Picosecond pulse induced transient molecular birefringence and dichroism

Abstract

Excitation of dye molecules in fluid solution can lead to transient birefringence (caused by change in the polarizability anisotropy) and transient dichroism (caused by change in the absorption cross section). In the anisotropic absorption method of Shank and Ippen, both effects are observed, and in some cases the measured signal is dominated by the birefringence contribution. Very small amounts of external birefringence can lead to erroneous values for the rotational reorientation time. We present an analysis of these effects in a Jones matrix formalism, and use the results to show how to obtain correct reorientation times. The changes in polarizability anisotropy for the dyes oxazine-725 and DODCI are evaluated as +75 and +4 Å3, respectively, at 585 nm and we believe these to be the first direct measurements of this change between ground and first singlet states. The rotational reorientation times of four molecules in ethanol are presented and compared with hydrodynamic predictions.