Ultrafast Dynamics Of Cationic Electronic States Of Polyatomic Molecule By Strong-field Ionization

The investigation of electronic states and dynamics of polyatomic molecular cations is of great significance for understanding the microscopic mechanism of photochemical and photophysical processes in the fields of interstellar,environmental and combustion science.Since the pioneering work of femtosecond chemistry by Zewail,ultrafast time-resolved pump-probe studies have blossomed to reveal ultrafast dynamics of excited neutral molecules.However,our knowledge about electronic states of molecular cations is rather limit,partly due to the difficulty in producing high-number density of the ground state cations and combining with ultrafast time-resolved measurement.It contains abundant physical effects when molecules are ionized by strong laser field.Combined with strong field ionization and femtosecond time-resolved measurement,it provides a powerful tool for investigation of the electronic states and dynamics of molecular cationsIn our study,the cations are prepared by a strong 800nm laser field.A delayed weak 400 nm laser field is used to excite the cations to electronic excited states that ultimately undergo fragmentation.The transients of both parent and daughter ions are obtained by recording the ion yields as a function of the pump-probe delay time.The Strong field ionization-photofragmentation（SFI-PF）method is used to explore the coupling between electronic states,multi-orbit effect and dynamic process of polyatomic molecular cations including benzene,toluene and ethylene bromide.The following innovations have been achieved:1.We demonstrate the C²A_2u state as the one-400nm-photon resonant excited state from the ground state of benzene cations in SFI-PF.With the different initial population in electronic states of parent ion by the strong field multi-orbit effect,we observe an additional relaxation process in the transients of daughter ions with a timescale about 250 fs to 270 fs,which is attributed to the population transfer via conical interaction of B²E_2g-X²E_1gof benzene cations.2.We obtain the transients of both parent and daughter ions of toluene,which show ultrafast exponential decays and out-of-phase oscillations.It is indicated that lifetime induced by the internal conversion of D₁-D₀cationic states is about 530 fs and Fermi resonance between the vibrational states in D₁ and D₀ leading to oscillations with a period of about 2.05 ps.3.In the study of vinyl bromide by SFI-PF,an exponential growth component is observed in the transient of Br⁺or C₂H₂⁺,which attributes to the internal conversion of A²A′-X²A″of the cations in a time of about 200fs.Concerning the C₂H₃⁺daughter ions,the additional exponential growth component around 300 fs,indicates the dissociation of A²A′state of the cations.Our study would be helpful to improve our understanding of the interaction and dynamics of electronic states in polyatomic molecular cations.On this basis,further investigation of dynamics of non-adiabatic ionization with vibronic coupling and ultrafast dissociation and channel identification can be extended to multiple ionization or non-adiabatic ionization in infrared laser field combining with ultrafast time-resolved image technique.