Ultrafast nonlinear optical responses and geometrical relaxations in π-conjugated polymers

Abstract

In the present paper the following subjects are discussed about conjugated polymers: the first is a model of the relaxation of excitons, the second is the Raman spectrum of the excitons with lps lifetime, and the third is the optical Stark effect of the Raman gain spectrum. (1) The decay kinetics of self-trapped (ST) excitons in one-dimensinal conjugated polymers were explained with a model in which potential crossing and tunneling between two potential curves of the ST exciton and the ground state are the major processes in the relaxation. The weak temperature dependence indicates that the activation process over the potential barrier between the ST exciton and the ground state is not dominant in the radiationless relaxation of the ST exciton. (2) Geometrical relaxation of main chain configuration in PDA was also investigated by time-resolved Raman gain spectroscopy with 300 fs resolution which is the highest time resolution of the vibrational spectrum ever reported. The Raman signal due to the ST exciton in PDA was observed at 1200 cm-1. The observed Raman signals offer most direct evidence of the structure of ST exciton being butatriene-like structure after geometrical relaxation from acetylene-like structure in free exciton state. (3) The stimulated Raman gain signals of a single-crystal PDA-DFMP, with the sidegroups 2, 5-bis trifluoromethyl-phenyl, were reported to exhibit an optical Stark shift of about 50 cm-1. Using an ultrashort, near-resonant high intensity laser, the intensity dependent signal shifts to higher probe photon energies for both the C=C and C=C stretching modes. Semi-quantitative agreement between the data and a 3-level system density matrix calculation is achieved using a nonlinear optical mechanism in which a pump laser field is optically coupled to both the exciton ←zero-phonon ground state and the exciton ←one-phonon ground state transitions. © 1995 IUPAC