Nonlinear phase excitations in the PPP model of polyacetylene

Abstract

The Pariser-Pam-Pople (PPP) method has revealed rich effects of the .n electron Coulomb interaction in (CH),. We discuss results obtained by the PPP model with the unrestricted Hartree-Fock (UHF) approximation and we show that concepts introduced by the PPP-UHF model give a new insight on Franck-Condontype electronically excited states in (CH),. In the first part of this paper, a brief review of the PPP-UHF model is given. We introduce phase and amplitude variables and show that solitons in the PPP-UHF model are phase excitations. In the second part of this paper, the effective Hamiltonian describing phase excitations is derived. We show that the phase Hamiltonian derived by the time-dependent Hartree-Fock approximation is the classical limit of the Hamiltonian derived by the bosonization method of Tomonaga, Luther, and others. The phase Hamiltonian has a family of new soliton solutions. These solitons are pure electronic excitations and correspond to the exciton and the magnon in (CH),.