Ionization And Cationic Dynamics Of Vinyl Bromide In A Femtosecond Intense Laser Field

The strong field–atom/molecule interaction is now a hot topic in the field of atomic and molecular physics.Strong-field ionization(SFI)is viewed as the fundamental process of many atomic or molecular strong field physical phenomena.Owing to their additional nuclear degrees-of-freedoms(vibrations and rotations),diverse geometric structure,complicated electronic states and alignment/orientation,the SFI of molecules is more complicated than that of atoms.The knowledge about molecular SFI would be of value to understand the physical mechanisms of the interaction between molecules with strong laser fields.In this study,we investigate SFI of vinyl bromide(C₂H₃Br)molecules using a time-of-flight mass spectrometer in 400 nm and 800 nm strong laser fields,and we also utilize femtosecond time-resolved strong field ionization-photo fragmentation(SFI-PF)method to study the electronic states evolutionary process of C₂H₃Br⁺cations.This study would shed some light on SFI and dissociation mechanism of vinyl bromide molecules and the ultrafast evolution dynamics of excited states of C2H3Br⁺cations.The main results of the study are summarized as following:1)We investigate the dependence of SFI and fragmentation of C₂H₃Br in 400nm and 800 nm strong laser fields.The angular distributions of the fragments are obtained with varying laser intensity and ellipticity.The results show that in the 400 nm laser field with the laser intensity range used in which we studied occurs mainly via the(2+2)resonance enhanced multiphoton ionization process.The angular distribution of fragment ions show obvious anisotropy,which may be caused by the geometrical alignment effect of the HOMO.In the 800 nm laser field,with the increase of the charge state,the dependence of the fragment ions on the ellipticity becomes more obvious.This shows the importance of the re-collision process in the ionization and dissociation process of C₂H₃Br molecules.2)The SFI-PF method is used to investigate the dynamic of the electronic state of vinyl bromide cations.Our results demonstrate experimentally the C²A'state as the one-400 nm photon resonant excited state.Under high SFI pulse energy,with the transient of the delay time of the two laser beams,the yield of fragment Br⁺exhibits an additional relaxation process with a time scale about 220 fs,which is interpreted in terms of population transfer via conical intersections of A²A'-X ²A"state of C₂H₃Br⁺ion.And the yield of fragment C₂H₃⁺exhibits an additional exponential increase process with a time scale about 300 fs which is interpreted in terms of the direct dissociation of the A²A'state.