Stimulated emission pumping spectroscopy of HCP near the isomerization barrier: EVIB≤25 315 cm−1

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Abstract
Rotation‐vibration levels in the energy region near the HCP↔HPC X̃ 1Σ+ isomerization barrier (i.e., the HPC geometry) were observed by HCP ÖX̃ and C̃–X̃ 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 ÖX̃ SEP spectra, the l=0 and 2 components of pure bend overtone levels (0,26≤v2≤42,0) and the bend–CP stretch combination levels, (0,24,1) and (0,26,1), were sampled. On the other hand, in the C̃–X̃ SEP spectra, which sample l=0 components exclusively, 2ω2:1ω3 polyads were identified. These polyads appeared in the SEP spectra as a regular, easily recognizable pattern. Since the C̃–X̃ 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 v2=42. However, an abrupt change was found in the v2‐dependence of all of the vibrational fine structure constants above v2=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 v2=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.