Contribution of electric quadrupole moment to high order harmonic generation in high-intensity laser field

The time-dependent Schrodinger equation of the single electron atom in three-dimensional spherical coordinates is numerically solved, and the contribution of the electric quadrupole moment to the generation of high-order harmonics is studied under the interaction of the intensity linearly polarized laser field and the hydrogen atom. We find that under the influence of the dipole moment the hydrogen atom emits odd harmonics in the intensity laser field while under the action of the electric quadrupole moment the hydrogen atom emits even harmonics in the intensity laser field. By analyzing the power spectrum and the picture of the time-frequency combined with semi-classical 'three-step model', the mechanism and reasons of the even high harmonics generation are explained reasonably. (authors)