SPECTROSCOPIC INVESTIGATION AF THE GENERATION OF ISOMERIZATION STATES- EIGENVECTOR ANALYSIS OF THE BEND-CP STRETCH POLYAD

The highly excited vibrational levels of HCP exhibit a regular energy level and intensity pattern characteristic of 2:1 bend-CP stretch poly ads. Stimulated by the experimental observation of vibrational levels with relational constants (B-values) 5%-10% larger than other observed levels, Schinke and co-workers noticed that these large-B levels were characterized by atypical nodal structures indicative of large amplit ude motion along the minimum energy HCP<->CPH isomerization path [J. C hem. Phys. 107, 9818 (1997)1. In this paper, we show that the transiti on from ''normal-mode-type'' to ''isomerization'' vibrational states a rises naturally out of a traditional spectroscopic (algebraic) effecti ve Hamiltonian polyad model. A global least squares fit, based on this polyad H-eff model, shows that all of the observed ''isomerization'' states belong to polyads and that the eigenvectors of this H-eff model have the qualitatively distinct nodal structure first noticed by Schi nke and co-workers. The ''isomerization'' stales are not indicative of a breakdown of the polyad model; rather they are a natural consequenc e of this traditional spectroscopic model. (C) 1998 American Institute of Physics.