The Ionization/Dissociation Of Triatomic Molecules In A Femtosecond Laser Field With Different Polarizations

The interaction of laser and material is one of the hot topic and frontier problem in the research of atomic and molecular optical physics.Under the action of femtosecond strong laser fields,atoms and molecules generate some nonlinear strong-field physical phenomena which are of great significance to the development of emerging discipline such as attosecond physics and quantum control.Strong field ionization is the most basic and important step in these strong field physical phenomena.Study on the strong field ionization is helpful to understand and reveal the new mechanism of the interaction between atoms molecules with strong laser fields.Compared with atoms,molecules have more diverse geometrical structures,additional nuclear degree-of-freedom and complex molecular orbitals,which make the dynamics of molecular strong field ionization become more complex,it has been proved that some effect like multi-centers interference,contributions from multiple orbitals and motion of molecular nuclear have effect on the molecular strong field ionization.This promotes scholars devoted to more detailed experimental and theoretical researches to reveal new phenomena and effects in the interaction of femtosecond laser and molecules.In this dissertation,the strong field ionization and fragmentation of some triatomic molecules with different structures CO₂,OCS,N₂O and NO₂ in a strong800nm 50fs laser field were measured by a time-of-flight mass spectrometry.According to the yields dependence of all ions on the laser parameters,combined with theoretical analysis,several factors that affecting the strong field ionization of the molecules were studied.This study is very important to understand the mechanism of strong field ionization for triatomic molecules,further learn the ionization and related physical processes of polyatomic molecules in the strong laser fields.The dissertation mainly obtained the following results:1)The angular distributions of the parent ions and all fragment ions of CO₂,OCS,N₂O and NO₂ molecules under the femtosecond laser field were studied.It is found that the terminal atoms are mainly emitted along the polarization direction of the laser field,while the angular distributions of the central atoms are totally different.The C ions from CO₂ have an elliptical angle distribution in the vertical direction,and the C ions from OCS exhibit the four-leaf pattern.The angular distributions of the N ions from N₂O and NO₂ molecules are both orthogonal which have maxima in both the parallel and vertical directions.Analysis shows that the strong anisotropy in the angular distribution of the terminal atoms originated from the geometric alignment of the molecular HOMO,and the angular distributions of the central atom depend not only on the molecular structure and electronic orbitals,but also on the contribution of low-lying orbitals to the strong field ionization.2)The dependence of the different ions yields from N₂O molecule on the laser intensity and ellipticity in a 50fs,800nm strong laser field were measured.It is found that tunneling ionization is dominant for the strong field ionization of N₂O in the range of 2.0×10¹³W/cm² to 3.6×10¹⁴W/cm².The yields of fragment ions depend on the laser ellipticity dramatically,and the degree of dependence increases with charge of ions.Analysis shows that the multiply charged ions are mainly formed by the dissociation or Coulomb explosion of the multi charged parent ions through non-sequential double/multi ionization,and laser-induced electron re-scattering process is very important for the formation of multiply charged ions.