Photodissociation and Reaction Dynamics of Molecular and Cluster Anions Using Femtosecond Photoelectron Spectroscopy

加州大学伯克利分校的丹·纽马克得到了实验物理化学计划和多学科活动办公室的支持，利用他最新开发的时间分辨实验技术--阴离子飞秒光电子能谱(FPES)来研究负离子的光解离和反应动力学。具体地说，他将使用这种方法来探索控制孤立和簇状负离子中光解离和双分子反应过程的势能面。在负离子FPES中，飞秒泵浦激光激发负离子到解离电子态；产生的演化系统用飞秒探测脉冲光分离，并测量光电子能谱。作为泵浦-探测延迟的函数的光电子能谱产生了由探测脉冲引发的动力学图像。Neumark将首先将FPES应用于孤立的阴离子，然后应用于大小选定的簇中的阴离子发色团。要研究的体系包括用二氧化碳或水溶解的碘二聚体阴离子簇，裸露或簇状的碘三聚体阴离子，臭氧阴离子和碳酸盐阴离子，以及时间分辨的阴离子-分子反应。负离子在大气和溶液相化学中起着重要的作用。这项研究的结果将立即应用于化学动力学和光谱学，随后将应用于对广泛的物理化学现象进行模拟，例如大气污染以及光学和无线电波在正常和扰动大气中的传输。此外，这些研究应该对溶液中化学反应的性质有新的认识，这些方法最终可能对蛋白质动力学的研究有用。

Dan Neumark of the University of California at Berkeley is supported by the Experimental Physical Chemistry Program and the Office of Multidisciplinary Activities to use his newly developed time-resolved experimental technique, anion femtosecond photoelectron spectroscopy (FPES) to investigate the photodissociation and reaction dynamics of negative ions. Specifically, he will use this method to probe the potential energy surfaces governing photodissociation and bimolecular reaction processes in isolated and clustered negative ions. In anion FPES, a femtosecond (fs) pump laser excites a negative ion to a dissociative electronic state; the evolving system that results is photodetached with a fs probe pulse and the photoelectron spectrum is measured. The photoelectron spectrum as a function of pump-probe delay yields a picture of the dynamics initiated by the probe pulse. Neumark will first apply FPES to the isolated anion, then to the anion chromophore in a size-selected cluster. Systems to be investigated include clusters of iodine dimer anions solvated with carbon dioxide or water, bare or clustered iodine trimer anions, ozone anions and carbonate anions, and time-resolved anion-molecule reactions. Negative ions play important roles in atmospheric and solution phase chemistry. Results of this research will find immediate application in chemical kinetics and spectroscopy, with subsequent application in modeling a wide range of physico-chemical phenomena, such as atmospheric pollution and transmission of optical and radio frequency waves in the normal and perturbed atmosphere. In addition, these studies should shed new light on the nature of chemical reactions in solution, and the methods may eventually be of utility in the study of protein dynamics.