Local electronic states in chains with two atoms in the unit cell

The method of local perturbations is applied to the local levels arising from distortion (substitution, deviation from coplanarity) in a polymer chain composed of two types of atoms. Allowance is made for possible bond alternation. The minimum perturbation needed to produce such a state is related to the number of the perturbing atom or bond. The condition for local states in the forbidden band is found to be substantially dependent on whether there is bond alternation, so the local levels provide a convenient means of studying the electronic structure of the upper unperturbed chain. Experiments are proposed to elucidate the relative contributions from bond alternation and difference in type of atom to the formation of a forbidden band. The results can be used to decide between different models for the electronic structure of a polyene.