Departamento de Química, Universidad Autónoma de Madrid, 28049 Madrid, Spain.
Max-Born-Institut, Max Born Strasse 2A, D-12489 Berlin, Germany.
Sci Rep. 2016 Sep 6;6:32653. doi: 10.1038/srep32653.
Isotropic gases irradiated by long pulses of intense IR light can generate very high harmonics of the incident field. It is generally accepted that, due to the symmetry of the generating medium, be it an atomic or an isotropic molecular gas, only odd harmonics of the driving field can be produced. Here we show how the interplay of electronic and nuclear dynamics can lead to a marked breakdown of this standard picture: a substantial part of the harmonic spectrum can consist of even rather than odd harmonics. We demonstrate the effect using ab-initio solutions of the time-dependent Schrödinger equation for and its isotopes in full dimensionality. By means of a simple analytical model, we identify its physical origin, which is the appearance of a permanent dipole moment in dissociating homonuclear molecules, caused by light-induced localization of the electric charge during dissociation. The effect arises for sufficiently long laser pulses and the region of the spectrum where even harmonics are produced is controlled by pulse duration. Our results (i) show how the interplay of femtosecond nuclear and attosecond electronic dynamics, which affects the charge flow inside the dissociating molecule, is reflected in the nonlinear response, and (ii) force one to augment standard selection rules found in nonlinear optics textbooks by considering light-induced modifications of the medium during the generation process.
各向同性气体受到强红外光长脉冲的照射可以产生入射场的非常高次谐波。通常认为，由于产生介质的对称性，无论是原子还是各向同性分子气体，只能产生驱动场的奇次谐波。在这里，我们展示了电子和核动力学的相互作用如何导致这种标准图像的明显崩溃：谐波谱的很大一部分可以由偶数谐波而不是奇数谐波组成。我们使用时间相关薛定谔方程的从头算解及其同位素在全维空间来证明这一效果。通过一个简单的分析模型，我们确定了其物理起源，即由于光诱导的电分离过程中电荷局域化，导致同核分子在解离时出现永久偶极矩。这种效应出现在足够长的激光脉冲中，并且产生偶数谐波的光谱区域由脉冲持续时间控制。我们的结果（i）表明飞秒核和阿秒电子动力学的相互作用如何在非线性响应中反映出来，这影响了在解离分子内部的电荷流动，以及（ii）迫使人们通过考虑在生成过程中光诱导对介质的修改来增强非线性光学教科书中的标准选择规则。
