标题： 高次谐波产生的理论研究：对称性、简并性和取向的影响 显示英文标题

标题： Theoretical studies of high-harmonic generation: Effects of symmetry, degeneracy and orientation

摘要： 利用量子力学的三步模型，我们展示了对四种多原子分子高次谐波产生的数值计算结果。乙烯（C$_2$H$_4$）作为一个例子，其中轨道对称性直接影响谐波产额。我们研究甲烷（CH$_4$）的情况，以探讨由简并轨道分子产生的高次谐波生成。为此，我们展示了单个轨道贡献如何体现在总谐波信号中。这个例子说明了将各个简并轨道的振幅贡献相干叠加的重要性。最后，我们研究了丙烷（C$_3$H$_8$）和丁烷（C$_4$H$_{10}$）的高次谐波产生。这两种分子由于结构扩展且远离球形，产生了具有非平凡取向依赖性的谐波。特别是，丙烷可以被定向，从而有利于高频谐波的产生，因此该分子有望用于生成紫外阿秒脉冲。 显示英文摘要

摘要： Using a quantum mechanical three-step model we present numerical calculations on the high-harmonic generation from four polyatomic molecules. Ethylene (C$_2$H$_4$) serves as an example where orbital symmetry directly affects the harmonic yield. We treat the case of methane (CH$_4$) to address the high-harmonic generation resulting from a molecule with degenerate orbitals. To this end we illustrate how the single orbital contributions show up in the total high-harmonic signal. This example illustrates the importance of adding coherently amplitude contributions from the individual degenerate orbitals. Finally, we study the high-harmonic generation from propane (C$_3$H$_8$) and butane (C$_4$H$_{10}$). These two molecules, being extended and far from spherical in structure, produce harmonics with non-trivial orientational dependencies. In particular, propane can be oriented so that very high-frequency harmonics are favorized, and thus the molecule contains prospects for the generation of UV attosecond pulses.