Molecular Alignment In Laser Field

In this thesis, molecular alignment induced by short pulse laser is investigated theoretically by solving time-dependent Schrodinger equation numerically using the pseudospectral method based on rigid rotor model of molecule.First of all, the results obtained show that the field-free rotational wave-packet of the molecules is produced after the interaction between the short laser pulse and molecules, then N₂ and O₂ molecules will align periodically. The alignment parameter of the molecules depends on the temperature of the ensemble. Both N₂ and O₂ molecules are better aligned at lower temperature, while they are poorly aligned at higher temperature. The behavior can be attributed to the population of the angular moment in the rotational wave packets of molecules at different temperatures.Alignment of N₂ molecules induced by two short-pulse lasers is also investigated. In this case, our results show that field-free alignment of N₂ molecule induced with intensity 1.5×10¹⁴W/cm²and duration 50fs can be optimized by dividing the laser pulse into two-pulse of the same duration. The degree of alignment can be maximum with an intensity ratio 0.3636 at the appropriate delay time of the second pulse relative to the peak of the first pulse, which is in good agreement with experimental measurement.Pulse duration effect on alignment of N₂ molecule induced by double pulse laser is also investigated. The results show that alignment is affected greatly by pulse duration of the two pulses in the case of fixation for the laser intensity. For I₁ =5×10¹³W/cm²,I₂= 1×10¹⁴W/cm², the first laser duration t_wt=50 fs, and the second laser duration t_w2=140 fs, alignment can be maximized as long as the delay time between the two laser pulses is chosen appropriately.