15min:

ANTHONY J. MERER, Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei, Taiwan 10617, Department of Chemistry, University of British Columbia, 6174 University Boulevard, Vancouver, B.C., Canada V6T 1Z3; NAMI YAMAKITA, Department of Chemical and Biological Sciences, Japan Women's University, Mejirodai, Bunkyo-ku, Tokyo 112-8681, Japan; ADAM H. STEEVES, HANS A. BECHTEL AND ROBERT. W. FIELD, Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139.

Acetylene is linear in its ground state, but becomes trans -bent in its A ¹A_u state. The vibrational structure of the A X system consists of long progressions in the ₃' ( trans -bending) vibration based on the origin and the ₂' (C-C stretch) fundamental. These progressions become irregular at a vibrational energy of about 5000 cm^-1, with many extra bands appearing. Spectra of jet-cooled acetylene showed recently that extra bands can also be found at much lower energy. Two of the extra levels were identified as the a_g combinations 1¹3¹ and 2² 3¹, but the other levels are combinations involving the two low frequency bending vibrations, ₄ (torsion) and ₆ (in-plane bending).

Two types of high resolution spectra have been recorded in order to assign these extra levels. Infrared-ultraviolet double resonance spectra via the X, ₃ level have identified large numbers of ungerade vibrational levels at energies of 2000-7000 cm^-1, while high sensitivity laser-induced fluorescence spectra of jet-cooled acetylene have given the positions of many low-lying gerade levels. The low-lying bending levels are affected by strong a -axis Coriolis coupling, which distorts their K -structures considerably. In addition, there is very strong Darling-Dennison resonance between ₄ and ₆, with k₄₄₆₆ ~ 50 cm^-1. Nevertheless a good fit to the structures of the pure bending polyads can be obtained, allowing for both the Darling-Dennison resonance and the Coriolis coupling.