Nucleation of optically excited solitons and breathers in trans-polyacetylene

Solitons and breathers are well known self-localized stable excitations in transpolyacetylene. Here we discuss numerically the optical excitation of soliton pairs and breathers in trans-polyacetylene by short (30 fs) and resonant (hw = 2.0 eV) laser pulses. In a perfect uniform chain the formation of self-localized excitations is suppressed by the decay into extended excitations like optical phonons even if the chain has absorbed sufficient energy. A small change of the Hamiltonian at one monomer and very small fluctuations of bond lengths may serve to create nucleation centres, which aid the formation of spatially localized excitations. We present soliton pairs and breathers directly created by the electric field of a laser pulse within the dipole approximation and mean-field theory for the electron-phonon coupling. The time development of the pi-electron system is solved in the framework of the single-particle Schrodinger equation with the SSH Hamiltonian. We neglect the Coulomb interaction of the pi-electrons.