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。

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.