Multidimensional Femtosecond Studies of Chemical Reaction Dynamics

在化学结构，动力学和机理-A（CSDM-A）计划的支持下，以及化学测量和成像（CMI）计划的部分支持下，无论是在化学系，大卫乔纳斯教授和他在科罗拉多大学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。 热力学能量决定了一个化学反应是否可能以及它的最大效率。 所有化学燃料最初都是由光激发的分子形成的，这一过程很难有效复制。 光激发的分子的热力学能量是无法测量的;相反，70多年来，化学家们被迫使用光谱学的量子能量来近似激发分子的热力学能量，尽管这种近似可能导致四个数量级的误差。 该项目的目的是能够测量被光激发的分子的热力学能量。这样的测量具有通过使得能够精确测量由光驱动的化学反应的热力学限制而使得能够从阳光有效地形成燃料的潜力。参加该项目的学生将接受培训，使用先进的飞秒激光技术和分子光谱学进行和解释测量，并获得数据分析和数值建模方面的技能。 该项目在光谱的光学区域使用二维傅里叶变换（2DFT）光谱学。 研究小组将在时域中测量四波混频信号的飞秒电场，然后使用傅立叶变换相对于飞秒脉冲之间的延迟来获得光谱。样品和飞秒脉冲的额外测量将用于转换2D光谱，使得它们独立于飞秒2D光谱仪。变换后的2D光谱表现出吸收光谱和发射光谱之间的关系，这些关系可用于在噪声限制内从发射中检索吸收，反之亦然。 该团队正在研究这些关系是否适用于分子溶液，以及这些关系是否为激发态相对于基态提供了热力学标准自由能。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具有快速热化的均匀分子溶液中的分子自由能变化; 2）使用二维光谱来确定具有缓慢热化的分子溶液中的分子自由能变化; 3）测试光谱自由能对热力学循环的影响。 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NSF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.