Theoretical Study On The Regulation Of LiH Molecular Orientation By Ultrafast Laser Pulses

The interaction between laser and molecules is one of the major study in photophysics and photochemistry.The process produces many physical phenomena,such as molecular population transfer,ionization,dissociation,alignment and orientation etc.Molecular alignment and orientation play an important role in the high harmonics generation,photoelectrons angular distribution and chemical reaction kinetics etc.The development of ultrafast laser pulse has driven the research of molecular alignment and orientation.In this thesis,the density matrix method is used to theoretically study the free-field orientation of LiH molecule by the combination of two-color femtosecond and THz laser pulses.The specific works are as follows:(1)First,we study the field-free molecular orientation by a two-color femtosecond laser pulse,with the strong hyperpolarizability LiH as the prototype molecule.After the irradiation of the two-color femtosecond laser pulse,LiH molecule was excited to not only the even rotational quantum number but also the odd rotational quantum number.Which is not possible for low hyperpolarizability molecules such as CO.The effects of the total intensity,pulse width,and center frequency of two-color femtosecond laser pulse as well as the relative intensity of the fundamental and second harmonics on the molecular population shift are discussed.Finally,it is found that the width of the two-color femtosecond laser pulse has almost no influence on the molecular orientation.We can effectively adjust the molecular population distribution by changing relevant parameters of two-color femtosecond laser pulse.(2)We added a THz laser pulse on the basis of the above.We use the combination of two-color femtosecond and THz laser pulses to control molecular orientation.First,the center frequency of the THz laser pulse is chosen according to the pre-excited population distribution by the two-color femtosecond laser pulse.The choice of the appropriate center frequency of the THz can significantly improve the molecular orientation degree.Then the width and carrier-envelope phase of the THz laser pulse as well as the delay time between the femtosecond laser pulse and THz laser pulse are adjusted to achieve high orientation degree.In addition,the temperature have an influence on molecular orientation.It is found that the orientation degree decreases sharply with increasing temperature.Finally,according to the population at room temperature,the parameters of laser pulses are re-optimized.And the maximal orientation degree is 0.22 at 300 K.It also shows that our scheme is also suitable for high temperature conditions.