High harmonic generation (HHG) takes place in all phases of matter. In gaseous atomic and molecular media, it has been extensively studied and is very well understood. In solids, research is ongoing, but a consensus is forming for the dominant microscopic HHG mechanisms. In liquids, on the other hand, no established theory yet exists, and approaches developed for gases and solids are generally inapplicable, hindering our current understanding. We develop here a powerful and reliable ab initio cluster-based approach for describing the nonlinear interactions between isotropic bulk liquids and intense laser pulses. The scheme is based on time-dependent density functional theory and utilizes several approximations that make it feasible yet accurate in realistic systems. We demonstrate our approach with HHG calculations in water, ammonia, and methane liquids and compare the characteristic response of polar and nonpolar liquids. We identify unique features in the HHG spectra of liquid methane that could be utilized for ultrafast spectroscopy of its chemical and physical properties, including a structural minimum at 15–17 eV that is associated solely with the liquid phase. Our results pave the way to accessible calculations of HHG in liquids and illustrate the unique nonlinear nature of liquid systems.

中文翻译：

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在液体中产生高次谐波的从头算聚类方法

高次谐波产生（HHG）发生在物质的所有阶段。在气态原子和分子介质中，它已被广泛研究并被很好地理解。在固体方面，研究正在进行中，但对主要的微观 HHG 机制正在形成共识。另一方面，在液体中，尚不存在既定的理论，为气体和固体开发的方法通常不适用，阻碍了我们目前的理解。我们在这里开发了一种强大且可靠的基于从头算簇的方法，用于描述各向同性散装液体和强激光脉冲之间的非线性相互作用。该方案基于时间相关的密度泛函理论，并利用了几个近似值，使其在现实系统中可行且准确。我们展示了我们在水、氨气中进行 HHG 计算的方法，和甲烷液体，并比较极性和非极性液体的特征响应。我们确定了液态甲烷 HHG 光谱中的独特特征，这些特征可用于其化学和物理性质的超快光谱，包括仅与液相相关的 15-17 eV 的结构最小值。我们的结果为液体中 HHG 的可访问计算铺平了道路，并说明了液体系统独特的非线性特性。