Tddft Method Simulation Of Hydrogen Atoms, Hydrogen And Nitrogen Molecules In High-harmonic Generation

Recently with the development of laser technology, the subject of interaction between strong laser and atoms, molecules has become a new hot topic. When atoms and molecules are radiated by ultra intense laser pulse, a series of new non-linear phenomena which can't be interpreted by perturbation theory appears. The most attractive one is high order harmonic generation (HHG), which can be used as a source of extreme ultraviolet (XUV) and soft X-ray lasers. The importance of studying HHG consists in two aspects. Firstly it can accelerate the development of non-perturbation theory and contribute to the further understanding of strong laser induced new phenomena and the underlying new physical mechanism. Secondly it may provide theory background for the development of simple and cheap short wavelength coherent light generators.The popular theoretic methods to describe the simplified interaction of laser-atoms and molecules are based on time-dependent Schr?dinger equations (TDSE). As the number of electrons and the degree of freedom increase, the solutions of the TDSE go beyond current computational ability. The developing time-dependent density-functional theory (TDDFT) has been attracting more and more attention since its intrinsic advantages, such as explicit physical conception, rigorous basis in theory, convenience in application and general applicability. It is hopeful to become a general method to deal with both linear and non-linear response of multi-electron systems. In this thesis, the HHG of hydrogen atom, hydrogen molecule and nitrogen molecule radiated by an ultra strong femto-second laser pulse are simulated by the TTDFT method with the general gradient approximation (GGA) of the exchange-correlation energy. Then we study the effects of pulse-shape on HHG of hydrogen atom. Effects of different laser polarization direction on HHG of hydrogen molecule and nitrogen molecule are discussed. The results indicate that the HHG spectra of H, H2 and N2 have a"Decrease-Plateau-Cutoff"structure, which are similar to the typical atom HHG spectrum. A kind of"odd allowed, even forbidden"selective phenomena of the order number of HHG are observed. The different pulse-shape may change the character of HHG for hydrogen atom. With the increasingθ(the angle between laser polarization direction and molecular axis), the spectral intensity of HHG yields for N2 and H2 decreases. Good agreement with the experimental and theoretical results .The basic works in this thesis may lead us to explore more interesting and fascinating strong laser induced nonlinear response of atoms and molecules.