大气VOCs的飞秒量级超快系间窜越动力学研究

The intersystem crossing induced by spin-orbit coupling changes the direction of the spin typically occurring at nanosecond and microsecond time scales. In recent years, the intersystem crossing processes in tens of femtoseconds are discovered in the atmospheric VOCs, which becomes a mystery and the interpretation challenges the widely accepted view. It is very necessary to explore the deep-level mechanism of such physical phenomenon. In this project, based on our preliminary observation of ultrafast intersystem in xylenes, we plan to employ a tunable ultraviolet (UV) femtosecond laser pulse for further investigations of femtosecond ultrafast intersystem crossing processes on a series of aromatic hydrocarbons. In the pump-probe photoelectron imaging experiment, a tunable UV-femtosecond laser is used to prepare the excited state, and the excitation wavelength is scanned to track the process of growing out from nothing for the observed intersystem channel. And the other femtosecond pulse is employed by ionization the prepared excited states for the probing with different time delay to obtain time-resolved photoelectron imaging. The intersystem dynamic can be tracked by such multi-dimensional time-resolved photoelectron imaging. At the same time, three kinds of photoionization detection technique of the radical fragments are carried out as the followings. Synchrotron VUV beam is employed for the measurement of the appearance energy. A femtosecond pulse is used for time-resolved detection of the rising time of the radical fragments, and a nanosecond REMPI pulse is employed for state-resolved of the radical fragments with obtaining the velocity map imaging. Moreover, VUV photoelectron spectra via tunable synchrotron radiation are also performed to obtain the accurate energy level of the ionic states to ensure the accurate analysis of the involved dynamics. From the perspective of multi-dimensional and high-precision, a variety of atmospheric VOCs are investigated systematically to reveal its deep physical and chemical mechanism of femtosecond ultrafast intersystem crossing.

由自旋轨道耦合引起的系间窜越涉及多重度的变化，一般发生在纳秒和微秒时间尺度，而近年在大气VOCs发现了几十飞秒量级的超快系间蹿越过程，极大挑战了传统观念。本项目基于二甲苯观测到的超快系间窜越的前期工作，将可调谐紫外飞秒激光脉冲运用在泵浦-探测光电子成像实验中，一束可调谐紫外飞秒激光制备激发态，并扫描激发波长，探索系间窜越通道由无到有的过程，另一束飞秒激光电离激发态，得到时间分辨光电子影像，追踪系间窜越的动力学过程。同时对产生的自由基碎片进行三种方案光电离探测：利用同步辐射VUV光源测量碎片出现势；利用飞秒脉冲测量碎片自由基生成时间；利用纳秒脉冲对碎片进行态分辨探测。并且联合同步辐射VUV光电子能谱技术获得准确的离子态能级，确保对动力学过程进行准确分析。从多维角度高精度并系统性地研究多种大气VOCs的飞秒量级超快系间窜越过程，揭示其深层次的物理和化学机理。

由自旋轨道耦合引起的系间窜越涉及多重度的变化，一般发生在纳秒和微秒时间尺度，而近年在大气VOCs发现了几十飞秒量级的超快系间蹿越过程，极大挑战了传统观念。本项目联合激光光电离和光解质谱成像技术和光谱技术（飞秒脉冲、纳秒脉冲）以及同步辐射VUV光电子能谱技术，聚焦于大气VOCs的光物理和光化学动力学研究。基于该研究目标，我们建立大气VOCs的在线探测方法和技术，基于激光诱导击穿光谱、单颗粒气溶胶质谱与激光拉曼光谱技术搭建了一套针对大气中VOCs的激光在线探测系统。利用自主开发的便携式拉曼光谱仪对上述VOCs样品进行分子结构的探测，通过拉曼光谱分析获取了各种VOCs分子的“光谱指纹”。再结合第一性原理计算，解析其拉曼指纹光谱对应的分子振动模式，成功实现探测卤代烷烃类VOCs分子的不同构象，为VOCs分子的结构探测与分析提供理论支撑。同时开展大气VOCs分子理论计算研究，采用第一性原理方法计算和构造高精度激发态势能面，并通过计算模拟分子解离动力学，为实验结果的分析提供重要的理论信息。理论与实验相结合，可以更清晰地阐明大气VOCs的动力学机理。并结合实验开展基于离子速度成像技术的VOCs分子光解离（飞秒激光脉冲和同步辐射）研究，通过离子速度成像技术，结合飞秒激光脉冲和同步辐射两种光源研究了VOCs分子光解离光电离动力学，特别是着重研究了重要的VOCs，如呋喃、氟溴苯、碘乙烷等VOCs分子的光解离光电离动力学。结合飞秒泵浦探测技术，采用飞行时间质谱技术研究大气VOCs在激光诱导下产生碎片离子的质谱以及随时间延迟到瞬态信号，采用光电子成像技术研究大气VOCs在激光诱导下时间分辨的光电子影像，反演光电子动能分布和角度分布，获得详尽的超快光物理和光化学动力学信息。发表高质量学术论文17篇（全部第一标注），其中中科院一区和二区论文8篇；授权专利4项（1项发明专利）；申请软件著作权6件；出版专著2部，培养博士研究生1名，硕士研究生6名，博士后2名。