Molecular Radiative and Relaxation Processes

University Of California-Irvine, Irvine CA

Investigators

Abstract

Shaul Mukamel of the University of California, Irvine is funded by the Chemical Theory, Models and Computational Methods program in the Chemistry division for research to develop techniques that are used to analyze the precise ways molecules absorb and redistribute light. This type of process happens during photosynthesis. Understanding the precise ways that solar energy is converted to chemical energy can help in the design of new devices which artificially convert light to more useful forms of energy. In addition, the detection of individual photons, particles of light which have interacted with molecules, can reveal a great deal of information about the structure and motion of the atoms which make up those molecules. By applying a series of short light pulses to a molecule, the investigators are finding ways to extract additional information about that molecule's structure and function. Additional research explores the use of entangled photons, a very promising tool for quantum computing, secure communication methods and other possible applications. New imaging methods that might be helpful in revealing the details of biological cells are also being developed. All of these studies are carried out in collaboration with other investigators who design equipment and experiments that are used to test the insights revealed by these theoretical studies. The work has a broad impact on the experimental science community through the development of new language and concepts that help to guide scientist's understanding of the fundamental nature of the process by which light interacts with matter. The investigators freely share the software they develop through the web. The response of complex molecules to sequences of ultrafast optical laser pulses provides a multidimensional view of electronic and vibrational dynamics and of correlations. Models and computational tools are developed for the design, interpretation and analysis of these signals. Quantum properties of light, such as photon entanglement, receive special consideration as tools for probing molecules and chromophore aggregates. In another project, nonadiabatic dynamics is studied by carefully designed coherent Raman signals. The interplay of temporal and spectral resolution in stimulated Raman signals is being investigated. New detection schemes that combine coincidence measurements of individual photons and interferometry can then be predicted. Experimental techniques involving novel pulse sequences are being designed, and theoretical and computational tools for their analysis are developed and broadly applied. Some of the applications are to complex molecules in the condensed phase, to excitons in semiconductor quantum dots, and to the elementary charge and energy migration processes in light harvesting in photosynthetic antennae and reaction centers. In particular, superoperator Green's function formalism is being developed that allows the computation of the response of molecules to both quantum and stochastic light. New spectroscopic signals that make use of entangled and squeezed photons offer temporal and spectral resolutions not possible with classical light. These techniques are being applied to the study of excitons in molecular aggregates. Measurements of the effect of interactions with molecules on photon statistics are also explored as a novel spectroscopic tool. Incoherent detection of coherent signals by fluorescence which can allow the study of single molecules is being investigated. A highly-intuitive loop diagram representation is developed for the design of pulse sequences by utilizing pulse shaping, phase control and molecular chirality. Generalized signals involving the combination of several impulsive excitations and measurements that can probe both the response and spontaneous fluctuations in classical and quantum systems at steady states are predicted and connected to nonlinear fluctuation theorems. New many-body approaches for describing and calculating excited states with multiple electron and hole particles are being developed using a composite Boson (co-boson) algebraic representation. The tedious explicit computation of multi-exciton wavefunctions usually required for predicting nonlinear signals is, thus, avoided with this new approach.