Molecular rydberg transitions. VIII. The geometry of ethylene in the R 1 s state

The vacuum ultraviolet absorption spectrum of 1,1‐C 2 H 2 D 2 has been reexamined. Using the symmetric top approximation and the Merer‐Schoonveld (A. J. Merer and L. Schoonveld, Can. J. Phys. 47 , 1731 (1969)) potential for the lowest‐energy s Rydberg ( R 1 s ) state of ethylene, the energies and relative intensities of the first few torsional vibrations ( v 4 ′) in the R 1 s state of 1,1‐C 2 H 2 D 2 have been computed. The agreement with experiment is excellent. This result negates the recent proposal (F. H. Watson, Jr. and M. N. Nycum, Spectrosc. Lett. 8 , 223 (1975)) that the N → R 1 s transition obtains allowedness only through vibronic coupling. The energies and relative intensities of v 4 ′ have also been computed for CH 3 D, 1,2‐C 2 H 2 D 2 ( cis and trans ) and C 2 HD 3 . The Merer‐Schoonveld potential is in excellent agreement with experiment in all instances. Some discussion of the photoelectron spectra is also provided.