Dynamics in Anion Clusters and Liquid Jets Using Ultraviolet and Extreme-Ultraviolet Time-Resolved Photoelectron Spectroscopy

在这个由化学部门化学结构动力学和机理（CSDM-A）项目资助的项目中，来自加州大学伯克利分校的Daniel Neumark教授使用超快激光脉冲来检测气相和水溶液中的化学反应。气相实验旨在了解受光激发的电子与核酸（NA）组分之间的相互作用；这种相互作用在辐射生物学中很重要，在细胞突变、癌症和细胞死亡中起着关键作用。水溶液中互补实验的重点是暴露于紫外光后核酸组分的变化。在这些实验中，紫外光激发分子中的电子，分子的稳定性取决于它们将光能转化为振动能的速度有多快（在反应发生之前，这些振动能被周围的溶剂分子耗散）。这些实验为这些松弛动力学提供了新的见解。除了对辐射生物学和其他医疗技术领域的影响外，这项研究项目还为下一代科学家提供培训。纽马克教授提倡女性科学家的培训，其最终目标是增加女性科学家在学术界的代表性。本项目主要利用飞秒时间分辨光电子能谱对气相离子配合物和液态水微射流的化学反应动力学进行互补研究。气相实验的目的是研究核碱基、核苷酸和核苷中的电子诱导化学。从碘离子阴离子（I-）与核酸组分结合的二元配合物开始，电子通过紫外线（UV）飞秒泵脉冲的激发从I-转移到核酸（NA）组分。随后的动力学是用飞秒探针脉冲光分离多余的电子，并测量得到的光电子动能分布。在液态水微射流实验中，飞秒高谐波产生（HHG）实验是利用光子能量高达100 eV的光电发射来研究溶液中反应动力学的一种非常通用的探针。例如，在这里，一种核酸成分被紫外泵脉冲激发，用HHG探针脉冲电离获得的时间分辨光电子能谱跟踪光激发物质的弛豫动力学，一直回到基态。人们还可以进行时间分辨的核心能级光电发射实验，该实验依赖于碘原子的四维电子结合能，显示出取决于其总体电荷和化学键环境的可测量的化学位移。这种方法被用于跟踪碘原子在溶液中电荷转移到溶剂激发后的电子复合。

In this project, funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Daniel Neumark from the University of California, Berkeley uses ultrafast laser pulses to examine chemical reactions in the gas phase and in aqueous solution. The gas phase experiments are aimed at understanding the interactions between electrons excited by light and nucleic acid (NA) constituents; this interaction is important in radiation biology and plays a key role in cell mutation, cancer, and cell death. Complementary experiments in aqueous solution are focusing on the changes of nucleic acid components subsequent to exposure to ultraviolet light. In these experiments, ultraviolet light excites the electrons in the molecules, and the stability of the molecules depends on how rapidly they can convert the light energy into vibrational energy (which can then be dissipated by the surrounding solvent molecules before reactions occur). The experiments are providing new insights into these relaxation dynamics. In addition to its implications for radiation biology and other areas of medical technology, this research project provides training for the next generation of scientists. Professor Neumark promotes the training of female scientists, with the ultimate goal of increasing the representation of women scientists in academia.This project focuses on complementary studies of chemical reaction dynamics in gas phase ion complexes and liquid water microjets by means of femtosecond time-resolved photoelectron spectroscopy. The goal of the gas phase experiments is to investigate electron-induced chemistry in nucleobases, nucleotides, and nucleosides. Starting with binary complexes of iodide anions (I-) bound to a nucleic acid constituent, an electron is transferred from the I- to the nucleic acid (NA) constituent through excitation by an ultraviolet (UV) femtosecond pump pulse. The subsequent dynamics are followed by photodetaching the excess electron with a femtosecond probe pulse and measuring the resulting photoelectron kinetic energy distribution. In the liquid water microjet experiments, femtosecond high harmonic generation (HHG)experiments are being developed as a very general probe of reaction dynamics in solution via photoemission at photon energies as high as 100 eV. Here, for example, a nucleic acid constituent is excited by a UV pump pulse, and time-resolved photoelectron spectra obtained by ionization with the HHG probe pulse tracks the relaxation dynamics of the photoexcited species all the way back to its ground state. One can also perform time-resolved core level photoemission experiments that rely on the 4d electron binding energy of atomic iodine exhibiting a measureable chemical shift that depends on its overall charge and chemical bonding environment. This method is being used to follow electron recombination with iodine atoms in solution subsequent to charge-transfer-to-solvent excitation.