Multidimensional Femtosecond Studies of Chemical Reaction Dynamics

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With supported from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) program, and with partial support from the Chemical Measurement and Imaging (CMI) program, both in the Division of Chemistry, Professor David Jonas and his research group at the University of Colorado are investigating methods to measure thermodynamic free energies using absorption and emission of light. Thermodynamic energies determine whether a chemical reaction is possible and its maximum efficiency. All chemical fuels were originally formed from molecules excited by light, a process that is difficult to efficiently replicate. It has not been possible to measure the thermodynamic energy of molecules that have been excited by light; instead, chemists have been forced to approximate the thermodynamic energies of excited molecules using quantum energies from spectroscopy for over 70 years even though this approximation can result in errors as large as four orders of magnitude. The aim of this project is to enable the measurement of thermodynamic energies for molecules that have been excited by light. Such measurements have the potential to enable the efficient formation of fuels from sunlight by enabling accurate measurements of the thermodynamic limitations on chemical reactions that are driven by light. Students participating in the project will be trained to perform and interpret measurements using advanced femtosecond laser techniques and molecular spectroscopy as well as gain skills in data analysis, and numerical modeling. This project uses two-dimensional Fourier transform (2DFT) spectroscopy in the optical region of the spectrum. The research team will measure the femtosecond electric field of four-wave mixing signals in the time domain, and then use Fourier transforms with respect to the delays between the femtosecond pulses in order to obtain the spectra. Additional measurements of the sample and femtosecond pulses will be used to transform the 2D spectra so that they are independent of the femtosecond 2D spectrometer. The transformed 2D spectra exhibit relationships between absorption and emission spectra that can be used to retrieve, within noise limits, absorption from emission and vice versa. The team is investigating whether these relationships apply to molecular solutions and whether the relationships provide thermodynamic standard free energies for the excited state relative to the ground state. To test these relationships for molecular solutions, the Jonas research group will carry out research in three broad areas: 1) the use of 1D spectra to determine molecular free energy changes in homogeneous molecular solutions with fast thermalization; 2) the use of 2D spectra to determine molecular free energy changes in molecular solutions with slow thermalization; 3) tests of spectroscopic free energies against a thermodynamic cycle. The team is also exploring whether these approaches can be extended to non-equilibrium thermodynamics in systems with slow thermalization, including measurements using 3DFT spectroscopy.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.