中文摘要：

结合时间分辨的飞秒光电子影像（TRPEI）技术和时间分辨的质谱技术，研究了氯化苄（BzCl）分子内转换动力学过程．从光电子影像中获得了光电子动能分布和角度分布．氯化苄分子吸收两个400nm的光子后从基态跃迁到S4态和S2态．获得的母体离子随泵浦-探测时间延迟变化的曲线可以用两个指数函数进行拟合，包括一个时间常数为50fs的快速组分和一个时间常数为910fs的慢速组分．通过分析光电子动能分布随延迟时间的变化，我们认为分子被激发到S4态后在很短的时问内与S2态发生耦合迅速弛豫到S2态，然后再经内转换（IC）弛豫到S1态．最初布居的激发态分子经过内转换弛豫到S1态的时间尺度为50fs．910fs的慢速时间组分反映了分子弛豫到S1态后，经内转换向基态S0的弛豫．光电子角度分布的各向异性参数从零时刻的0．87增加到25fs时的0．94，然后逐渐减小到190fs时刻的0.59的现象，也反映了氯化苄分子从S4态耦合到S2态，然后内转换到S1态的动力学过程．

英文摘要：

The ultrafast internal conversion of benzyl chloride （BzCl） was studied with femtosecond time-resolved photoelectron imaging （TRPEI） coupled with time-resolved mass spectroscopy. Time-energy maps of the photoelectron intensity and the angular anisotropy were generated from a series of photoelectron images. Upon absorption of two 400 nm photons, benzyl chloride was excited to the S4 and S2 states at the same time. The time evolution of the parent ion with different pump-probe delays can be well described by biexponential decay. The fit yielded τ1=50 fs and τ2=910 fs. By analysis of time-resolved photoelectron kinetic energy distributions, it is concluded that the excited S, state has coupled with and decayed to the S2 state in a short time scale and then converted to the S1 state through ultrafast internal conversion （IC）, Within 50 fs, the molecule electronically relaxes into S1 through IC and from there, decays to the So ground state with the relatively slow time constant of 910 fs. The anisotropy parameters of photoelectron angular distributions changed from 0.87 at the delay time of 0 fs to 0.94 at 25 fs and then to 0.59 at 190 fs, which also reflects the coupling from the S, state to the S2 state and the following IC to the S1 state.