Photodissociation Dynamics of Transient Species: Hydrogen-Bonded Dimers and Trimers

在该项目中，来自南加州大学的Hanna Reisler教授将阐明氢键配合物在气相中预解离的机制，特别是二聚体和三聚体，该项目由化学系化学结构、动力学和机制A项目支持。尽管它们的相互作用强度很弱，但氢键在从活细胞到太阳系冰体的各种环境中都至关重要。为了深入了解预解离机制并确定氢键强度，将利用光片段离子成像的状态分辨率和检测灵敏度来获得激光激发氢键二聚体或三聚体的一个亚基后片段的成对相关分布。循环三聚体将作为更大的氢键网络中振动能量耗散的原型，并用于加强氢键的合作相互作用的实验估计。导致键断裂的特定状态的能量流模式将从碎片量子态分布中推断出来，并以光谱精度获得键解离能。例如，在HCl-water混合团簇中，HCl的初始质子转移对预解动力学的影响将被揭示。在三聚体和更大的簇中，2体和3体破碎过程的贡献将被确定。振动预解离机制将与埃默里大学Joel Bowman教授的理论小组合作阐明。这种合作已经导致了对水二聚体断键机制的描述，将扩展到包括其他二聚体和三聚体。最终目标是了解导致键断裂的振动能量流中尚未解释的精致状态特异性，并将其扩展到更大的氢键网络。理解通过氢键网络的能量流动一直是气体和凝聚态分子行为实验和理论研究的主要目标。然而，对于这些弱键是如何形成和断裂的，它们的灵活性是什么，以及由许多分子组成的网络是如何促进键的强度的，目前所知甚少。氢键在诸如生物系统、分子固体、中性分子和离子的水合溶液等环境中很重要。所选择的分子系统也与影响气候变化的大气过程有关。参与本研究的学生观察光谱学和动力学课程中所学概念的实验表现，将实验结果与理论进行比较，并构建强调物理洞察力的动力学模型。Reisler教授开发并领导了各种各样的活动，旨在鼓励科学和工程领域的女性从事研究工作，并将这项研究的结果纳入她的本科和研究生课程。

In this project, supported by the Chemical Structure, Dynamics and Mechanisms - A Program of the Division of Chemistry, Professor Hanna Reisler from the University of Southern California will elucidate mechanisms of predissociation of hydrogen-bonded complexes in the gas phase, in particular dimers and trimers. Despite their weak interaction strength, hydrogen bonds are crucially important in environments ranging from living cells to icy bodies in the solar system. To gain insight into the predissociation mechanisms and determine the hydrogen bond strength, the state resolution and detection sensitivity of photofragment ion imaging will be exploited to obtain pair-correlated distributions of fragments following laser excitation of one subunit of hydrogen-bonded dimers or trimers. Cyclic trimers will serve as prototypes of vibrational energy dissipation in larger hydrogen-bonded networks and for experimental estimates of cooperative interactions that strengthen hydrogen bonding. State-specific energy flow patterns that lead to bond breaking will be inferred from fragment quantum state distributions, and bond dissociation energies will be obtained with spectroscopic accuracy. For example, in HCl-water mixed clusters, the influence of incipient proton transfer from HCl on predissociation dynamics will be revealed. In trimers and larger clusters, the contributions of 2- and 3-body fragmentation processes will be determined. Vibrational predissociation mechanisms will be elucidated in collaboration with the theory group of Professor Joel Bowman at Emory University. This collaboration, which has already resulted in a description of the bond breaking mechanism in water dimers, will be extended to include other dimers and trimers. The ultimate goal is to understand the yet unexplained exquisite state specificity in vibrational energy flow that leads to bond breaking, and to extend to work to larger hydrogen bonding networks.Understanding energy flow through hydrogen-bonded networks has been a major goal of experimental and theoretical studies of molecular behavior in the gas and condensed phases. However, little is currently known about how these weak bonds are formed and broken, what is their flexibility, and how networks, made of many molecules, contribute to the strength of the bonds. Hydrogen bonds are important in environments such as biological systems, molecular solids, hydrated solutions of neutral molecules and ions. The chosen molecular systems are also relevant to atmospheric processes that affect climate change. Students participating in this research observe experimental manifestations of concepts learned in courses on spectroscopy and dynamics, compare their experimental results to theory, and construct dynamical models that emphasize physical insight. Professor Reisler has developed and leads a wide variety of activities aimed at encouraging women in science and engineering to pursue research careers, and incorporates results from this research in her undergraduate and graduate courses.