The importance of impurity states in doped transpolyacetylene

Abstract

The authors assess the importance of the impurity states of a doped trans-polyacetylene chain. The impurity potential is modelled by a point charge that is located off the chain and screened phenomenologically with the bulk dielectric constant. The common assumption that the impurity levels of a dimerised chain closely approximate the hydrogenic levels of a point charge impurity in invalid when the impurity is not on the chain. Additional nonhydrogenic states occur. One of the new states is much deeper in the gap. The formation energies for charged soliton and charged polaron lattice distortions are found by solving the Su-Schrieffer-Haeger model for polyacetylene with an impurity added. The impurity states severely modify the structure of the soliton distortion. The charged polaron distortion, not the charged soliton distortion, is the most stable distortion that can be formed during doping with gap states consistent with the observed mid-gap absorption.