Controlling Molecular Orientation By Two-color Slow Turn-on And Rapid Turn-off Laser Pulses

The process of interaction between laser and matter will produce a series of physical phenomena such as molecular alignment,orientation,ionization,dissociation,population transfer,etc.Molecular alignment and orientation are regarded as key prerequisites for laser-controlled molecular dynamics studies in many other directions,such as the high harmonics generation,attosecond physics,photoelectrons angular distribution,ionization,and dissociation.In this thesis,the molecular ensemble orientation induced by the laser pulse has been studied by the density matrix theory.The main works are summarized as follows:（1）The molecular orientation induced by two-color slow turn-on and rapid turn-off laser pulses is theoretically investigated.In the double-well potential,the energy levels located in the deep place are in a nearly degenerate state and appear in pairs.When the adiabatic evolution is completed,the molecular orientations generated by this pair of instantaneous eigenstates are just the opposite.It explains why there is a significant decrease in molecular orientation when the temperature is considered.Meanwhile,the effect of temperature on the orientation of the molecular also depends on the molecular parameters.High degrees of orientation can be easily achieved for molecules with a large △α/B_e（Ratio of the anisotropic polarizability of the molecule to the rotational constant）even under relatively low laser peak intensities.For molecules with larger B_e values,the degree of orientation decreases more rapidly with increasing temperature.The rise time σ_r of the two-color slow turn-on and rapid turn-off laser pulses is a key parameter to maintaining adiabatic evolution,and a small fall time σ_f results in a high degree of field-free orientation.The decreasing trend of <cosθ>_max as a function of time is the same at different temperatures.For molecules with smaller B_e,the orientation of the molecular is more sensitive to the fall timeσ_f.（2）The combination of THz positive chirped laser pulse and two-color slow turn-on and rapid turn-off laser pulses is used to improve the orientation of the HCN molecular.Firstly,the THz positive chirped laser pulse is used for preexcitation,so that after the individual adiabatic evolution of the molecular,each instantaneous eigenstate has the same sign of orientation degree,and then the two-color slow turn-on and rapid turn-off laser pulse is used to improve the orientation of the molecular.However,since the rotations of J≠0 is multiple degenerates,it is difficult to realize the above process only by adiabatic population transfer.As a result,the orientation degree of the HCN molecular did not improve after the combined pulse action.