Soliton and exciton effects in femtosecond pump probe spectroscopy of conjugated polymers

Abstract

Summary form only given. Photogeneration of charged solitons is one of the most important experiments which support soliton theory. However, photoexcitation and lattice relaxation processes of polyacetylene are not yet under stood well. Su and Schrieffer (SS) have performed a numerical simulation of the dynamical response to creation of an electron-hole pair using the Su-Schrieffer-Heeger (SSH) model. However, some important features of photoinduced absorption spectra can not be explained by the theory. In the SSH model, the electron-phonon coupling is considered but the Coulomb interaction between electrons is neglected. However, it is generally accepted that the Coulomb interaction plays important roles in the physics of conjugated polymers. In particular, the exciton effect, which dominates their optical properties, can not be described without it. Thus, we adopt the Pariser-Parr-Pople model, which includes both Coulomb interaction and the coupling of electron motion with lattice dynamics. Furthermore, since the coupling to the external field is treated explicitly in our model, we can obtain comprehensive understanding of the excitation process. The coherent pump-prove signals are calculated by deriving equations of motion for the single electron density matrix and lattice displacements, and by solving them using the time dependent Hartree-Fock and classical approximations, respectively. We found the following results: 1) charge transfer excitons, which come from Coulomb interaction, are generated by photoexcitation. 2) the charge transfer exciton accompanies soliton-pair like lattice distortion (excitonic polaron). 3) the formation of the excitonic polaron shows up as an additional feature in the femtosecond pump-prove signal.