Large amplitude local-bending eigenstates in the SEP spectrum of acetylene

The bending dynamics of acetylene undergo a normal-to-local transition in the vicinity of twelve quanta of excitation. We report the observation, by stimulated emission pumping, of the first few eigenstates exhibiting local-mode behavior. Assignments are made on the basis of complementary information in spectra recorded from intermediate levels with {\it gerade} and {\it ungerade} vibrational symmetry. The highest energy state currently assigned ($N_{\mathrm{bend}}=16$) is characterized by a large ($> 75^{\circ}$) distortion from the linear geometry at its classical turning point. Large geometrical distortions will lead to unique electronic, collisional, and chemical properties for these eigenstates.