X-ray Studies of Exotic Novel States of Solid-Density Matter Created with 4th Generation Light Sources

在过去一年左右的时间里，X射线科学发生了一场革命，研发出了比同步加速器产生的X射线亮度高100亿倍的软、硬X射线的新来源。这些新颖的光源发出极短(低于100fsec)的X射线脉冲，并且可以聚焦到非常小的光点。由于脉冲如此短，光中的能量是巨大的--在发射的短暂持续时间内，光中的能量相当于一个中等规模的发电站的能量。当所有这些能量被聚焦到一个小光点时，巨大强度的X射线照射到目标的路径上--这是迄今为止在X射线制度下从未产生过的强度。在过去的几个月里，我们进行了一些最初的实验，旨在了解物质如何对如此强烈的X射线光做出反应，而这一提议的目的是极大地促进这种理解。我们已经发现，这种强度如此之大，以至于靶中每个原子的一个电子都可以被X射线击穿，这可以改变材料本身的X射线性质--实际上，通过这种方法，我们已经制造了一种所谓的可饱和吸收体。我们根本感兴趣的是，随着电子重新填充核心空穴，它们提供了关于这种奇异和高度电离状态的电子结构的信息，为非常密集但非常热的材料的物理提供了全新的见解。这种材料(热的致密物质)很有趣，因为它的热能和电子能(库仑势)是相似的，这使得它的性质极难计算。这种情况--热能和库仑能竞争--也发生在惯性约束聚变的初始阶段，也是与理解巨行星内部相关的物理学的一部分-因此，有很多理由希望更好地理解它。强烈的X射线为理解这种物质提供了一个独特的机会，因为在飞秒内，它们就会形成一种特殊的状态--密度很明确的非常热的电子，但却是冷离子。观察这种状态的演变(通过观察荧光，并监测吸收随时间的变化)可以提供关于电子结构的详细信息。例如，对于高度电离的铝，我们在最高强度下遇到了一种不寻常的情况，在这种情况下，经历了复合的特定铝离子现在正在与自己仍处于电离状态的邻居进行复合。这极大地改变了荧光发射的形状，这与合金的荧光信号随着化合物成分的变化有很大关系。因此，这项研究将提供独特的洞察物质的电子结构，在数十万开尔文，但仍然在固体密度。

Over the past year or so there has been a revolution in X-ray science, in that new sources of soft and hard X-rays have been developed that are ten billion times brighter than those produced by synchrotrons. These novel sources emit extremely short (sub 100fsec) pulses of x-rays, and can be focussed to very small spots. As the pulses are so short, the power in the light is enormous - for the brief duration of the emission the power in the light is equivalent to that in a fair-sized electrical power station. When all this power is focussed to a small spot, enormous intensities of x-rays impinge upon the target in its path - intensities that have hitherto never been produced in the X-ray regime. In the last few months we have performed some of the first experiments aimed at understanding how matter reacts to such intense X-ray light, and the aim of this proposal is to vastly further that understanding. What we have already found is that the intensity is so great that an electron from every atom in the target can be knocked out by the X-rays, and this can alter the X-ray properties of the material itself - indeed, by this method we have made a so-called saturable absorber. What is of fundamental interest to us is that as the electrons re-fill the core holes, they provide information about the electronic structure of this exotic and highly-ionized state, providing completely new insight into the physics of very dense, yet very hot material. This material (warm dense matter) is of interest in that the thermal energies and electronic energies (the coulomb potential) are comparable, making its properties extremely difficult to calclulate. This situation - where the thermal and coulombic energies compete - also occurs in the initial stages of inertial confinement fusion, and is also part of the physics that is relevant to the understanding of the interior of the giant planets - thus there are many reasons for wishing to understand it better. The intense X-rays give a unique opportunity to understand such matter, as within femtoseconds they make a particular state - very hot electrons but cold ions, at a well defined density. Watching this state evolve ( by looking at the fluorescence, and monitoring the absorption as a function of time) gives detailed information on the electronic structure. For example, with highly ionized aluminium, we have an unusual situation at the highest intensities where a particular aluminium ion that undergoes recombination is now doing so with neighbours that themselves are still ionized. This drastically alters the shape of the fluorescence emission in a way which has much to do with how the fluorescence signal from an alloy is altered as the compound composition changes. Thus this research will provide unique insight into the electronic structure of matter at hundreds of thousands of degrees kelvin, yet still at solid density.