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

吡嗪在无碰撞条件下，在超声分子束相干激发光激发与飞秒激光脉冲，和随后的复苏，从而创建的旋转波包的时间分辨光电子成像询问。在光电子强度和角分布上都可以看到明显的复苏信号。通过泵浦激光脉冲光学制备的S1吡嗪的观察到的特征完全由Felker和Zewail报道的用于激光诱导荧光检测的旋转相干光谱的理论来解释，而从S1状态通过系间穿越产生的T1状态的特征没有被理论完全再现。由于20三重态能级耦合到一个单一的S1水平在吡嗪，这些三重态电子振动能级之间的转动常数的微小差异，推测减少了在一个旋转周期的S1状态的复苏程度。原则上，T1中的旋转波包应该在一个较长的时间段内完全恢复，这在我们的实验中无法测量。本工作中获得的另一个新数据是吡嗪分子随时间变化的光电子角分布。结果表明，只有当吡嗪完全取向时，光电子才沿沿着N-N轴散射，而当吡嗪几乎完全随机取向时，光电子的散射信号完全消失。

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.