THE GEOMETRY OF ETHYLEHE IN THE $R_{1S}$ STATE

There has been considerable work on the first Rydberg state ($R_{1s}$) of ethylene, which is elegantly summarized in a review by Merer and $Mulliken,^{1}$ According to a careful analysis of the intensities of vibronic transitions involving the torsional mode $\\nu_{4}$, Merer and $Schoonveld^{2}$ have derived an upper state potential which indicates a nearly planar structure for ethylene in the $R_{1s}$ state. Foo and $Innes^{3}$ have essentially confirmed the results of the vibronic analysis by a study of the rotational envelopes of some of the $R_{1s}$ vibronic transitions for the series of deuterated ethylenes. Thus, the state of affairs seemed settled. However, Watson and $Nycum^{4}$ recently proposed a different explanation. In order to retain the same isotopic ratios ($C_{2}H_{4}:C_{2}D_{4}$) for the ground and Rydberg states, they suggested that the first of the two prominent peaks (57 336 $cm^{-1}$ in $C_{2}H_{4}$) is only vibronically allowed by coupling with $\\nu_{8}$ (Herzberg’s notation), and that the second peak (57 808 $cm^{-1}$ in $C_{2}H_{4}$) is built on a very weak origin (at 56 900 $cm^{-1}$ in $C_{2}H_{4}$) and becomes allowed by coupling with $\\nu_{4} + \\nu_{10}$. Their argument is based on the non-constancy of $\\Delta/{\\nu^{\\prime\\prime}_{4}}$ and the constancy of $\\Delta/(\\nu^{\\prime\\prime}_{4} + \\nu^{\\prime\\prime}_{10} - \\nu^{\\prime\\prime}_{8})$, where $\\Delta$ is the energy spacing between the two prominent peaks. This suggestion prompted us to reinvestigate the VUV spectrum of $1,1-C_{2}H_{2}D_{2}$. In addition, we have extended the theoretical treatment of $C_{2}H_{4}$ and $C_{2}D_{4}$ by Merer and Schoonveld to $1,1-C_{2}H_{2}D_{2}$ and approximately so to cis- and trans-l,2-$C_{2}H_{2}D_{2}, C_{2}H_{3}D$ and $C_{2}HD_{3}$. Briefly, the results indicate that Merer and Schoonveld’s potential provides an excellent description of the $R_{1s}$ torsional potential in the mono-, di-, and tri-deuterated ethylenes.