Abstract
Recent advances in the theory of the giant dipole resonance in nuclei suggest that systems of a small number of fermions interacting through a repulsive residual force should have collective states. The theory is adapted to small molecules, and applied to ethylene (C2H4) with the Coulomb potential replaced by a dipole-dipole interaction. This approximation should reproduce effects due to correlations arising from the long range of the Coulomb potential. The excitation energies of `single-particle-single-hole' triplet states are taken either from experiment or from rough non-empirical values available in the literature. The calculation yields a collective singlet state at about 50 ev, in a region which has not yet been explored experimentally. It suggests also that long range correlations lead to reductions by about a factor 2 in the spacing of the lowest singlet and triplet excited states (as compared with the value for a pure π -> π* excitation) and for the dipole transition moment from the lowest excited singlet to the ground state. These results arise from co-operative interactions of several electrons. The reduction of the strength of the π-π interaction can be expressed as a dielectric effect due to the other electrons.