30min:

H. ISHIKAWA, C. NAGAO AND N. MIKAMI, Department of Chemistry, Graduate School of Science, Tohoku University, Sendai, JAPAN 980-77 ; R. W. FIELD, Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139.

Among various kinds of bond-breaking isomerization reactions, the isomerization of a triatomic monohydride, HAB, is the simplest one: HAB \longleftrightarrow ABH. This system is most fundamental for both experimental and theoretical studies. On the basis of a time- independent picture, the isomerization process can be characterized by analysis of rotation-vibration eigenstates in the high energy region. In this study, highly excited vibrational states of HCP \widetilde X~^{1 +} were investigated by dispersed fluorescence and stimulated emission pumping spectroscopies. As a result, two distinct families of vibrational states were observed in the 13400 - 17500 cm^-1 energy region. One of them is well characterized by polyads involving the bending (₂) and CP stretching (₃) vibrations. This 1:2 polyad structure is very robust and valid in the lower energy region. This family can be referred to as ``normal-mode-type'' states. On the other hand, vibrational levels of the other family appear suddenly at 13400 cm<sup>-1</sup>. The rotational constants of these levels are much larger than those of the ``normal-mode-type'' states. Moreover, these levels exhibit strong anharmonicity. We have assigned this family of vibrational levels as delocalized ``isomerization'' states whose existence was predicted by a recent theoretical study. This assignment is based on the good agreement between the present spectroscopic observations and the theoretical predictions. Thus, ``isomerization'' states have been identified experimentally for the first time.