Spectroscopic investigation of the generation of “isomerization” states: Eigenvector analysis of the bend-CP stretch polyad

Abstract

The highly excited vibrational levels of HCP exhibit a regular energy level and intensity pattern characteristic of 2:1 bend-CP stretch polyads. Stimulated by the experimental observation of vibrational levels with rotational 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 amplitude motion along the minimum energy HCP↔CPH isomerization path [J. Chem. Phys. 107, 9818 (1997)]. In this paper, we show that the transition from “normal-mode-type” to “isomerization” vibrational states arises naturally out of a traditional spectroscopic (algebraic) effective Hamiltonian polyad model. A global least squares fit, based on this polyad Heff model, shows that all of the observed “isomerization” states belong to polyads and that the eigenvectors of this Heff model have the qualitatively distinct nodal structure first noticed by Schinke and co-workers. The “isomerization” states are not indicative of a breakdown of the polyad model; rather they are a natural consequence of this traditional spectroscopic model.