U.V. OPTICAL DOUBLE RESONANCE STUDY OF THE PREDISSOCIATED $\tilde{C}{'} ^{1}A_{g} \leftarrow \bar{A}^{1}A_{u} \leftarrow \tilde{X} ^{1}\Sigma_{g}^{+}$ STATE OF ACETYLENE

A predissociated vibronic progression in the trans-bending ($v_{3}$) mode of HCCH with vibrational origin at $61,340 cm^{-1}$ has been observed using the $\\tilde{C} ^{1}A_{g} \\leftarrow \\bar{A} ^{1}A_{u}\\leftarrow \\tilde{X}^{1}\\Sigma_{g}^{+}$ excitation scheme. We have determined the $\\tilde{C}$ state to be a strongly bent valence state with electron configuration $(la_{g})^{2} (lb_{u})^{2} (2a_{g})^{2} (2b_{u})^{2} (3a_{g})^{2} (3b_{u})^{2} (4a_{g})^{2}$, and electronic symmetry $^{1}A_{g}$. Analysis of the J and K resolved spectra have allowed us to determine the $B \\equiv C$ and A rotational constants of this state to be $0.85(6) cm^{-1}$ and $7.5(6) cm^{-1}$ respectively. We are currently recording the spectra of DCCD to obtain bond lengths and angles for this state. The results of these experiments will be presented and the vibrational and J. K-dependence of the predissociation rate will be discussed.