Research On The Alignment And High Order Harmonic Generation Of Linear Molecules By Ultrashort Laser Pulses

Alignment of linear molecules by ultrashort laser pulses has great effects on the ionization and high-order harmonic generation (HHG) in laser fields. The dissertation aims to calculate and analyze the alignment of linear molecules by femtosecond laser pulses in theory, and study the HHG from linear molecules by using molecules of CO₂ as samples in ultrashort pulse laser fields in experiment.When a molecule or an ensemble of molecules is excited by a linearly polarized ultrashort laser pulse, it can create broad time-evolution wave packets. The coherent superim-position of rotation eigenstates of wave packets makes a molecule or an ensemble of molecules align periodically along the polarizing direction of laser field. The alignment of nitrogen molecules and an ensemble of nitrogen molecules, carbon monoxide molecules and carbon dioxide molecules by one single linearly polarized laser pulse has been calculated, and the effects of laser intensity, initial rotational temperatures of molecules, odd state and even state on the alignment of molecules have been analyzed.A ultrashort laser pulse train can enhance the alignment of molecules. The dissertation has studied the alignment of nitrogen molecules and an ensemble of nitrogen molecules and carbon monoxide molecules by a train including two femtosecond laser pulses, and analyzed the effects on the alignment by the direction of laser that is parallel or perpendicular to an ensemble of molecules and laser intensity when the second laser pulse arrives.The alignment by an elliptically polarized laser pulses is different from a linearly polarized laser pulse. The dissertation has analyzed the influence of eccentricity and laser intensity on the alignment of molecules, and exhibited intuitively the alignment of molecules by using angle distribution of molecule wave functions. The results show that distribution probability of molecules that appears near Z axis and XOZ plane is largest.In Experiment, HHG from linear molecules by using molecules of CO₂ as samples in a static garget has been studied. The experiment results show that gas pressures and the position of gas cell have great influences on HHG from CO₂.Finally, the dissertation sums up all of the research work.