Multidimensional Femtosecond Studies of Chemical Reaction Dynamics

在化学结构、动力学和机理a （CSDM-A）项目的支持下，以及化学测量和成像（CMI）项目的部分支持下，科罗拉多大学化学系的David Jonas教授和他的研究小组正在研究利用光的吸收和发射来测量热力学自由能的方法。热力学能决定一个化学反应是否可能发生以及它的最大效率。所有的化学燃料最初都是由受光激发的分子形成的，这个过程很难有效地复制。测量受光激发的分子的热力学能是不可能的；70多年来，化学家们一直被迫使用光谱学中的量子能来近似激发态分子的热力学能，尽管这种近似可能导致高达四个数量级的误差。这个项目的目的是测量被光激发的分子的热力学能量。这种测量有可能通过精确测量由光驱动的化学反应的热力学限制，使从阳光中有效地形成燃料成为可能。参与该项目的学生将接受训练，使用先进的飞秒激光技术和分子光谱学来执行和解释测量结果，并获得数据分析和数值建模的技能。该项目在光谱的光学区域使用二维傅立叶变换（2DFT）光谱。研究小组将在时域测量四波混频信号的飞秒电场，然后对飞秒脉冲之间的延迟进行傅里叶变换，以获得光谱。对样品和飞秒脉冲的额外测量将用于转换二维光谱，使它们独立于飞秒二维光谱仪。变换后的二维光谱显示了吸收光谱和发射光谱之间的关系，可用于在噪声限制内从发射中提取吸收，反之亦然。研究小组正在研究这些关系是否适用于分子溶液，以及这些关系是否提供了激发态相对于基态的热力学标准自由能。为了测试分子溶液的这些关系，Jonas研究小组将在三个广泛的领域进行研究：1)使用一维光谱来确定快速热化的均匀分子溶液中的分子自由能变化；2)利用二维光谱测定慢热化分子溶液中分子自由能的变化；热力学循环下的光谱自由能测试。该团队还在探索这些方法是否可以扩展到缓慢热化系统中的非平衡热力学，包括使用3DFT光谱进行测量。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.