Multidimensional Femtosecond Studies of Chemical Reaction Dynamics

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This project is funded by the Chemical Structure, Dynamics and Mechanism (CSDM-A) program of the Chemistry Division. Professor David Jonas of the University of Colorado at Boulder is developing femtosecond laser techniques to study the motion of electrons in molecules. A femtosecond is a millionth of a billionth of a second. Inside molecules, atoms and slow electrons move on the femtosecond timescale. The goal of the project is to measure how atomic and electronic motions are linked when the forces driving their motion are weak. Such weak forces can arise from energies on the order of the thermal energy. This project explores the smallest energy required to drive motion of electrons. Similar electronic motions are involved in lighting, displays, and photosynthesis. Three dimensional spectra correlate electronic and atomic motion. High symmetry molecules simplify the study of electronic and atomic motions.In 3D Fourier transform spectroscopy, four-wave mixing is used to record transient signals as a function of three time delays between pulses. Triple Fourier transformation with respect to the three time delays generates a spectrum that is a function of three frequency axes. Unlike most 2D spectra, the pulse spectra and pulse propagation effects can be divided out in 3D. This reveals the underlying 3D nonlinear response, which should be independent of experimental conditions. The signal from cross-phase modulation in transparent solvents produces compact time-domain transients for initial experimental recovery of the underlying 3D response. 2D and 3D electronic spectroscopy of high symmetry molecules with pseudo Jahn-Teller effects can reveal the mechanism of their femtosecond electronic population transfer through comparison to quantum mechanical calculations. The calculations fully incorporate non-adiabatic interactions between motion of electrons and the vibrational motion of atoms. The broader impacts of this project include training of students in modern experimental and computational techniques plus potential societal benefits from a better understanding of the minimum energy required to drive electronic motion.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.