不同核轴取向的O2的高次谐波

分子的高次谐波是强场超快物理的重要研究课题. 采用建立在形式散射理论基础上的频域方法计算了O₂在线偏振激光场下的高次谐波, 探讨了核轴被准直在与激光传输方向垂直的平面内时, 高次谐波随核轴与光电场偏振方向所成夹角θ₀的依赖关系. 结果表明: 各次谐波都是在θ₀约为45°时强度最大, 并有较宽的峰值宽度; 当偏离此角度, 高次谐波的强度变小; 到达平行或垂直取向时, 降到最低. 分析表明, 这是由于高次谐波的强度取决于分子基态的电子在动量空间中的电场方向的布居. 针对核轴被准直在激光传输方向与电场偏振方向所确定的平面内的情况, 计算了高次谐波随θ₀的依赖关系, 结果与前一种情况基本相同. 分析发现, 当核轴被准直固定后, 分子绕核轴旋转的角度ψ没有固定, 所以最后的高次谐波强度需要对不同的ψ 时的高次谐波的贡献求和平均. 平均后相当于波函数相对于核轴旋转对称, 从而导致O₂的高次谐波仅与θ₀有关, 而与核轴被准直在哪个面上无关.

High-order harmonic generation of O2 molecules with different nuclear axis orientations

High-order harmonic generation (HHG) is one of the hottest topics in strong field atomic and molecular physics. In this paper, frequency domain theory which is based on formal scattering theory is extended to study the HHG of O₂ molecules under a linearly polarized single mode laser field. The dependence of HHG on the angle θ₀ between the laser polarization direction and nuclear axis is investigated. In our calculation, we only consider the contribution of highest occupied molecular orbital (HOMO) and use the single electron approximation. The HOMO is obtained from quantum chemical software Molpro. The intensity of the laser is 5.18×10¹⁴ W/cm² and the wavelength is 800 nm. On the one hand, in the case that the nuclear axis lies in the plane perpendicular to the laser propagation direction, we find that the yields of all order harmonics increase with θ₀ increasing until the yields reach the maximum values when θ₀ is equal to about 45°. Then the yields decrease with θ₀ increasing and have the minimum values when θ₀ is equal to about 90°. The analysis shows that the yield of HHG is dominated by the density of electrons in HOMO along the laser polarizing direction in momentum space. On the other hand, in the case that the nuclear axis lies in the plane parallel to laser propagation direction, the dependence of HHG on θ₀ is the same as that when the nuclear axis is in the plane perpendicular to laser propagation direction. The reasons for the same results for the two cases lie in the following fact. The HOMO of O₂ molecule has π_g symmetry which is not rotationally symmetric around nuclear axis. So HHG yield relies on the π_g extension orientation. Since the π_g extension orientation cannot be fixed, the HHG of O₂ should be averaged over the contributions to HHG at all possible π_g extension orientations. This average is equivalent to that the electron density is rotationally symmetric around the nuclear axis and hence leads to the fact that the HHG yield of O₂ depends on θ₀ rather than the plane that the nuclear axis lies in.