High-order Harmonic Generation From Diatomic Molecules At A Intense Laser Field

The high-order harmonic generation(HHG) from atoms and molecules has been one of the most intensely studied aspects of strong-field physics,this is due to:1) it can be applied as a coherent ultrashort radiation source in the extreme ultraviolet(XUV) and soft x-ray regions;2) it is the preferred choice for shaping the attosecond pulse;3) it can be used to molecular orbital tomography;4) the study on the HHG greatly improves the theory of the strong field physics. Presently,the investigation of the HHG focuses on molecules.Due to the multi-center characteristics and greater freedom,molecules show more complicated structures in their HHG spectra than atoms and the dynamics of molecules in the external field is more difficult to investigate.In this thesis,we investigate the high-order harmonic generation(HHG) from diatomic molecules. We study the mechanism of the HHG from diatomic molecules numerically and analytically.We address the new theory models that illuminate the molecular HHG.Further,we explain the present novel experimental phenomena on moleclar HHG.In the second chapter,we develop an analytic theory for the harmonic generation of symmetric diatomic molecular ions beyond two-level model,emphasizing on the influence of the chargeresonance (CR) states that are strongly coupled to electromagnetic fields for the case of large internuclear distance.With taking into account the continuum states that are ignored in the two-level model and become important for the intense laser case,our model is capable to produce the spectrum for the whole range of harmonic orders that consists of a molecular plateau due to the CR transition and an atomic-like plateau for a long-wavelength excitation.Our analytic results are in good agreement with the numerical results from directly solving the Schr(o|¨)dinger equation. Our theory also identifies the crucial role of the CR states in the fine structure of the harmonic spectrum and shows that the harmonic generation in molecular system can be effectively controlled by adjusting the internuclear distance.In the third chapter,using the strong field approximation(SFA) model developed in the above chapter,we investigate the high-order harmonic generation(HHG) from diatomic molecules with large internuclear distance.We find that the hump and dip structure emerges in the plateau region of the harmonic spectrum,and the location of this striking structure is sensitive to the laser intensity.Our model analysis reveals that two-center interference as well as the interference between different recombination electron trajectories are responsible for the unusual enhanced or suppressed harmonic yield at a certain order,and these interference effects are greatly influenced by the laser parameters such as intensity.In the four chapter,based on the investigation in the third chapter,we investigate molecular orbital tomography using the HHG.The calculations show that the molecular recollision electronic wave packets(REWPs) in the HHG are closely related to the ionization potential as well as the particular orbital from which it ionized.As a result,the spectral amplitude of the molecular REWP could be largely different from its reference atom(i.e.,with the same ionization potential as the molecule under study) in some energy regions due to the interference between the atomic cores of the molecules.This finding implies that molecular orbital tomography experiment using HHG[Nature 432,867(2004)]that is applicable for N₂ can not be generalized to other molecules such as CO₂.In the fifths chapter,based on the limitation of molecular orbital tomography illuminated in the four chapter,we investigate the new method for probing the molecular structure using the HHG.The calculations on the orientation dependence of high-order harmonic generation(HHG) from H₂⁺ with different internuclear distances and laser intensities show,that harmonic spectra at different orientation angles have striking intersections in the plateau region,the locations of which are weakly dependent on the laser parameters.The model analysis on the formations of the intersections reveals that two-center interference has an important role in both the recombination and the ionization process of the molecular HHG.We demonstrate that molecular interior messages can be read from the intersections on an ultrafast time scale.In the sixth chapter,based on the above investigation of the HHG from small molecules H₂⁺, we investigate the HHG from larger molecules.In this chapter,using a strong field approximation model that considers the translation invariance,we investigate the orientation dependence of molecular high-order harmonic generation(ODM-HHG) of O₂ and CO₂.The calculations show that for O₂,the harmonic yields peak atθ=50°(θthe angle between the molecular axis and the laser polarization),and for CO₂,those peak atθ=50°andθ=60°for the low and the high order harmonics,respectively.While the ionization peaks of them are all arrived atθ=45°.The results for CO₂ are in significant disagreement with current theories.We demonstrate that it is the interplay of two-center interference and molecular orbital in the ionization step that has an important role in the unusual ODM-HHG of CO₂.Especially,due to the interplay,rich molecular information is printed on the intersections of harmonic spectra at differentθs.In the seventh chapter,based on the investigation in the sixth chapter,we investigate the ionization process in the molecular HHG in detail.In the chapter,taking into account the translation iavariance in strong-field approximation(SFA),we introduce the modified molecular SFA (MM-SFA).The model developed here is applied to both neutral diatomic molecules and diatomic molecular ions.Using it we investigate the orientation dependence of molecular ionization for H₂⁺, O₂ and CO₂.We compare the results obtained by it to that obtained by the standard molecular SFA(SM-SFA),the MO-ADK theory,etc.The predictions of the MM-SFA agree with the numerical simulations and the experimental measurements.Especially,the preference for ionization of CO₂ while aligned at 45°to the laser field revealed by the experimental measurement[Phys. Rev.Lett 98,243001(2007)]is reproduced by the theory,and can be attributed to the effect of two-center interference.The above investigations give the semiclassical picture of the molecular HHG.In the eighth chapter,through the numerical simulation of the recollision process in the HHG,that accurately incorporates the Coulomb effects,the ground state depletion as well as the contribution of higher bound states to the HHG,we demonstrate a complete quantum picture of harmonic emission beyond the semiclassical one.The simulation reveals that the primary contribution to a certain harmonic comes from the electrons with a broad region of energy beyond the energy conservation. Moreover,based on the simulation,we address the quantum mechanism of two-center interference in the molecular HHG,and further,we identify the important role of the first excited state in that.