Vibronic coupling effects in the photoelectron spectrum of ethylene

The vibrational structure of the first band in the photoelectron spectrum of ethylene is calculated taking into account the vibronic coupling between the ground state and first excited state of the ion. The vibronic Hamiltonian describes linear coupling to the totally symmetric vibrational modes ν₁-ν₃ as well as to the non-totally symmetric torsional mode ν₄. The energies and coupling constants entering the calculation are computed by ab initio Hartree-Fock and many-body methods. Qualitative agreement between the theoretical and the experimental spectrum is found. By slightly readjusting some of the parameters, the experimental spectrum can be reproduced accurately. It turns out that nonadiabatic and intensity borrowing effects are small. The vibronic coupling results mainly in a pronounced anharmonicity of the adiabatic potential energy surface. In particular, a nonplanar equilibrium geometry is found for the ionic ground state, the equilibrium torsional angle being ∼25°. Although the corrections to the Franck-Condon principle are small, the calculation of the vibrational structure is greatly complicated by the nonseparability of the totally symmetric and the non-totally symmetric vibrations. A decoupling procedure is presented which approximately makes possible the separate treatment of the modes. The results obtained with this procedure are in good agreement with the full vibronic treatment for ethylene.