It is shown that the alignment dependence of the yields of high-order harmonic generation (HHG) from molecules by intense lasers is governed by the orbital symmetry of the outermost electron(s). For N2, with its outermost σg electron, the HHG yield is maximal when the molecules are aligned with the laser polarization direction, in agreement with the recent experiment of Itatani et al. [Nature 432, 867 (2004)]. For O2, with its outermost пg electron, the HHG yield peaks when the molecules are aligned at about 45° from the polarization axis. We emphasize that the alignment dependence is determined mostly by the orbital symmetry and weakly on the laser parameters or the species.

用数值方法求解含时薛定谔方程，研究了具有长程势和短程势的一维原子在强激光场中的高次谐波和电离特性。在强激光场中，长程势和短程势原子产生的高次谐波具有相似的特性，对应的平台和截止位置相同，但是短程势原子没有低阶的高次谐波，而长程势和短程势原子在激光场中的电离概率明显不同。研究结果表明，原子的激发态结构对低阶的高次谐波和原子的电离概率有重要影响。

We present a linear-least-squares-fitting method to solve the time-dependent Schr

利用基函数展开结合线性最小二乘法方法求解含时薛定谔方程，研究了一维原子在强激光场中产生高次谐波过程中，束缚态和连续态以及它们的干涉效应对高次谐波的影响。结果表明，低能的高次谐波是由束缚态-束缚态之间的相互作用产生的，高能的高次谐波是由束缚态连续态之间的相互作用产生的，而截断频率后消失的高次谐波是由于束缚态-束缚态与束缚态-连续态之间存在干涉效应的结果。

An alternative theoretical approach for solving the time-dependent Schro dinger equation for atoms in an intense laser field is presented.In this method the time-dependent wave function is expanded in a basis set but the expansion coefficients are determined by linear-least-squares fitting of the wave function on discrete mesh points in configuration space,thus avoiding the need of evaluating a large number of matrix elements.We illustrate the method by computing wave functions,above-threshold ionization spectra,and harmonic generation spectra of a model atom and compare the results with those obtained using the split-operator method.