Intersystem crossing of 2-Methlypyrazine studied by femtosecond photoelectron imaging

Abstract

The ultrafast nonadibatic relaxation dynamics of the excited state of 2-methylpyrazine has been studied by using femtosecond time-resolved photoelectron imaging and femtosecond time-resolved mass spectrometry. The first excited state S₁ of 2-methylpyrazine was excited by 323 nm pump light, and the excited state deactivation process is detected by 400 nm probe light. The lifetime of S₁ state 98 ps is obtained by time-resolved mass spectroscopy. The intersystem crossing from the S₁ state to the T₁ state is observed on real time. The relaxation dynamics of S₁ state of 2-methlypyrazine is different from that of pyrazine, the results show that the intersystem crossing process between S₁ and T₁ is the main relaxation channel of S₁ state of 2-methlypyrazine, but the internal conversion process between S₁ and S₀ is also a main relaxation channel of S₁ state. By using the advantages of femtosecond time-resolved photoelectron imaging, the photoelectron angular distribution at different pump-probe time delay was obtained experimentally. From the photoelectron angle distribution combined with photoelectron kinetic energy distributions, we tried to observe the field-free nonadiabatic alignment. However, due to the fact that the molecular symmetry of 2-methylpyrazine is lower than that of pyrazine, it is more challenging to observe the phenomenon of molecular nonadiabatic alignment with lower symmetry. Therefore, it is fail to observe nonadiabatic alignment feature of 2-methylpyrazine in this experiment. This work provides a clearer physical picture for S₁ state nonadibatic relaxation dynamics of 2-methylpyrazine.