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Among various kinds of bond-breaking isomerization reactions, the isomerization of a triatomic monohydride, HAB, is the simplest one: $HAB⟷ABH$. 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 $X~1\Sigma +$ were investigated by dispersed fluorescence and stimulated emission pumping spectroscopies. As a result, two distinct families of vibrational states were observed in the $13400-17500cm-1$ energy $region.h$ One of them is well characterized by poly ads involving the bending $(\nu 2)$ and CP stretching $(\nu 3)$ 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 $13400 cm^{-1}$. The rotational constants of these levels are much larger than those of the normal-mode-type” states. Moreover, these level exhibit strong anhamonicity. We have assigned this family of vibrational levels as deocalized isomerization” states whose existence whose 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.