The geometry of ethylene in the ground ionic state

Abstract Franck—Condon analyses were carried out on the first photoelectron band of ethylene to deduce the CC bond length, the CH bond length and the HCH bond angle in the ground ionic state 1 B 3u |to be 1.42 ± 0.01 A, 1.09 ± 0.01 A and 120 ± 1° respectively. These structructural parameters have values close to those previously obtained in the 1 B 3u Rydberg state (see ref. 10). With regard to the vibrational energies of the torsional mode of ethylene and its deuterated species in the 2 B 3u and 1 B 3u Rydberg states, they were analysed by using a one-deminsional potential function constructed from the harmonic oscillator term perturbed by a Gaussian barrier. The equilibrium torsional angles of ethylene thus obtained are found to be 28° and 25°, respectively, in the 2 B 3u and 1 B 3u states. In addition, the observed vibrational intensity distributions of the torsional mode accompanying electronic transitions from the ground state to the two 2 B 3u and 1 B 3u states are well reproduced by the proposed potential. Moreover, it is found that these vibrational transition probabilities are sensitive to the magnitude of the barrier heights, the shape of the potentials and the frequency changes upon electronic transitions and thus should be useful in providing structural information about the molecules under study.