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

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在化学系化学结构、动力学和机制-A（CSDM-A）项目的支持下，正在进行探测核酸成分与紫外光相互作用的实验。 所有核酸在紫外线中具有强吸收带，其光子能量足以将这些物质解离成活性自由基，这是一个具有显著负面生物学后果的过程。 然而，已知电子弛豫与解离竞争，将电子激发的核酸快速引导到其基态电子状态，并无害地释放多余的能量作为热量。Neumark教授领导的团队将进行新颖的时间分辨实验，以绘制出从光激发瞬间到最终产生基态分子的水溶液中核酸电子弛豫的途径，从而为这些物种的电子弛豫提供新的见解。 这些研究结果不仅在基础化学物理领域具有重要意义，而且在辐射化学和生物学领域也具有潜在的影响。该项目的更广泛影响还包括为学生提供高级培训机会，包括旨在提高女科学家地位的辅导工作，以及从科学界代表性不足的群体中招募研究生。在液态水中，将进行微喷射，其中溶质分子被紫外泵浦脉冲激发，并使用飞秒极端电离。紫外（XUV）脉冲，22 eV。 这些实验探测单和二核苷酸的非绝热弛豫动力学后，光激发他们的π-π * 跃迁约4.7 eV。 22 eV的探测光子的能量足够高，以使这些物质不仅处于由UV激发形成的初始激发态，而且处于作为整体弛豫机制中的中间体以及处于基态的任何电子态。 因此，这些实验有可能提供一个完整的动态映射的电子状态，发挥作用的光激发的单和二核苷酸的放松。 具体来说，研究小组将研究光激发的核碱基是否通过光学暗态弛豫，并探测光激发的二核苷酸中瞬态激基复合物的形成。该项目的第二个目标是产生大约100 eV的飞秒脉冲，这是一种足以从卤素和过渡金属中发射核心电子的光子能量。在目前的资助期内，将朝着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，采取初步措施。该奖项反映了NSF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.