Stimulated emission pumping spectroscopy of HCP near the isomerization barrier: E(VIB)<=25,315 cm(-1)

Rotation-vibration levels in the energy region near the HCP<->HPC (X) over tilde(1) Sigma(+) isomerization barrier (i.e., the HPC geometry) were observed by HCP (A) over tilde-(X) over tilde and (C) over tilde-(X) over tilde stimulated emission pumping (SEP) spectroscopy. Unlike HCN<->HNC, where the unstable isomer corresponds to a local minimum on the potential energy surface, the HPC isomer corresponds to a saddle point. In the (A) over tilde-(X) over tilde SEP spectra, the l=0 and 2 components of purl bend overtone levels (0,26 less than or equal to upsilon(2) less than or equal to 42,0) and the bend-CP stretch combination levels, (0,24,1) and (0,26,1), were sampled. On the other hand, in the (C) over tilde-(X) over tilde SEP spectra, which sample l=0 components exclusively, 2 omega(2):1 omega(3) polyads were identified. These polyads appeared in the SEP spectra as a regular, easily recognizable pattern. Since the (C) over tilde-(X) over tilde SEP spectra appeared to be almost totally Franck-Condon nonselective (a large fraction of the predicted total density of l=0 levels was observed), the polyad pattern was the key to vibrational assignments of highly excited vibrational levels. It was found that the bending vibration exhibits very regular (Morse-type) behavior up to at least upsilon(2)=42. However, an abrupt change was found in the upsilon(2)-dependence of all of the vibrational fine structure constants above upsilon(2) = 36, E((0,36,0)) = 22 048 cm(-1). In addition, a sudden turning on of perturbations in the pure bend overtone levels was also observed to occur at upsilon(2)=32. These abrupt changes in the level structure could be related to an abrupt change in curvature of the potential energy surface along directions perpendicular to the bending coordinate when the bending coordinate is far from equilibrium. (C) 1996 American Institute of Physics.