Quantum Path Interference Effects In The Process Of High Order Harmonic Generation From Oxygen Molecule

The high order harmonic generation(HHG)is an important subject in the field of strong fields,an important extreme deep ultraviolet light source,and an important tool for detecting atomic and molecular dynamics and obtaining molecular structure information.This paper studies HHG spectrum of oxygen molecule,extends the Lewenstein's theory of calculating atomic HHG to calculate the HHG of molecule,uses strong field approximation,solves the schrodinger equation,obtains the dipole moment,and then obtains the intensity of HHG proportional to its square.In the integral operation of the momentum to obtain the dipole moment,the saddle point method is used.According to different saddle points,the dipole moment is divided into four terms,corresponding to the two types of channels generated by the classic HHG,one is electronic ionization and recombination occur at the same atomic center,the other is where electrons ionize at one atomic center and recombine at another atomic center.so the final high order harmonic is the superposition of the two types of channel contributions.This paper calculates HHG spectrum under different angles between the nuclear axis and the direction of the laser field.It is found that at 45 degree,the intensity of HHG is the largest,and at zero degree and 90 degree,the intensity of HHG is almost zero,which is consistent with the results of the experiment.In addition,we have seen that there is an interference minimum in the HHG spectrum.The order of the HHG corresponding to this minimum is a function of the angle between the nuclear axis and the laser polarization.The greater the angle,the greater the order of the HHG corresponding to the minimum value.Analysis shows that this minimum value is caused by the interference of two types of channels.For this minimum value,we predict its position by calculating the phase difference between the two types of channels.This paper analyzes the conditions of zero degree and 90 degree,and shows that the physical images with small HHG are different in the two cases.For zero degree,the low harmonic intensity is due to the atomic wave function in the zero degree direction,and the small electron density makes the amplitude of the two types of channels small,so that the amplitude of the two types of channels is still small after the superposition of the two types of channels.In the 90 degree direction,the amplitude of the two types of channels is very large,but the phases of the two types of channels are opposite to all the harmonic orders,that is,the two types of channels produce destructive interference,resulting in the intensity of HHG at 90 degree to be low.Based on this,it can be predicted that for N₂ molecules,the intensity of HHG is the strongest at zero degree.For all symmetric linear molecules,the intensity of HHG will be very low in the direction perpendicular to the molecular axis.This paper also discusses the atomic unit and gives the conversion relationship between the two atomic units and the international unit.