NEW EXPERIMENTAL TESTS OF EXISTING INTERPRETATIONS OF ELECTRONIC TRANSITIONS OF ETHYLENE

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1974

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Ohio State University

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The absorption spectra of the gases ethylene-$d_{0}$, $d_{1}$, 1, 1-$d_{2}$, cis-$d_{2}$, trans-$d_{2}$, $d_{3}$ and $d_{4}$ have been photographed, using the first order of a 21-foot vacuum spectrograph. Analyses of both the Rydberg system at 1750 {\AA} and the $\pi^{*} - \pi$ system between 1500 and 2100 {\AA} have been extended. Band contour analysis has confirmed the assignment of the Rydberg system as $^{1}B_{3u} - ^{1}A_{g}$. The resulting rotational constants of the excited state lead to an inertial defect indicative of a nonplanar molecule. Assuming point group $D_{2}$, we have combined the constants with Franck-Condon calculations in a complete structure determination of the Rydberg state: $r_{CC}= 1.41$ {\AA}, $r_{CH} = 1.08$ {\AA}, $)\kern -9 pt<$$HCH = 124.4^{\circ}$ and an azimuthal angle of $37^{\circ}$. Line widths in the successful simulation of the $C_{2}D_{4}$ 0 - 0 band indicate a lifetime greater than $10^{-13}$ sec, for molecules in the zero-level of the Rydberg state. For the $\pi^{*} - \pi$ system, all observed bands for all molecules studied have been assigned. Most striking is the relative inactivity of the C - C stretching vibration; except in $C_{2}D_{4}$, only the torsional mode is active. It is concluded that $r_{cc}$ for the $\pi^{*} - \pi$ state is not very different than that for the Rydberg state. Finally, the extrapolation to the (unobserved) origin of the $\pi^{*} - \pi$ system is revised to give 5.5 ev (2285 {\AA}).

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This work was supported by the National Science Foundation.
Author Institution: Department of Chemistry, State University of New York

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