Attosecond Electron Dynamics

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In this project funded by the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program of the Chemistry Division, Professor Stephen Leone of the University of California, Berkeley, uses sophisticated laser techniques to study some of the shortest time processes in molecules. Modern laser technology allows us to track the motion of the atomic nuclei in molecules as chemical reactions take place. However, the electrons that make up the bonds between atoms are thousands of times less massive than nuclei and therefore move much faster. This research project seeks to follow electron processes inside molecules that occur within 100 – 200 attoseconds (One attosecond is a decimal point followed by seventeen zeros and then a “1”). The fast measurements are accomplished by using laser light with wavelengths in the extreme ultraviolet and x-ray regions of the spectrum. The goal of this project is to characterize the motions of electrons in molecules undergoing chemical reactions, and perhaps catch the electron cloud in the moment when it is about to choose one reaction path over another. The research is important to understand the fundamental principles of charge flow (negative electrons and positive nuclei) and the breaking and reforming of chemical bonds for utilization of sunlight energy. Further benefits to society arise from the development of laser measurement capabilities that push the boundaries of short time durations in chemical dynamics using tools that require precision stability of lasers and optical platforms. These capabilities are certain find use in many areas of science and technology. In addition to the three graduate students who are directly involved in this project, members of the Leone research group engage in informal educational activities, including laboratory tours for high school students and teachers, and outreach events in coordination with the Lawrence Hall of Science.Ultrafast transient absorption in the extreme ultraviolet and X-ray, techniques developed in the Leone laboratory, are employed to make attosecond time-resolved dynamics measurements as a way to probe chemical dynamics that can separate electron dynamics timescales from nuclear motion. Electron correlation and superposition states play central roles in chemical processes on these short timescales. Attosecond probe pulses are produced via the process of high harmonic generation using carrier-envelope-phase stabilized driver laser pulses, and ultraviolet pulses are used to excite molecules by well-defined one-photon absorption. Core level transitions probed in reporter atoms are site- and oxidation-state specific, bond-length-sensitive, and electronic-state-dependent, providing an orbital-specific way to measure ultrafast dynamics. Objectives are to study attosecond and few-femtosecond time processes of curve crossing, passage through conical intersections, electronic and vibrational quantum mechanical superpositions, and charge migration recurrences. Molecules are chosen for study that have very different electronic configurations in multiple excited states or chemical environments around the molecule that are recognizable by spectroscopic means for charge flow, or for their relevance in chemical processing. The broader impacts of this work involve the development of tools that push the limits of time, important as both the dimensions and speed of devices decrease and as quantum principles in molecules are considered for computing architectures. Students who participate in this project learn an array of principles relevant for high technology professions that enable the speed and dimensions of ubiquitous electronic devices to decrease.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.