Theoretical And Experimental Study On Molecular Alignment And Harmonic Emission Of Diatomic Molecules

High-order harmonic generation (HHG) from aligned molecules in strong laser fields of femtosecond duration has attracted increasing interest due to the extra degrees of freedom and nonspherical symmetry of molecules as compared to atoms. Therefore current study of harmonic emission of aligned molecules is motivated by the properties of molecules which may lead to richer physics phenomena and a higher degree modulation of HHG. Firstly in this paper the influences of the highest occupied molecular orbitals (HOMOs) and rotational temperature on molecular spatial alignment are discussed. Secondly based on intense field ionization and time-dependent Schr?dinger equation we present two innovative methods of calculating the harmonic emission of diatomic molecule. One is the valence orbital method which not only can describe the properties of molecular alignment-dependent harmonic emission from diatomic molecule, but also can analyze the contributions to harmonic emission from different valence orbitals. The other is the molecular wave function method which can precisely calculate the harmonic emission with different initial condition including the rotational temperature, spatial alignment and the excited electronic states. The comparisons are carried out between theoretical calculations of molecular wave function method and a series of experimental results.Firstly molecular rotational excitation and field-free spatial alignment in a nonresonant intense laser field are studied numerically and analytically by using time-dependent Schr?dinger equation. The broad rotational wave packets excited by the femtosecond pulse are defined in the conjugate angle space, and their coefficients are obtained by solving a set of coupled linear equations. The temperature-dependent properties of angular distributions of CO and O2 molecules are obtained through analyzing the influences of the HOMOs on molecular spacial alignment.Secondly we present a valence orbital method for high-order harmonic generation from diatomic molecule with arbitrary orientation by using space rotation operator. We evaluated the effects of each valence orbital of N2 and O2 on harmonic emission with different molecular alignment in detail. The calculation results show that the bonding orbital of N2 decides the maximum of harmonic emission when the molecular axis of N2 is aligned parallel to laser vector. For O2 molecule the two antibonding orbitals contribute the maximum of harmonic yield and two bonding orbitals slightly influence the alignment angle of maximum of harmonic radiation not exactly at 450 which confirm the experimental results in references. Furthermore we also present a method of molecular wave functions for calculating harmonic emission of diatomic molecules based on time-dependent Schr?dinger equation. The molecular wave functions are superposed by a series of wave functions of electron terms weighted by time-dependent coefficients for different molecular orbitals, rotational and vibrational levels. The molecular alignment and rotational temperature dependent properties of harmonic emissions are calculated in detail when the initial conditions are excited states or mixed states. We put forward the theory of manipulating the harmonic emission with excited states of molecule based on a lot of calculated results.Finally we carry out a series of harmonic emission experiments of N2 and O2 under different molecular alignment, different rotational temperature, different intensity of femtosecond pulse, different ellipticity of fundamental pulse, and different gas pressure in two color field. The properties of harmonic emission under different conditions are analyzed in detail. We find a well agreement between theoretical calculations and experimental results including those in foreign references.