ICC: Disentangling deactivation pathways in DNA bases and model systems using time-resolved photoelectron spectroscopy and three pulse techniques

虽然已经有大量的实验旨在研究生物分子的光阻特性，但由于观察到的结果越来越复杂，加上对理论模拟的计算要求越来越高，这一领域的最新进展一直受到阻碍。飞秒时间分辨光电子能谱(TRPES)为研究光诱导过程提供了独特的能力，并已成功地应用于小多原子分子。当扫描泵浦脉冲和探测脉冲之间的延迟时，通过两步激发-电离方案获得光电子光谱。获得的二维数据提供了关于分子系统演化的双重信息：参与驰豫和衰变相关光电子光谱的电子激发态的寿命，用于光谱鉴定。在将这项技术应用于生物分子等尺寸和复杂性不断增加的分子的过程中，下列挑战变得明显：(1)由于多个自由度和越来越多的低激发态，弛豫过程是复杂的。(2)发生在类似的超快时间尺度上并呈现光谱重叠特征的过程几乎不可能解开。Ullrich/Stavros的实验利用了两种互补的光谱技术，具有高度的协同性，这将使对弛豫路径的明确识别与波长相关的研究成为可能。该项目的预期结果包括两个方面：(1)详细的分子水平了解这些分子的光物理，包括竞争路径、失活通道的集合，以及结构修改对动力学的影响；(2)设计三个脉冲TRPES实验的关键信息，例如预期的信号能级和使用固有的宽带飞秒脉冲激发特定反应坐标和窄谱线的可行性。虽然这项建议侧重于实验，但从头计算研究也将极大地受益于在这种逐步方法中获得的光谱细节水平。由于有关研究所的大部分设备已经到位，所有实验将由本地学生和来访学生在PIS实验室长期停留期间进行，他们将在数据分析和结果公布期间继续合作。这两个私人投资机构一直在将本科生和少数族裔学生纳入他们的研究，这个项目也不会例外。除了科学方面的合作，学生们还将从接触不同的学术环境和外国文化中受益。该项目还寻求交流私人投资机构在外联活动中的经验，以便在其本国机构开展类似的活动。例如，一名前英国教师(现在是华威的外展协调员)将参加佐治亚州教师实习计划，该计划将高中教师置于研究环境中，旨在将他们的经验整合到教学计划中。作为交换，UGA物理将了解华威高中的化学路演及其电子学习方法，以向更广泛的社区受众宣传研究。

While there has been a large flurry of experiments aimed at studying the photoresistive properties of biomolecules, recent progress in this field has been hampered by the growing complexity of the results observed, coupled to the increasing computational demands on theoretical simulations. Femtosecond (fs) time-resolved photoelectron spectroscopy (TRPES) provides unique capabilities for studying photoinduced processes and has successfully been applied to small polyatomic molecules. Photoelectron spectra are obtained via a two-step excitation-ionization scheme as the delay between the pump and probe pulses is scanned. The two-dimensional data obtained provides two-fold information on the evolution of the molecular system: lifetimes of the electronically excited states participating in the relaxation and decay associated photoelectron spectra for their spectroscopic identification. In the endeavour of applying this technique to molecules of increasing size and complexity such as biomolecules the following challenges become apparent: (1) Relaxation processes are complex due to the many degrees of freedom and an increasing number of low-lying states. (2) Processes that occur on similar, ultrafast time-scales and exhibit spectrally overlapping features are almost impossible to disentangle. The experiments by Ullrich/Stavros make use of two complimentary spectroscopic techniques at a high level of synergy that will allow wavelength-dependency studies with unambiguous identification of relaxation pathways. The expected outcome of this project is two-fold: (1) a detailed molecular level understanding of the photophysics of these molecules including competing pathways, onsets of deactivation channels, and the effect of structural modifications on the dynamics, (2) critical information for the design of three pulse TRPES experiments, such as expected signal levels and the feasibility of using inherently broadband fs pulses for excitation of specific reaction coordinates and narrow spectral lines. While this proposal focuses on experiments, ab initio computational studies will also tremendously benefit from the level of spectroscopic detail obtained in this stepwise approach. As most of the equipment is already in place at the respective institutes, all experiments will be performed by local and visiting students during extended stays in the PIs' laboratories and their collaboration will continue throughout data analysis and publication of their results. Both PIs have been integrating undergraduate and minority students in their research and this project will be no exception. Beyond the scientific aspect of this collaboration students will benefit from the exposure to a different academic environment and foreign culture. The project also seeks to exchange the PIs' experiences in outreach activities with the intention of implementing similar activities at their home institutions. For example, a former UK teacher (now outreach coordinator at Warwick) will participate in the Georgia Internship for Teachers program that places high school teachers into a research environment and aims at integrating their experiences into a lesson plan. In exchange UGA Physics will learn about Warwick's chemistry road shows for high schools and their E-learning approaches to publicize research to audiences in the wider community.