Studies on structure and dynamics of transient molecules by ultrafast molecular structure determination method

用飞秒激光脉冲相干激发超音速分子束中无碰撞条件下的吡嗪，并利用时间分辨光电子成像对由此产生的旋转波包的恢复进行了研究。在光电子强度和角分布上都可以看到明显的再生特征。Felker和Zewail用激光诱导荧光检测旋转相干光谱的理论完全解释了用泵浦激光脉冲光学制备S1吡嗪的特征，而从S1态通过系统间交叉产生的T1态的特征没有得到理论的完全再现。在吡嗪中，由于有20个三重态能级耦合到一个S1能级上，因此这些三重态振动能级之间旋转常数的微小差异可能会降低S1态在一个旋转周期内的恢复程度。原则上，T1中的旋转波包应该在较长的时间内完全复苏，这在我们的实验中无法测量到。本工作还获得了吡嗪分子随时间排列的光电子角分布的新数据。结果表明，只有在吡嗪完全排列时，光电子才会沿着N-N轴散射，而在吡嗪几乎完全随机排列时，这种特征完全消失。

Pyrazine under collision free conditions in a supersonic molecular beam was coherently excited by photoexcitation with a femtosecond laser pulse, and subsequent revivals of rotational wavepacket thus created were interrogated by time-resolved photoelectron imaging. The clear signature of revival was seen both in photoelectron intensity and angular distributions. The feature observed for S1 pyrazine that was optically prepared by a pump laser pulse was completely explained by the theory reported by Felker and Zewail for rotational coherence spectroscopy with laser induced fluorescence detection, whereas the feature of T1 states that are created via intersystem crossing from S1 state was not fully reproduced by theory. Since 20 triplet levels are coupled to a single S1 level in pyrazine, slight difference in rotational constant among these triplet vibronic levels presumably decreased the degree of revival in one rotational period of S1 state. In principle, rotational wavepacket in T1 should fully revive in a longer time period, this could not be measured in our experiment. Another new data obtained in the present work were photoelectron angular distribution measured for time-dependent molecular alignment of pyrazine. As a result, we have seen that photoelectrons are scattered along N-N axis only when pyrazine shows full alignment, and this signature completely disappeared at other time where pyrazine was almost completely randomly aligned.