准直的O_(2)分子高次谐波谱中的干涉效应

Interference effect in high order harmonic generation by aligned O_(2)

高次谐波是强场物理的重要现象,不仅是潜在的深紫外光源,而且已经成为分析原子分子动力学以及获取分子结构信息的重要工具.本文采用Lewenstein理论,考察了准直氧分子在激光场中的高次谐波谱对激光偏振方向与核轴所成的夹角的依赖关系.通过计算电离电子在不同的原子中心复合的量子通道对高次谐波贡献之间的相位差,分析了高次谐波随激光场偏振方向与核轴的夹角的变化,并给出了谐波谱上最小值的形成机制.另外,通过分析高次谐波产生过程中不同量子通道的贡献,发现当偏振方向与核轴成0°和90°时,谐波谱强度都小,但它们背后的物理图像是不同的:0°时,产生谐波的每个量子通道的幅度都很小,造成谐波谱的强度较低;而90°时,每个通道贡献的幅度都很高,但是由于通道间的干涉相消,造成谐波谱的强度接近于零.

High order harmonic generation(HHG)is an important phenomenon when atoms or molecules interact with an intense laser field.It can be used to generate ultrashort laser source,and can also be used to investigate the atomic and molecular dynamics and obtain the electric structure information of molecules.All these require to understand in depth the mechanism of HHG.There are complicated interference effects in HHG spectra of molecules due to multiple re-collision atomic centers in the molecule.In this paper,spectra of aligned O2 molecule in linearly polarized laser field is investigated by using the Lewenstein’s model.The dependence of the spectrum on the angleθbetween the nuclear axis of the molecule and the laser polarization direction is obtained.It is shown that the maximum yield of HHG occurs atθof 45°,which is in consistence with the experimental result.In addition,it is found that there exists a minimum value in the HHG spectrum for any given value ofθ.The harmonic order corresponding to the minimum increases withθincreasing.It is found that the minimum comes from the coherent superposition of contributions from two channels.One channel refers to that the ionized electron from one atomic center,subjected to the electric field of the laser,moves back to its parent atomic center and there it combines with the molecule and emits harmonics;while the other channel is that the ionized electron generated from one atomic center move back to the other atomic center to complete the combination and emission of harmonics.The angleθ-dependent phase difference between contributions from these two channels is calculated and the harmonic order corresponding to the minimum value is obtained.Finally,the reason why the yield of HHG is low for the case of the molecular axis parallel to the laser polarization direction is different from that for the case of the molecular axis perpendicular to the polarization direction.For the parallel case,the contributions to HHG from the two channels are both small so that the amplitude of their coherent superposition is small.While for the perpendicular case,the individual contribution from each channel is not small but their destructive interference leads to small yield in harmonicspectrum.