Two-Dimensional Transient Spectroscopy Measuring Heterogeneity in Electronic and Rotational Dynamics

Multiple population-period transient spectroscopy (MUPPETS) is a picosecond, time resolved experiment that uses a sequence of six laser pulses. It was previously known that MUPPETS could measure heterogeneity in electronic-state decay. This dissertation presents two projects that extend MUPPETS to new processes. One process is the extension from 2-level system into 3-level system, another new process extends the kinetics from electronic decay to rotational decay. In addition, a third, ongoing project on rotational dynamics in ionic liquids will also be discussed briefly. The first project consisted of two main parts. The first part focused on the biexciton decay in semiconductor nanoparticles. The power dependence of the excited state decay in nanoparticles has been attributed to biexcitons, but those measurements are easily contaminated with other species. New theoretical work in excitonic systems shows that MUPPETS can measure biexciton decays free from contaminations. Our experiments successfully isolate the biexciton decay of CdSe/ZnS core–shell nanoparticles. The biexciton signal shows a highly dispersed, nonexponential dynamics, which is inconsistent with current theories of Auger recombination. The second part of the first project investigated the heterogeneity of exciton decay. There is a fast, nonradiative decay in the exciton decay of core–shell nanoparticles, which has been attributed to a subset of poorly passivated particles. Using a new theory of multi-level systems, our MUPPETS experiments showed that such a subpopulation does not exist. We suggest that the early component in exciton decay is caused by surface relaxation. The second project probed heterogeneity in the local dynamics of polymers, as sensed by solute rotation. The rotation of a solute in a small molecule solvent is exponential, but it becomes nonexponential in a polymer melt. This nonexponential behavior may be explained by either variations in the local viscosity of the polymer—a heterogeneous model—or local anisotropy of the polymer structure—a homogeneous model. To measure heterogeneity in rotation rates, we extended the original MUPPETS experiment to a polarized version. The new method was demonstrated on the anisotropy decay of Pyrromethene 597 in poly(dimethylsiloxane) (PDMS). The results show strong molecule-to-molecule variation in the rotation rate. They are consistent with local, shortlength scale variations in viscosity within the polymer. No evidence for local anisotropy was found. In the final projects, the rotational dynamics of a solute in ionic liquids was measured with 1D polarization experiments. Experiments and simulations have suggested that heterogeneous microstructures exist in ionic liquids. A new signal normalization channel was built to reduce noise, increase long term stability and improve the ability to detect nonexponential decay. Rotational decays are measured for ionic liquids with different chain lengths and different mixture ratio with acetonitrile. Weakly nonexponential decays were found for long chains, but none was found for short chains. Experiments and analysis are ongoing