Shaul Mukamel of the University of California Irvine is supported by an award from the Theoretical and Computational Chemistry program to continue his research program aimed at developing theoretical and computational methods for predicting the response of molecules to various external fields, such as light, electric current and other molecules. New experimental approaches related to multidimensional coherent ultrafast laser spectroscopy and to electric current measurements on molecules in quantum junctions are suggested by this theoretical work. A set of tools for their analysis in real space and real time is being developed in the course of the research: signatures of exciton transport in coherent femtosecond optical spectroscopy of chromophore aggregates are being identified; the statistics of single electron transfer events in junctions and of photons emitted in single molecule spectroscopy are being predicted and will be used to explore quantum generalizations of fluctuation theorems connected with the distribution of work and entropy production of driven systems; fluctuations and response functions of quantum systems in nonequilibrium steady states and undergoing transitions between steady states are being studied; and techniques of quantum electrodynamics are being used to predict photon counting statistics and establish fundamental connections between time and frequency resolved nonlinear response functions. This is providing a microscopic description of the incoming and signal fields and allowing the PI and his students to develop new approximation schemes for optical signals.
The dynamics of electrons and nuclei in molecules can be effectively probed by monitoring their response to external perturbations such as optical fields, electrical currents and even the response of the probed molecule to interactions with neighboring molecules. Mukamel's work is providing the theoretical tools needed by experimental scientists for analyzing the data produced by different types of experiments. The work is having a broader impact through its application by scientists around the world as well as through the highly interdisciplinary training received by the students in the group. The results of this research have been widely disseminated through Mukamel's textbook, "Principles of Nonlinear Optical Spectroscopy," which he plans to update and expand in the next period of support. This is expected to further broaden the impact of the funded work.
Elementary motions of electrons and nuclei in molecules can be investigated by their response to sequences of ultrafast laser pulses. These can prepare molecules in specific electronic and vibrational states and provide a multidimensional view of their dynamics and correlations. Multidimensional signals are obtained by studying the variation of the response with several time delay periods between pulses. Nuclear magnetic resonance (NMR) uses similar concepts for studying spins in the radio wave regime. Over the past 20 years laser technology had advanced to allow the extension of these ideas to probe femtosecond molecular processes using infrared and visible pulses. This program focuses on the design of novel optical experiments for molecules in the condensed phase and the development of theoretical and computational tools for their interpretation. Energy and Charge Separation in Photosynthetic Complexes were investigated. The harvesting of solar energy and its conversion to chemical energy is essential for all forms of life. The primary photon absorption, transport, and charge separation events, which trigger a chain of chemical reactions, take place in membrane-bound photosynthetic complexes. Whether quantum effects, stemming from entanglement of chromophores, persist in the energy transport at room temperature, despite the rapid de-coherence effects caused by environment ?uctuations, is under current active debate. We made use of the complex chirality and fundamental symmetries of multidimensional optical signals to design new sequences of ultra-short laser pulses that can distinguish between coherent quantum oscillations and incoherent energy dissipation during the exciton relaxation. Signatures of quantum transport have been observed by two- dimensional coherent optical spectroscopy. Two dimensional infrared (2DIR) studies of proteins reveal how they fold into secondary structures that allow them to perform these functions. Applications were made to small model peptides and to the folding of the Villin Headpiece. The forming and breaking of hydrogen bonds in proteins and in liquid water were investigated. Ordinary measurements are carried out on large ensembles of molecules where much of the desired information is averaged out. These difficulties can be overcome by studying individual molecules. Novel pulse sequences and detection schemes that can probe single molecules and molecules in current-carrying states on the femtosecond time scale were developed. The counting statistics of single electron transfer events in junctions and of photons emitted in single molecule spectroscopy triggered by sequences of phase locked laser pulses were predicted and analyzed using fluctuation theorems that constrain the distribution of work and entropy production in driven open systems. By making use of the quantum nature of light it is possible to perform optical experiments using entangled photons which offer an unusual combination of bandwidths and temporal resolution, not possible by classical beams. Contributions from desired resonances can be selected by varying the parameters of the photon wave function. The signals scale linearly rather than quadratically with the laser ?eld intensity, which allow performing the measurements at low powers. This is useful for biomedical imaging applications. This research program has broad impacts in several areas. An essential component is the training of undergraduate and graduate students and postdoctoral fellows and preparing them to assume faculty positions in research universities, four-year colleges, and government labs. The textbook Principles of Nonlinear Optical Spectroscopy, first published in 1995, provides a unifying framework for the interpretation of optical measurements on molecules in the condensed phase. A graduate course based on the book is taught at UCI and in short courses and summer schools at other institutions. These lectures and the constant feedback from students and users of the current text, form the basis for a new considerably expanded textbook that is being written. Extensive collaborations and student exchanges with many leading experimental groups have been established. A computer code SPECTRON developed for multidimensional optical electronic and vibrational spectroscopy has been made available to the research community. The P.I. had served on review panels for federal funding agencies and on the scientific advisory boards of several research centers in the area of ultrafast spectroscopy, in the United States and Europe.
