A simple analogy -- which light on a Christmas tree will light up next? Dr. Rieger's doctoral archives:

• computational data and source files
• experimental data files
• dissertation, "Computational and Ultrafast Studies of Electronic Energy Transport in Spatially Disordered Systems"; also covers simple ordered systems; (600 dpi bitmap in PostScript)
• paper, "On the Forster Model: Computational and Ultrafast Studies of Electronic Energy Transport", Chem. Phys. 221, 85 (1997); condensed version of dissertation: exclusively covers spatially disordered systems; (600 dpi bitmap in PostScript)
• unformatted text of dissertation (TeX and Encapsulated PostScript files)
• unformatted text of paper (TeX and Encapsulated PostScript files; requires dissertation's EPS files)
• miscellaneous text: poster, proposals, orals exam, etc. (plain text and TeX files)

To study the transport of electronic energy in discrete, spatially disordered systems in the condensed phase, this work uses the Förster model: Absorption of a photon electronically excites a donor molecule. The donor either emits this energy or transfers it to an acceptor molecule. Radiationless transfer of the excitation occurs due to an electronic dipole-dipole interaction between the molecules. The interaction between these molecules is weak compared to the thermal energy of the system, thus the electronic energy is localized on a single molecule (chromophore).

This work introduces a computational benchmark to compare several analytical theories and determine their range of applicability over an expansive domain of the model parameters (reduced concentration, Förster distance, and chromophore diameter) for systems consisting of up to 5000 chromophores.

This work also examines the survival probability and long-time diffusion coefficient in concentrated dye solutions using transient grating methods.

There are three reasons for studying the transport of electronic energy from excited to unexcited particles in systems (such as solutions and amorphous solids) in which the distribution of the particles is spatially disordered. First, this fundamental process has numerous applications in condensed-matter physics and chemistry. For example, investigation can lead to a better understanding of photosynthesis, photosensitive devices, liquid solvation dynamics, and high-frequency dielectric response. Additionally, insight into the latter is needed for a complete description of another fundamental chemical process, electron transfer in a homogeneous phase or at an interface.

Second, it is an example of the many-body problem in nonequilibrium statistical mechanics. This problem is important since it occurs in much of physics and chemistry: the formulation of hydrodynamics; rate and relaxation processes; and transport phenomena; all of which involve an approach to equilibrium. But the many-body problem is correspondingly difficult to solve. This is because of the large number of interacting particles involved in systems of physical interest. Moreover, the spatial disorder in our many-body problem only adds to the difficulty.

Third, disordered systems present a challenge in their own right. The primary consequence of aperiodicity is that the system's state cannot be labeled with a single wavevector. This requires that a description of the system's properties be in terms of stochastic variables or its structure in real space. Accordingly, the study of disordered systems is important in relating the function of biological molecules to their structure; fabricating materials for solar energy conversion; polymer science; and basic solid-state physics.

Still, the main interest is the many-body nature of the interactions rather than the spatial disorder of the system: We are interested in the correlations between the chromophores, not their coordinates and momenta; we are interested in the dynamics of the system.

"When you are in a strategic transformation, you kind of get lost. Part of you would want to retreat back to doing what you know how to do, because it's familiar, because you know what you're good at, you know where the problems are. But your intellect tells you that's not where you really want to be. So you strike out in a new direction." -- Andy Grove

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