Collaborative Research: Understanding Ultrafast Observables

在化学系的化学结构、动力学和机理-A（CSDM-A）和化学理论、模型和计算方法（CTMC）项目的支持下，斯托尼布鲁克大学的Allison、Levine和Weinacht教授以及坦普尔大学的Matsika教授正在开发新的方法来理解从分子动力学的复杂测量中获得的信息。分子的结构和行为受量子力学规则的支配。量子化学领域将量子力学的原理应用于分子问题，几十年来一直基于实验和理论之间的严格比较而发展，产生了可靠的计算机代码，非专家可以使用这些代码来计算分子在最低能量状态下的性质。然而，类似的量子化学计算对于已经被激发的分子来说更具挑战性，例如通过从光中吸收能量，并且能够经历非常快的化学转变。开发激发态分子的量子化学方法的部分困难在于，实验测量更难解释，并且与理论的比较通常比基态分子要严格得多。这个合作研究小组正在努力通过研究使用不同类型实验以相同方式制备的分子，并通过将这些观测值与模拟测量过程和激发态动力学的量子化学计算进行直接比较，以更好地了解实验观测值。除了为几种代表性分子制作一套基准测量外，该团队还在努力建立一种新的范式，以理解分子动力学的测量，包括一种新的数据共享格式。除了这些更广泛的科学影响外，该项目还为研究生提供了高度协作环境中的高级培训。超快光谱提供了直接探测激发后分子动力学的机会。然而，超快光谱数据的解释仍然是一个挑战，因为高维动力学投影到一个低得多的维度信号是不可避免的。原则上，如果一个探测器能将随时间变化的分子波包投射到所有可能状态的集合上，那么它就能提供一个完整的（如果难以解释的话）所讨论的动力学图像。由Allison，Levine，Weinacht和Matsika教授领导的研究小组正在通过应用多个最近开发的实验和理论工具来测量和计算相同制备的气相分子的动力学来解决这个问题。互补的时间分辨光电子和可见瞬态吸收探针将分子波包投射到希尔伯特空间的一个宽阔的区域上，提供了比任何一种方法本身都更多的动力学信息。测量结果进行了比较从头算模拟的动态，从相同的预测进行。最近的方法学进展，包括气相瞬态吸收光谱仪和新的从头算工具，有效地计算探针信号从大分子动力学数据集的发展，使测量和计算光谱利用互补探针之间的这种严格的比较。这些系统性的研究正在产生关于原型分子系统的基准数据集，这些原型分子系统在量子化学和分子动力学的前沿提出了具有挑战性的问题，包括非绝热动力学和系统间交叉。研究中的基本过程在光驱动的各种化学反应中发挥着重要作用。通过这一合作努力，该团队还致力于开发和传播一种新的数据格式，用于共享基于FAIR原则（可查找、可访问、可互操作、可重复使用）的理论和实验超快动力学结果。该项目的研究生学习如何在实验和理论前沿的合作研究的基础上处理化学中的复杂问题。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估而被认为值得支持。

With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) and Chemical Theory, Models, and Computational Methods (CTMC) programs in the Division of Chemistry, Professors Allison, Levine, and Weinacht at Stony Brook University, and Professor Matsika at Temple University are developing new ways to understand the information obtained from sophisticated measurements of the dynamics of molecules. The structure and behavior of molecules are governed by the rules of quantum mechanics. The field of quantum chemistry, which applies the principles of quantum mechanics to molecular problems, has developed over decades based on rigorous comparison between experiments and theory, resulting in reliable computer codes that can be used by non-experts to calculate the properties of molecules in their lowest-energy states. However, similar quantum chemistry calculations are far more challenging for molecules that have been excited, for example by absorbing energy from light, and are able to undergo very fast chemical transformations. Part of the difficulty in developing quantum chemistry methods for excited molecules is that the experimental measurements are much harder to interpret, and comparisons with theory are generally much less rigorous than for molecules in their ground state. This collaborative research team is working to better understand the experimental observables by studying molecules prepared in the same way using different types of experiments, and by making direct comparisons of those observables with quantum chemical calculations that simulate both the measurement process and the excited-state dynamics. In addition to producing a set of benchmark measurements for several representative molecules, the team is working toward a new paradigm for understanding measurements of the dynamics of molecules, including a new format for sharing data. Beyond these scientific broader impacts, the project also provides advanced training for graduate students in a highly collaborative environment.Ultrafast spectroscopy offers the opportunity to directly probe the dynamics of molecules after excitation. However, the interpretation of data from ultrafast spectroscopy remains a challenge because projection of high dimensional dynamics into a much lower dimensional signal is unavoidable. In principle, a probe that projects the time-dependent molecular wave packet onto the set of all possible states provides a complete, if difficult to interpret, picture of the dynamics in question. The research team led by Professors Allison, Levine, Weinacht, and Matsika is addressing this problem by applying multiple recently developed experimental and theoretical tools to measure and calculate the dynamics of identically prepared gas-phase molecules. Complementary time-resolved photoelectron and visible transient absorption probes project the molecular wave packet onto a broad swath of Hilbert space, providing more information about the dynamics than is possible with either method on its own. The measurements are compared with ab initio simulations of the dynamics, from which identical projections are performed. This rigorous comparison between measured and calculated spectra utilizing complementary probes is enabled by recent methodological advances, including the development of a gas-phase transient absorption spectrometer and novel ab initio tools for efficiently computing probe signals from large molecular dynamics data sets. These systematic studies are producing benchmark datasets on archetypal molecular systems that present challenging problems at the vanguard of quantum chemistry and molecular dynamics, including non-adiabatic dynamics and intersystem crossing. The fundamental processes under investigation play an important role across a wide range of chemical reactions that are driven by light. Through this collaborative effort, the team is also working to develop and disseminate a new data format for sharing both theoretical and experimental ultrafast dynamics results based on the FAIR principle (findable, accessible, interoperable, reusable). Graduate students working on the project learn how to approach complex problems in chemistry based on collaborative research at the forefront of both experiment and theory.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.