Photoexcitation and relaxation processes in polyacetylene

The dynamical response of a trans-polyacetylene chain after photoexcitation by the laser pulse which resonantly excites the lowest optically allowed excited (1B(u)) state is investigated using the Pariser-Parr-Pople model. The time evolution of electronic and lattice structures is calculated by solving the time-dependent Hartree-Fock equation for pi electrons and the Newton's equation of motion for the lattice simultaneously. The 1B(u) excitation reduces the lattice order parameters (LOP's) and the signs of the LOP's, except for around the ends of the chain, are reversed at a certain time when the pulse external field is strong enough. The characteristic charge density (CD) oscillation induced by the 1B(u) excitation continues until this time but subsequently the oscillation pattern changes drastically and abruptly as a result of the lattice relaxation. In several ranges of the magnitude of the external field, the induced CD's localize around the zeros of the LOP and a charged soliton pair is generated. The collision between the charged solitons results in formation of the breather. The breather decays into a complicated CD oscillation pattern: and the charged solitons disappear hy geminate recombination. The obtained life time of a charged soliton is less than 250 fs.