Femtosecond studies of the influence of solvent on chemical reaction dynamics

该奖项由化学部门化学结构、动力学和机制项目资助，南加州大学的Stephen Bradforth教授和他的学生将阐明液相小分子的超快电离、电荷转移和自旋交叉的途径。宽带瞬态吸收和液体微射流时间分辨光电子能谱的结合将用于了解溶剂在控制激发态动力学中的作用。该溶剂有望调节溶质电子结构，稳定电荷分离状态，并改变控制光化学分支的锥形交叉点的方法。在此框架下，将通过绘制和关联这些DNA碱基电离的开始与在非极性溶剂、气相和理论中表现出的解离反应性，来寻求对水鸟嘌呤和腺嘌呤光氧化的机制理解。此外，激发态自旋交叉有机铁配合物的中间态级联将通过时间分辨光学和光电子探针的组合来解开，后者对配体场态具有独特的敏感性。这些特定的化学研究将通过构建极紫外飞秒脉冲源来补充，以扩大稳态和瞬态光电子能谱的电子结合能范围。该项目揭示的基本机制将影响光生物学社区将紫外线波长与细胞DNA中病变形成联系起来的努力。此外，在自旋交叉系统中应用的新光谱工具应该有助于优化有机金属发色团，利用地球上丰富的金属进行太阳能转换。PI博士、博士后和本科生的培养美国的实验室将培养新一代的物理科学家来设计和分析使用现代物质光谱探针的实验。学生将有很多机会通过合作网络与国内外的国际科学家一起工作。Bradforth将继续扩大参与项目，为洛杉矶县社区大学学生提供为期8周的暑期实习，主题是光能转换研究。将在参与项目的各级强调不同群体的参与和广泛合作。

In this award, funded by the Chemical Structure, Dynamics and Mechanisms Program of the Chemistry Division, Professor Stephen Bradforth of the University of Southern California and his students will elucidate pathways for ultrafast ionization, charge transfer and spin crossover in solution phase small molecules. A combination of broadband transient absorption and liquid-microjet time-resolved photoelectron spectroscopy will be used to understand the role that solvent plays in controlling excited state dynamics. The solvent is expected to modulate solute electronic structure, stabilize charge-separated states and modify the approach to conical intersections that control photochemical branching. In this framework, a mechanistic understanding for the photooxidation of aqueous guanine and adenine will be sought by mapping and correlating the onset for ionization of these DNA bases with the dissociative reactivity exhibited in non-polar solvents, the gas phase and theory. In addition, the cascade of intermediate states in excited-state spin crossover organo-iron complexes will be unraveled through a combination of time-resolved optical and photoelectron probes, the latter being uniquely sensitive to ligand field states. These specific chemical studies will complemented by construction of a source of extreme UV femtosecond pulses to expand the electron binding energy range for both steady-state and transient photoelectron spectroscopy. The fundamental mechanisms revealed in this project will impact efforts in the photobiology community to correlate UV wavelength and lesion formation in cellular DNA. In addition, the new spectroscopic tools applied here to spin-crossovers systems should help to optimize organometallic chromophores utilizing earth-abundant metals for solar energy conversion. Training of PhD, postdoctoral and undergraduate students in the PI?s lab will prepare a new generation of physical scientists in designing and analyzing experiments that use contemporary spectroscopic probes of matter. Students will have abundant opportunities to work with international scientists both here and abroad through a network of collaborations. A broadening participation program with proven success will be continued whereby Bradforth offers Los Angeles county community college students 8-week summer internships themed on light-to-energy conversion research. Participation of diverse groups and extensive collaboration will be emphasized at all levels of involvement in the project.