SEP spectra obtained by PUMPing two different vibrational levels of the $\\widetilde{A}$ state were pooled and sorted into complete, pure sequences of all levels having the same values of the rigorously good quantum numbers J and parity. Complete, pure sequences of energy levels exhibit a level repulsion and spectral rigidity in the semiclassical limit for certain extreme forms of classical chaos. Analysis of the level spacing statistics for these pure sequences indicates that the spectrum of acetylene at $15,000 cm^{-1}$ is not as rigid as predicted for strongly chaotic dynamics.