Theoretical and experimental investigation of Kramers-Henneberger states in alkaline and noble gas atoms

该项目的主要目标是为自由电子的束缚态提供直接的光谱证据，这是一种由吸引核心势和强激光场协同作用产生的不寻常的量子态。处于这种状态的电子对激光场的反应几乎和自由电子一样，但平均而言，它们仍然被束缚在离子核上。这些状态通常被称为克雷默斯-亨内伯格（KH）状态，这是大约50年前首次提出的理论预测。几十年来，KH国家看起来像是一个纯粹的学术兴趣的理论概念。然而，今天有越来越多的间接实验证据表明，这些状态是普遍存在的，几乎在任何时候，一个原子或分子暴露在足够强的红外激光场下，都可以出现。吸引核心电位和激光电场的共同作用产生了一个势垒，束缚电子可以通过势垒逃脱。随着激光强度的增加，势垒的顶部可以下降到束缚电子态的能量以下。然而，国家并不一定是自由的。当这种状态下的电子振荡幅度达到10埃左右时，它对电离保持稳定。至关重要的是，这种情况对于暴露在强度在1013 W/cm2或更高的红外激光场中的所有激发态都是典型的。原子光谱的这种重组对非线性光-物质相互作用，包括激光成丝等过程具有重要意义。克雷默斯-亨内伯格状态在激光灯丝内的出现是极有可能的，它们在灯丝形成过程中的作用可能是实质性的。我们项目的关键目标之一是研究这些状态的出现，它们在成丝过程中的作用，并提供它们存在的结论性，直接的光谱证据。我们的目标是提供两种互补的光谱观测，基于孤立原子的角分辨光电子能谱和激光细丝中的瞬态吸收能谱，来解决这个有趣的情况。

The main objective of this project is to provide direct spectroscopic evidence for the bound states of a free electron an unusual quantum state created by the concerted actionof the attractive core potential and a strong laser field. Electrons in such states respond to the laser field almost like free electrons, yet on average they remain bound to the ionic core. The states are often referred to as the Kramers-Henneberger (KH) states, after first theoretical predictions made about 50 years ago. For many decades, the KH states looked like a purely theoretical concept of academic interest. However, today there is mounting indirect experimental evidence suggesting that these states are ubiquitous and can emerge almost any time an atom or a molecule is exposed to sufficiently intense infrared laser fields. The combined action of the attractive core potential and the laser electric field creates a potential barrier through which a bound electron can escape. As the laser intensity increases, the top of the potential barrier can descend below the energy of a bound electronic state. Yet, the state does not necessarily become free. While the electron oscillation amplitude in such a state reaches some ten angstroms, it remains stable against ionization. Crucially, this situation is typical for all excited atomic states exposed to infrared laser fields with intensities in mid-1013 W/cm2 and higher. Such restructuring of the atomic spectrum has important implications for nonlinear light-matter interaction, including such processes as laser filamentation. The emergence of Kramers-Henneberger states inside a laser filament is extremely likely, and their role in the filamentation process can be substantial. One of the key goals of our project is to investigate the emergence of these states, their role in the filamentation process, and provide conclusive, direct spectroscopic evidence of their existence. Our goal is to provide two complementary spectroscopic observations, based on angle-resolved photo-electron spectroscopy of isolated atoms and transient absorption spectroscopy in laser filaments, to resolve this intriguing situation.