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

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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.