Theoretical Study Of The Characteristics Of Molecular High-order Harmonic Generation

When an intense laser pulse focused into atomic or molecular gas will produceultra-wideband coherent radiation ranged from the infrared to the extreme ultraviolet(EUV), namely the so-called high-order harmonic generation (HHG). HHG has manyimportant applications, for example, it is the compact and relatively inexpensivesource of coherent EUV, is the most effective schemes to generate attosecond (10-18s)pulse, and is the imaging tools for electronic orbits in atoms and molecules, and so on.Molecular system under the irradiation of strong laser will emits high-order harmonic,an important application of molecular high-order harmonic is the imaging ofmolecular orbital. This thesis proceeds from theory, using the strong-fieldapproximation (SFA) method to explore the characteristics of molecular high-orderharmonics. The main contents of this thesis are as follows:First, we use the SFA method to simulate the HHG of different molecular orbitalofN2,C2H2,CO and O2, under the influence of the intense laser pulse. It is foundthat the harmonic spectra for the HOMO and the HOMO-1orbital of these differentmolecules are showed a significant minimum structure, the correspondence betweenthe minimum and harmonic frequencies depends on the type of molecules and theelectron orbital.Combined with two-center interference model, we have systematically studied theposition of molecular harmonic minimum. It is found that the position of interferenceminimum not only depends on the internuclear distance of two atoms which playing amajor contribution in the molecular HHG, but also depends on the symmetry ofmolecular orbital. For the harmonic of bonding molecular orbital, if the orbital is a σ orbital, the initial phase difference between two wave sources of two-centerinterference mode is π, such as the HOMO ofN2and CO, the HOMO-1ofC2H2.However, while the orbital of the bonding molecules is a π orbital, the initial phasedifference between two wave sources is0, such as the HOMO-1ofN2and theHOMO ofC2H2. Compared with the bonding molecular orbital, the correspondingorbital type of anti-bonding molecular orbital, their initial phase difference betweentwo wave sources should be added π. For example, the HOMO of O2is a π orbital, itis also a anti-bonding orbital, the phase difference between two sources is π.According to the above description, taken the initial phase difference into account, itis found that the position of minima in the spectra predicted by the two-centreinterference model are agreed well with the calculation given by the SFA method.The study also found that the harmonic of bonding molecular orbital with π-type,when the angle between molecular axis and the polarization direction of laser electricfield is0oor180o, the harmonic intensity in the plateau is much lower than theintensity of the other angle. However, the bonding molecular orbital of σ-type, theharmonic intensity of the plateau at90oand270oare much lower than the intensity ofthe other angle. This is arises from the opposite phase of different wave source, whichinduce a destructive interference in the spectrum.Now that the existence of a close correlation between the molecular harmonicminimum and molecular orbital symmetry, if the whole harmonic intensitydistribution of different frequency and angles (between molecular axis and thepolarization direction of laser electric field) is directly related to the shape of themolecular orbital? In this regard, we have changed the harmonic frequency to themomentum of return electron, the image of harmonic intensity vary with themomentum of return electron are obtained.The study found that the converted imageare striking similar with the image of corresponding molecular orbital wave functionin the momentum space. Such as the minima position of the harmonic spectra are fullyconsistent with the distribution of molecular orbital nodes in the momentum space,and the intensity contrast at the different areas are also basically similar.The overall structure of the harmonic spectra are completely dependent on themolecular orbital wave function in the momentum space, the image of molecularharmonic vary with the orientation angle can well reproduced molecular orbitalstructure in the momentum space. Using the correspondence between the harmonic spectra and orbital wave function in the momentum space, on this basis we using theminimum distribution of molecular harmonics to artificially determine the relationshipof harmonic phases, without the need for the reference atom with same ionizationenergy and prior assumption for the spatial symmetry of the molecular orbital, it ispossible to achieve accurate reconstruction of molecular orbital.