15min:

JAMES K. G. WATSON, Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R6.

The A- X transition of acetylene was the first electronic transition for which a change of point group was confirmed in detail (1,2). This paper presents harmonic calculations of intensities of the vibrational structure of this transition, using the force field of Tobiason \sl et al.~(3) for the A state. The A- X transition correlates with the forbidden ¹_u^--¹_g⁺ at linear geometries, and the transition moment is assumed to be proportional to the q₄'' bending coordinate. There are difficulties in the calculation because the usual harmonic wavefunctions of the upper state do not behave correctly at linear geometries. To correct this, a different calculation would have to be done for each K_a-value. Approximate results for the intensity integrals are presented. These agree well with measured values for the v₃'-0 bands in the absorption spectrum with v₃' up to 5 (1), but the agreement is only qualitative for recently measured emission bands (4) which peak near v₄''~ 14, and must be seriously affected by anharmonicity.

\beginlist em1. C.~K.~Ingold and G.~W.~King, J.~Chem.~Soc., 2702--2755 (1953). em2. K.~K.~Innes, J.~Chem.~Phys.~ 22, 863--876 (1954). em3. J.~D.~Tobiason, A.~L.~Utz, E.~L.~Sibert III, and F.~F.~Crim, J.~Chem.~Phys.~ 99, 5762--5767 (1993). em4. M.~P.~Jacobson, R.~W.~Field, \sl et al., personal communication. \endlist