THE GROUND-STATE OF AN EXTRA ELECTRON INTERACTING WITH ACOUSTIC PHONONS IN A MOLECULAR CHAIN

The ground state of a quasi-particle (exciton, electron or hole) interacting with acoustic phonons in a one-dimensional chain, is investigated using the variational method. The diagram of states is obtained which shows the regions of the electron-phonon coupling constant and of the nonadiabaticity parameter where the ground state of a quasiparticle is described as an almost free electron state, a ''small polaron'', or a spontaneously localized state. It is shown that the formation of a soliton-like state has a threshold with respect to the value of the electron-phonon interaction, whose critical value increases with increasing nonadiabaticity parameter.