Femtosecond studies of the influence of solvent on chemical reaction dynamics

在这项由化学系化学结构、动力学和机制计划资助的奖项中，南加州大学的斯蒂芬·布拉德福斯教授和他的学生将阐明溶液相小分子中超快电离、电荷转移和自旋交叉的途径。宽带瞬时吸收和液体微束时间分辨光电子能谱的结合将被用来理解溶剂在控制激发态动力学中所起的作用。这种溶剂有望调节溶质的电子结构，稳定电荷分离状态，并修改控制光化学分支的锥形交叉点的方法。在这个框架中，通过绘制和关联这些DNA碱基的起始电离与非极性溶剂、气相和理论中表现出的解离反应活性，将寻求对水中鸟嘌呤和腺嘌呤的光氧化的机理理解。此外，激发态自旋交叉有机铁络合物中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，后者对配位场状态唯一敏感。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，以扩大稳态和瞬时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界将紫外线波长与细胞DNA损伤形成联系起来的努力。此外，这里应用于自旋交叉系统的新的光谱工具应该有助于优化利用地球上丰富的金属进行太阳能转换的有机金属发色团。在皮？S实验室对博士、博士后和本科生的培训将培养新一代物理学家，使他们能够设计和分析使用当代物质光谱探测器的实验。通过合作网络，学生们将有大量机会与国内外的国际科学家合作。布拉德福斯将继续扩大参与计划，并取得成功，为洛杉矶县社区学院的学生提供为期8周的暑期实习机会，主题是光能转换研究。在参与该项目的各级都将强调不同群体的参与和广泛的合作。

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.