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

在化学系化学结构、动力学和机制(CSDM-A)计划的支持下，加州大学伯克利分校的Daniel Neumark教授和他的研究小组正在进行探索核酸成分与紫外光相互作用的实验。所有的核酸在紫外光的光子能量下都有很强的吸收带，足以将这些物种解离为反应性自由基，这一过程将产生重大的负面生物学后果。然而，众所周知，电子弛豫与解离竞争，将电子激发的核酸迅速引导到它们的基电子态，并以无害的方式释放多余的能量作为热量。纽马克教授领导的团队将进行新颖的时间分辨实验，绘制出从光激发瞬间到基态分子最终产生的水溶液中核酸的电子松弛路径，从而为这些物种的电子松弛提供新的见解。这些研究的结果不仅对基础化学物理感兴趣，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，包括为提高女科学家的地位而进行的辅导工作，以及从科学界代表性不足的群体中招募研究学生。将在液态水微射流中进行时间分辨光电子能谱，其中溶质分子用紫外线泵浦脉冲激发，并使用22 eV的飞秒极紫外光脉冲进行电离。这些实验探索了单核苷酸和双核苷酸在4.7 eV附近的pi-pi*跃迁受光激发后的非绝热驰豫动力学。22 eV探测光子的能量足够高，不仅在UV激发形成的初始激发态中电离这些物种，而且在作为整体弛豫机制中间体的任何电子态以及基态中也能电离这些物种。因此，这些实验有可能对在光激发单核苷酸和双核苷酸的驰豫过程中起作用的电子态提供一个完整的动态映射。具体地说，研究小组将检查光激发的核苷酸碱基是否通过光学暗态松弛，还将探索光激发的二核苷酸中瞬时激基复合体的形成。该项目的第二个目标是产生约100 eV的飞秒脉冲，这是一个足以从卤素和过渡金属中射出核心电子的光子能量。这一能力的初步步骤将在当前的资助期内进行，以实现使用核心电离实现对溶剂分子激发态动力学的变革性新测量的长期目标。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.