An Introduction to Transient Absorption Spectroscopy and Nonlinear Photochemical Behavior of Polymer Systems

Abstract

Transient absorption spectral measurements in the ns and ps time regions are described and their importance as photophysical and photochemical tools are discussed. Various artifacts such as spatial stray light effects, depletion effects on the ground state molecules, transient inner filter effects, nonlinear refractive index changes, thermal lensing effects, Resonance Raman scattering due to solvent, and two-photon excitation of solvent have to be examined in order to obtain a reliable and accurate absorption spectrum. Some of these factors are nonlinear with respect to excitation intensity and result in a characteristic behavior of polymers. Intrapolymer interactions between the excited singlet states, formation of a transient polyelectrolyte, and intrapolymer multiphoton charge separation are demonstrated and discussed by correlating the results with the polymer structure. Simultaneous two-photon absorption of the ps excitation pulse makes it possible to measure the absorption spectrum of the excited state of polymer solids. A combination of a polymer with pendant aromatic groups and a laser pulse with high intensity leads to new dynamic behavior and brings about a new methodology in polymer science.