Strong-field ionization and Rydberg-state excitation (RSE) near the continuum threshold exhibit two phenomena that have attracted a lot of recent attention: the low-energy structure (LES) just above and frustrated tunneling ionization just below the threshold. The former becomes apparent for longer laser wavelengths, while the latter has been especially investigated in the near infrared; both have been treated as separate phenomena so far. Here we present a unified perspective based on electron trajectories, which emphasizes the very important role of the electron-ion Coulomb interaction as expected in this energy region. Namely, those trajectories that generate the LES can also be recaptured into a Rydberg state. The coherent superposition of the contributions of such trajectories with different travel times (each generating one of the various LES peaks) causes an oscillation in the intensity dependence of the RSE yield, which is especially noticeable for longer wavelengths. The theory is illustrated by RSE experiments at 1800 nm, which agree very well with the theory with respect to position and period of the oscillation. The wavelength scaling of the RSE oscillation is also discussed. Our work establishes a solid relationship between processes below and above the threshold and sheds new light on atomic dynamics driven by intense laser fields in this critical energy region.

中文翻译：

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连续谱阈值附近激光驱动原子的电子动力学

连续谱阈值附近的强场电离和里德堡态激发 (RSE) 表现出两种最近引起广泛关注的现象：刚好在阈值上方的低能结构 (LES) 和刚好低于阈值的受挫隧道电离。前者对于较长的激光波长变得明显，而后者已在近红外区域进行了特别研究；到目前为止，两者都被视为单独的现象。在这里，我们提出了一个基于电子轨迹的统一视角，它强调了电子 - 离子库仑相互作用在该能量区域中所预期的非常重要的作用。也就是说，那些产生 LES 的轨迹也可以被重新捕获到里德堡状态。具有不同旅行时间的这种轨迹的贡献的相干叠加（每个产生各种 LES 峰值之一）导致 RSE 产率的强度依赖性振荡，这对于较长的波长尤其明显。1800 nm 处的 RSE 实验说明了该理论，其在振荡的位置和周期方面与理论非常吻合。还讨论了 RSE 振荡的波长缩放。我们的工作在阈值以下和以上的过程之间建立了牢固的关系，并为这一关键能量区域中由强激光场驱动的原子动力学提供了新的思路。1800 nm 处的 RSE 实验说明了该理论，其在振荡的位置和周期方面与理论非常吻合。还讨论了 RSE 振荡的波长缩放。我们的工作在阈值以下和以上的过程之间建立了牢固的关系，并为这个关键能量区域中由强激光场驱动的原子动力学提供了新的思路。1800 nm 处的 RSE 实验说明了该理论，其在振荡的位置和周期方面与理论非常吻合。还讨论了 RSE 振荡的波长缩放。我们的工作在阈值以下和以上的过程之间建立了牢固的关系，并为这一关键能量区域中由强激光场驱动的原子动力学提供了新的思路。