Non-Adiabatic Dynamics in Liquid Jets Studied by Time-Resolved XUV Photoelectronic Spectroscopy

在化学系化学结构、动力学和机理a （CSDM-A）项目的支持下，加州大学伯克利分校的丹尼尔·纽马克教授和他的研究小组正在进行实验，探索核酸成分与紫外线的相互作用。所有的核酸在紫外光子能量下都有很强的吸收带，足以将这些物种解离成活性自由基，这一过程将产生显著的负面生物学后果。然而，众所周知，电子弛豫与离解相竞争，迅速将电子激发的核酸引导到它们的基电子状态，并以无害的热量释放多余的能量。由Neumark教授领导的团队将进行新颖的时间分辨实验，以绘制水溶液中核酸从光激发瞬间到最终产生基态分子的电子弛豫途径，从而为这些物种的电子弛豫提供新的见解。这些研究结果不仅对基础化学物理学有重要意义，而且对辐射化学和生物学也有潜在的影响。该项目的更广泛影响还包括为学生提供高级培训机会，包括寻求提高女性科学家地位的指导工作，以及从科学界代表性不足的群体中招募研究生。用紫外泵脉冲激发溶质分子，用飞秒极紫外（XUV）脉冲在22 eV下电离溶质分子，对液态水微射流进行时间分辨光电子能谱（TRPES）研究。这些实验探讨了单核苷酸和双核苷酸在4.7 eV左右的pi-pi*跃迁光激发后的非绝热弛豫动力学。22 eV探针光子的能量足够高，不仅可以在UV激发形成的初始激发态中电离这些物质，还可以在整个弛豫机制中作为中间体的任何电子态以及基态中电离这些物质。因此，这些实验有可能提供在光激发单核苷酸和双核苷酸弛豫中起作用的电子态的完整动态映射。具体来说，研究小组将研究光激发核碱基是否通过光学暗态松弛，并将探索光激发二核苷酸中瞬态杂化的形成。该项目的第二个目标是产生100 eV左右的飞秒脉冲，这种光子能量足以从卤素和过渡金属中射出核心电子。在当前的资助期内，将采取初步措施实现这一能力，以实现利用核心电离对溶剂化分子的激发态动力学进行变革性新测量的长期目标。该奖项反映了美国国家科学基金会的法定使命，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 support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) Program in the Division of Chemistry, Professor Daniel Neumark and his research group at the University of California-Berkeley are carrying out experiments that probe the interaction of nucleic acid constituents with ultraviolet light. All nucleic acids have strong absorption bands in the ultraviolet at photon energies that are sufficient to dissociate these species into reactive free radicals, a process that would have significant negative biological consequences. However, electronic relaxation is known to compete with dissociation, rapidly channeling the electronically excited nucleic acids to their ground electronic states and releasing the excess energy harmlessly as heat. The team led by Prof. Neumark will perform novel time-resolved experiments to map out the pathways for electronic relaxation of nucleic acids in aqueous solution from the instant of photoexcitation to the ultimate production of ground state molecules, thus providing new insights into the electronic relaxation of these species. The results of these studies are not only of interest in fundamental chemical physics but also have potential impact in radiation chemistry and biology. The broader impacts of the project also include advanced training opportunities for students, including mentoring efforts that seek to enhance the advancement of female scientists and the recruitment of research students from groups that are underrepresented in science.Time-resolved photoelectron spectroscopy (TRPES) in liquid water microjets will be carried out in which solute molecules are excited with ultraviolet pump pulses and ionized using femtosecond extreme ultraviolet (XUV) pulses at 22 eV. These experiments probe the non-adiabatic relaxation dynamics of mono- and di-nucleotides subsequent to photoexcitation of their pi-pi* transitions around 4.7 eV. The energy of the 22 eV probe photons is sufficiently high to ionize these species not only in the initially excited states formed by UV excitation, but also in any electronic states that serve as intermediates in the overall relaxation mechanism as well as in the ground state. Hence, these experiments have the potential to provide a complete dynamical mapping of the electronic states that play a role in the relaxation of photoexcited mono- and di-nucleotides. Specifically, the research team will examine whether photoexcited nucleobases relax through optically dark states and also will probe the formation of transient exciplexes in photoexcited dinucleotides. A secondary goal of this project is to generate femtosecond pulses around 100 eV, a photon energy sufficient to eject core electrons from halogens and transition metals. Initial steps toward this capability will be undertaken during the current grant period toward the longer-term goal of enabling transformative new measurements of the excited-state dynamics of solvated molecules using core ionization.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.