The ?~1A_u state of acetylene: ungerade vibrational levels in the region 45,800–46,550?cm~(-1)

摘要

The ungerade vibrational levels of the ?~1A_u(S_1-trans) state of C_2H_2 lying in the region 45,800–46,550cm~(-1)have been assigned from IR–UV double resonance spectra. The aim has been to classify the complete manifold of S~1-trans levels in this region, so as to facilitate the assignment of the bands of S~1-cis C_2H_2. The rotational structure is complicated because of the overlapping of vibrational polyads with different Coriolis and Darling– Dennison parameters, but assignments have been possible with the help of predictions based on the properties of polyads at lower energy. An important result is that the analysis of the (1~14~1,1~16~1) polyad determines the anharmonicity constants x_(14)and x_(16), which will be needed to proceed to higher energies. Some regions of impressive complexity occur. Among these is the band given by the 3~36~1, K=1 state at 45,945cm~(-1), where a three-level interaction within the S_1 state is confused by triplet perturbations. Several probable S_1-cis states have beenobserved,includingcis-6~2,K=1;thisvibrationallevelappearstoshowaK-staggering,ofthetypethatarises when quantum mechanical tunnelling through the barrier to cis-trans isomerization is possible. The total number of identified cis vibrational states is now 6 out of an expected 10 up to the energies discussed in this paper.