A Study Of Even Order Harmonics Generation Of Electric Quadrupole Moment

The interaction between light and matter has always been an important subject in physics.In recent years,with the rapid development and wide application of laser technology,the interaction between light and matter has been deeply studied in the field of strong field physics.The results of the interaction of intense laser pulses with matter show a series of nonlinear physical phenomena: suprathreshold ionization,burst ionization,multi-photon ionization,ionization suppression,high-order harmonic and so on.These new phenomena bring great challenges to the theoretical and experimental research of strong field physics,but also promote the rapid development of modern laser technology.Because high-order harmonic can synthesize far-ultraviolet(XUV)light source,X-ray,ultra-short pulse and other important applications,it has attracted more and more attention in the field of atomic and molecular physics in recent years.The high-order harmonic emission is due to the high-order nonlinear effect and eventually emits coherent radiation waves whose frequency is an integer multiple of the laser field.While the harmonic spectrum decreases rapidly in the low-order region and then appears in the platform region,finally,the structural characteristics of the cut-off region,these can not be explained by the traditional perturbation theory.According to the semi-classical "three-step model" :(1)the trapped electrons in the atom are ionized into free electrons by multi-photon ionization or breakdown ionization under the action of intense laser pulses;(2)the ionized electrons move in a classical way in the laser field,and at first they are accelerated by the electric field and away from the parent nucleus;(3)when the laser field is reversed,some of the moving electrons will move to the vicinity of the nucleus,and be captured by the nucleus,then return to the ground state and emit high-energy photons.The emission mechanism of high-order harmonic and the physical image are explained successfully.At present,the research of high-order harmonic emission from the interaction between high-power laser and atomic molecules is mainly focused on three aspects:(1)the emission mechanism of high-order harmonic is studied under different driving laser fields;(2)to study ascheme to emit longer and shorter pulses by changing the potential field or other factors;(3)to explore ways to improve the efficiency of high-order harmonic emission and to enhance the harmonic signal so that it can be detected by existing instruments.The basis of studying the nonlinear interaction between laser and atom is to solve the time-dependent Schrodinger equation in the atomic system.We solve the time-dependent Schrodinger equation of the hydrogen atom in a monochromatic laser field by using the pseudo-spectrum-split operator method,which can be solved directly without establishing a model.The proposed method has two outstanding advantages:(1)non-uniform spatial grid points are adopted.More grid points are selected near the nucleus,and fewer grid points are selected in the region farther away from the nucleus.Even a few grid points can be used for high-precision calculation,which greatly improves the running efficiency of the program.(2)when r is close to 0,Coulomb singularity problem will appear in Coulomb potential,and quasispectral method can deal with Coulomb odd point problem effectively by solving range of transformation space,so that the electron wave function of real atomic system can be calculated.In this paper,we numerically solve the time-dependent Schrodinger equation of an order electron atom in the strong field approximation,and investigate the contribution of the electric quadrupole moment of hydrogen atom to the generation of high-order harmonic spectrum under the strong linear polarization laser field.The physical mechanism of even harmonic emission is analyzed theoretically.The concrete implementation is as follows:(1)solving the wave function of the ground state of hydrogen atom as the initial wave function in the time evolution,(2)using the pseudo-spectrum-split operator method to carry on the time evolution of the initial wave function,and obtaining the distribution of the wave function of each time in space;(3)the electronic dipole moment form of the desired observation system is obtained for the required physical quantities,(4)the time-dependent physical quantities after all evolution are transformed into the spectral space by Fourier transform,and the corresponding high-order harmonic spectra are observed.(5)thetime-frequency analysis of harmonic spectrum is made by using wavelet transform,and the generation mechanism of harmonic spectrum is analyzed by semi-classical "three-step model".