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Research in the Trinity Physics Laser Laboratory

An Introductory Summary

Random-Walk Motion and Forced Rayleigh Scattering

We are interested in the study of molecular motion in fluids. This type of motion is important in many fields, including medicine (where, for example, we might want to know how quickly a drug can be transported through the body) and chemical engineering (where one might want to use differences in transport rates to separate one type of molecule from another). In a liquid system, the path of an individual particle is very complicated. The particle collides with a large number of other particles in a short period of time, with each collision leading to a sharp change in momentum. Such collisions lead to a so-called "random-walk path" for the particle.

How can we hope to follow the complicated motion of random-walk particles experimentally? Several experimental techniques invoke an interaction between moving particles and light. One of the most powerful methods of this type is called the laser-induced transient grating (TG) technique, which we employ in the Trinity Physics Laser Laboratory. The type of TG experiment used in our work is usually called "forced Rayleigh scattering" (FRS), but is sometimes also referred to as "holographic grating relaxation spectroscopy."

To understand the FRS technique, suppose that we are interested in determining the rate at which dye molecules diffuse (due to random-walk motion) in a liquid. Two coherent laser beams (the "pump" beams) intersect in the fluid sample, creating a periodic interference pattern in which some locations of the sample (the interference maxima) are exposed to high laser intensity, while other locations (the interference minima) experience zero laser intensity. The laser wavelength employed is absorbed by the dye molecules, so that the interference fringes create long-lived, or "metastable" photoproduct states that form a grating-like spatially periodic pattern. If the photoproduct states are optically different from the unperturbed ground states, then the photoproduct molecules can be distinguished from their neighbors, and we can, in principle, diffract a third laser beam (the "probe" beam) from the photoproduct grating formed by the pump beams. As a specific example, suppose that the photoproduct molecules absorb red light, but that red light is transmitted by the ground-state molecules; then a red laser beam will diffract from the photoproduct grating. If we now turn off the pump beams, the grating will decay as the periodic photoproduct population profile dissipates due to random-walk motion. By measuring the time required for the diffracted intensity to decay to zero, we can infer the rate at which the dye molecules are diffusing due to their random-walk motion. Forced Rayleigh scattering has been used throughout the world for studies of diffusion in a wide variety of systems, including polymer solutions, liquid crystals, magnetic fluids, and supercooled liquids, to name a few.

Our Recent FRS Research at Trinity University

In our laboratory, we have attempted to develop a better understanding of the optical physics that underlies the FRS technique. It should be noted that the description above of the grating formation is actually somewhat oversimplified. Each time a photoproduct is created, a ground state is depleted. The ground-state and photoproduct molecules will not diffuse at exactly the same rate. There are thus two out-of-phase gratings created, and the detected signal is a superposition of the electric fields diffracted from each. This can lead to rather complicated time profiles for the diffracted signal; sometimes, in fact, the decay of the diffracted light is nonmonotonic. The basic mechanism for the formation of the two "complementary" gratings has been understood for some time, but extraction of quantitative information on diffusion from complicated profiles has proved difficult. We have carried out experimental studies showing it is possible obtain mean diffusion rates from even nonmonotonic profiles without difficulty. Details on this work can be found in some of our recent Publications.