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.

Different Real and Imaginary Components of the Resonant Third-Order Polarization Revealed by Optical Heterodyne Detected Transient Grating Spectroscopic Studies of Crystal Violet: Model and Experiment

“…14,15 Our previous study of the isomerization of 1,1′-diethyl-4,4′-cyanine (1144C) with different FWM techniques 15 showed strikingly different wavelength dependence in the decay behavior for the PP and homodyne TG signals, suggesting that different decay profiles can be associated with the real and imaginary components of the P (3) . Similar differences in the wavelength dependence of PP and homodyne TG signals were also observed in crystal violet and malachite green, 16 leading us to suspect that such differences are a common feature of systems with ultrafast decay to the ground state.…”

Rotational dynamics of neutral red: Do ionic and neutral solutes experience the same friction?

“…In a majority of the studies highly asymmetric charged molecules were employed as solutes. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] Though these molecules have less symmetry, their large absorption cross sections at wavelengths convenient for laser excitation made their choice ideal as probes for rotational diffusional studies. Often they are modeled either as prolate or oblate ellipsoids.…”

“…In low viscosity solvents, the experimentally measured reorientation times, were in agreement with the calculated ones using the SED theory with the stick boundary condition. 10,[14][15][16][17][18][19][20][21] However, in high viscosity solvents, especially in higher alcohols, slower rotation times were observed than predicted by the SED theory. [8][9][10][11]13,15,18 The observed slower rotation times were explained by specific solute-solvent interactions and hydrogen bonding effects which increase the volume of the rotating molecule.…”

Linear dichroism amplification: Adapting a long-known technique for ultrasensitive femtosecond IR spectroscopy

“…On a longer time scale, the decay of the polarization-dependence of the transient signals can be related to diffusion constants that depend on the shape of the molecule and its interaction with the environment. 4,5 Linear dichroism (LD) and anisotropy are essentially two metrics of the same quantity. LD is usually defined as (Jensen et al 6 )…”

“…Linear dichroism measurements in picosecond timeresolved electronic spectroscopy were originally carried out with exactly crossed polarizers (β = 0), 4,5 in which case the background-free pump-induced signal I 0 • depends quadratically on LD, but also on linear birefringence (LB). When one of the two polarizers is slightly tilted (β > 0), a small fraction of the probe light leaks to the detector and can selectively heterodyne the additional field due to pump-induced linear dichroism.…”