The helium atom in intense UV and XUV laser fields: rescattering-induced double ionization and ionization scaling laws

在足够高的激光强度下，多电子原子经历了非连续双电离(NSDI)，这是一个电子以高度关联(纠缠)对几乎同时弹射的过程。氦是所有多电子原子中最简单的，它为理论家提供了一个独特的机会，以前所未有的精度研究这些高度关联的强场过程。尽管氦的结构相对简单，但控制系统动力学的氦-激光薛定谔方程在高强度激光辐射的极限内几乎没有任何近似。在用于产生NSDI的强场中，氦激光薛定谔方程的所有项和所有自由度都是同时进行的。在过去的十年里，我们发展了在并行计算机上解决这一问题所必需的计算方法和计算机程序。我们的程序能够在很宽的激光频率和强度范围内求解全(含时的和全维的)氦激光薛定谔方程。在这一能力方面，我们在世界范围内没有对手。我们的代码仍然是唯一的，它能够在从光学到XUV的频率范围内生成准确可靠的解决方案，并在实际实验中尽可能高的激光强度。我们的目的是计算激光驱动的氦在UV到XUV波长(大约100 nm到7 nm)范围内的详细电离动力学。在强场限制下，这是非常未知的领域，因为直到最近还不可能在这些波长建立强场激光器。六个月前，随着汉堡附近DESY自由电子激光(FEL)第一阶段的完成，这种情况发生了变化。另一个更雄心勃勃的自由电子激光正在斯坦福大学建造中。作为原子-激光物理在这一极限中的高度重要性的一个例证，我们注意到上述非顺序双电离(NSDI)最近的一个成功的实际应用，其中持续时间为100阿秒(有史以来最短的)的光脉冲序列是NSDI的直接结果，并使用相同量级的时间分辨测量。用于驱动NSDI的激光为800 Nm。利用强紫外光激光产生更短脉冲的前景是激光驱动氦在紫外光中的一个特别令人兴奋的潜在应用。

At sufficiently high laser intensities, multi-electron atoms undergonon-sequential double ionization (NSDI), a process in which electronsare ejected near-simulteously in highly correlated (entangled) pairs.Helium, the simplest of all multi-electron atoms, presentstheorists with a unique opportunity to study these highly correlatedintense-field processes with an unprecedented accuracy. Despite therelatively simple structure of helium, the helium-laser Schroedingerequation, governing the dynamics of the system, admits few if anyapproximations in the limit of high intensity laser radiation. In theintense fields used to create NSDI, all terms of the helium-laserSchroedinger equation and all degrees of freedom are exercisedsimultaneously. The only reliable treatment of this problem has provedto be via high-accuracy numerical solution of the full Schroedinger equation.Over the last decade we have developed the computational methods, and thecomputer codes necessary to solve this problem on parallel computers.Our code is capable of solving the full (time-dependent and full-dimensional)helium-laser Schroedinger equation over a wide range of laserfrequencies and intensities. We have no rivals in this capability world-wide.Our code is still alone in its ability to generate accurate reliable solutionsat frequencies that range from the optical to the XUV, and atlaser intensities as high as can be achieved in practical experiments.Our intention is to calculate the detailed ionization dynamics of laser-drivenhelium in the UV to XUV range of laser wavelengths (roughly 100 nm to 7nm).In the intense field limit, this is very much uncharted territory,because until very recently it was impossible to build intense-fieldlasers at these wavelengths. Six months ago this changed with thecompletion of the first phase of DESY Free Electron Laser (FEL) nearHamburg. Another more ambitious FEL is under construction at Stanford.As just one illustration of the high importance of atom-laser physicsin this limit, we note a recent successful practical application of thenon-sequential double ionization (NSDI) mentioned above, in which trainsof light pulses of duration 100 attoseconds (the shortest everobserved) were created as a direct result of NSDI and used toperform time-resolved measurements of the same order. The laserused to drive the NSDI was 800nm. The prospect of producing evenshorter pulses by using intense UV lasers is a particularly exciting potentialapplication of laser-driven helium in the UV.