PHOTOEXCITATION AND RELAXATION PROCESSES IN POLYACETYLENE

The dynamical response of a trans-polyacetylene chain after photoexcit ation 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 i s calculated by solving the time-dependent Hartree-Fock equation for p i electrons and the Newton's equation of motion for the lattice simult aneously. The 1B(u) excitation reduces the lattice order parameters (L OP's) and the signs of the LOP's, except for around the ends of the ch ain, are reversed at a certain time when the pulse external field is s trong enough. The characteristic charge density (CD) oscillation induc ed by the 1B(u) excitation continues until this time but subsequently the oscillation pattern changes drastically and abruptly as a result o f the lattice relaxation. In several ranges of the magnitude of the ex ternal field, the induced CD's localize around the zeros of the LOP an d a charged soliton pair is generated. The collision between the charg ed solitons results in formation of the breather. The breather decays into a complicated CD oscillation pattern: and the charged solitons di sappear hy geminate recombination. The obtained life time of a charged soliton is less than 250 fs.