Probing Coupled Electronic and Vibrational Motions in Ultrafast Proton Transfer Processes with Mixed-Frequency Multidimensional Vibronic and Soft X-ray Spectroscopies

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In this project funded by the Chemical Structure and Dynamics-A (CSDM-A) program of the Chemistry Division, Professor Munira Khalil of the University of Washington is using advanced laser techniques to understand how a proton (a hydrogen atom without its electron) moves from an oxygen atom (proton donor) to a nitrogen atom (proton acceptor) in the same molecule. This process is termed intramolecular (within the molecule) proton transfer and is a fundamental step in natural photosynthesis, as well as some energy conversion processes like hydrogen fuel generation from light energy. Some important proton transfer processes occur extremely quickly, for example in less than two hundred femtoseconds (one quadrillionth of a second). The rapidity of this process makes it difficult to measure, and to answer such questions as, "how does the speed of intramolecular proton transfer depend on the structure of the molecule?". To overcome this challenge, Professor Khalil is designing experimental tools that utilize a wide range of light wavelengths, from the infrared spectrum (long wavelengths that we feel as "heat") to X-rays (very short wavelengths). These tools provide detailed information about the motions of electrons and atoms within molecules. This research provides new information to develop more efficient molecules for converting light into chemical energy. The effort also creates new femtosecond technologies and trains graduate students in cutting-edge optical techniques. Professor Khalil conducts an outreach program that includes a hands-on optics module for middle school girls from STEM-disadvantaged backgrounds. This program is part of the "Girls on Science" program conducted through the University of Washington's Burke Museum. The first objective of this research program is to map the relationship between electronic excitation and vibrational motion in a series of intramolecular hydrogen-bonded complexes with different hydrogen bond strengths. The second objective is to understand how a molecule shakes off excess energy after absorbing a photon (this happens in DNA complexes) as a function of the distance and angle between the hydrogen bond donor and acceptor. The first two objectives are accomplished using advanced laser techniques developed in the Professor Khalil's laboratory. The final goal is to map the electron distribution at the proton donor and acceptor sites with element specific soft X-ray spectroscopy. The X-ray experiments are conducted at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (Berkeley, CA). The broader impacts include an increased understanding of light-induced electron-proton transfer processes, which are important reactions in several natural and artificial energy conversion processes that may lead to the development of new molecules, materials and devices for alternative energy production. The team is also creating new femtosecond technologies and training the next generation of scientists in state-of-the art ultrafast techniques in the laboratory and at X-ray synchrotrons.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.