Abstract Free-electron lasers produce extremely brief, coherent, and bright laser-like photon pulses that allow to image matter at atomic resolution and at timescales faster than the characteristic atomic motions. In pulses of about 50 femtoseconds duration they provide as many photons as one gets in 1 s from modern storage ring synchrotron radiation facilities. FLASH, the Free-Electron Laser at DESY in Hamburg was the first FEL in the XUV/soft X-ray spectral range, started operation as a user facility in summer 2005, and was for almost 5 years the only short wavelength FEL facility worldwide. Hence, most of the technological developments as well as the scientific experiments performed by the user community were new and unique as outlined below. FLASH was driving FEL science and technology and paved the way for many new ideas. Because of using a linear accelerator in superconducting RF technology FLASH combines the extreme peak brightness characteristic for FELs with very high average brightness. It also was the prototype for the European XFEL located in the Hamburg metropolitan area, which started user operation in summer 2017. The present review provides an overview of the progress made with accelerator science and technology at FLASH for the production of stable beams of well characterized electron pulses, reduction of the pulse jitter to the femtosecond level, generation of ultra-short photon pulses, adequate synchronization of the machine parameters with the experiment, and demonstrating advanced FEL schemes using variable gap undulators. Much of this was done in the very exciting early days of FEL science when it was even not clear if the FEL concept could be realized for X-rays. The development and the operation of the FLASH user facility is described, as well as the techniques developed to make use of the new type of X-ray beams including photon beam diagnostics and damage studies of the optical elements. The review emphasizes breakthrough experiments which demonstrated that many of the ideas collected in the world-wide discussion of the scientific case of free-electron lasers could indeed be realized and they often produced unexpected results. The first experiment on Coulomb explosion of Xe clusters performed in 2002 was a clear demonstration of the feasibility of experiments with free-electron laser beams and opened a lively discussion in the atom, molecular and optical physics community (AMO). Time resolved single-shot single-particle imaging, summarized in the slogan “Take movies instead of pictures”, was one of the most popular science drivers for the construction of free-electron X-ray lasers. As a first step in this direction experiments using a highly focused beam of FLASH demonstrated that pictures of 2 dimensional objects could be reconstructed from single-shot single-particle diffraction patterns. Explosion dynamics of nano-size particles hit by an intense FEL pulse were studied. This method, called “diffraction before destruction”, is now very successfully applied with hard X-rays and, to a large extent, solves the radiation damage problem in structural biology. A long term goal is to determine the 3 dimensional structure of a large molecule from a single-shot diffraction pattern. Along these lines the 3D architecture of free Ag nanoparticles could be determined from one diffraction pattern only using soft X-rays from FLASH. To understand light–matter interactions in this new parameter space a number of pioneering AMO experiments have been performed including non-linear interactions in atoms, molecules and clusters. Multiphoton photoionization processes in the presence of intense optical fields have been studied, as well as photo-absorption of XUV photon energies on molecular ions important for astrophysics. The nature of formation and breaking of molecular bonds was investigated in VUV pump–VUV probe experiments using a reaction microscope and a specific delay line. As an example the process of ultrafast isomerization of acetylene molecules C 2 H2 triggered by single photon excitation has been studied. The structural changes during the isomerization process were visualized and an isomerization time of 52 +/- 15 fs was found. Clusters of variable size, which can be produced routinely, allow distinguish between inter- and intra-atomic effects and are considered model systems for the investigation of light–matter interactions in multi-atom objects. As an example such experimental studies provided instructive data for benchmarking theoretical models describing cluster ionization in intense short-wavelength laser pulses. The combination of single-shot single-particle imaging for determination of the cluster size with spectroscopy was crucial for success of these experiments. The investigations could later be extended to very large Xe clusters providing new insights into the nanoplasma formation and explosion dynamics of such large systems From early on, studies of high energy density plasmas and warm dense matter have been one of the most prominent research fields in building the scientific case for X-ray free-electron lasers. A good understanding of this complex regime between cold solids and hot dilute plasmas is important for high pressure studies, applied materials studies, inertial fusion, and planetary interiors. With the first observation of saturable absorption of an L-shell transition in Aluminum and pioneering studies of warm dense hydrogen FLASH kicked off research of matter in extreme conditions with free-electron lasers. In condensed matter experiments the emphasis is not so much on the peak power of the FEL beam and extreme focusing, but on beam properties like polarization and pulse duration. The sample has to stay intact in the beam over hours and the number of photons per pulse impinging on the sample has to be limited to avoid space charge effects. After demonstrating the possibility to record single-shot resonant magnetic scattering images with FELs the first time-resolved demagnetization study using a pump–probe approach with an IR-pump pulse and an XUV probe pulse to record a resonant magnetic scattering pattern as a function of pump–probe delay was also performed at FLASH. Free-electron lasers offer the possibility to extend the well-established X-ray spectroscopic techniques for the investigation of the static electronic structure of matter to probing the evolution of the electronic structure in the time domain after controlled excitation. At FLASH first time resolved core level photoemission (TR-XPS) experiments have been performed which are element specific and provide information on the dynamics of the local charge state around a specific center. Using 198 eV photons in a surface study at Ir single crystals it was possible to separate surface and bulk contributions in the Ir 4f levels with sufficient instrumental resolution. Time and angular resolved photoelectron spectroscopy (TR-ARPES) is a very powerful tool to study non-equilibrium electron dynamics of condensed matter systems, since it offers the possibility to follow the dynamics of the full band structure of a material. In another pioneering experiment the photo-induced dynamics of the Mott insulator 1T-TaS2 was studied at FLASH by investigating the dynamics of the Ta 4f photoemission. The formation of a commensurate charge density wave (CCDW) leads to a splitting of the Ta 4f level which decreases first on a sub-picosecond time scale due to electronic melting of the CCDW and afterwards on a picosecond lifetime due to electron–phonon coupling. This leads to transfer of energy from the electronic system to the lattice and a partial melting of the periodic lattice distortions accompanying the periodic charge arrangement in the CCDW phase. In materials science X-ray absorption and emission spectroscopy are among the most powerful spectroscopies to study the electronic structure of matter. The wavelength of the radiation is scanned over certain element specific resonances which at FLASH 1 can only be done by scanning the electron energy. This is time consuming and makes the experiments difficult. Nevertheless, the first time-resolved X-ray emission spectroscopy (XES) experiment was done at FLASH 1 in order to study non-thermal melting of a silicon sample. From a comparison of the observed valence electronic structure at different times after the photoexcitation it became clear that in the melting process in the first few ps a non-equilibrium low density liquid state is reached. The existence of such a metastable low density liquid state had been postulated for many systems that show tetragonal bonding in the crystalline phase like water for example, but spectroscopically the time-resolved silicon XES data taken at FLASH verified its existence for the first time. FLASH 2 has tunable undulators and it was demonstrated that scanning of the wavelength is very easy there.

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10 年在 DESY 的自由电子激光 FLASH 中开拓 X 射线科学

摘要 自由电子激光器产生极其短暂、相干和明亮的类似激光的光子脉冲，允许以原子分辨率和比特征原子运动更快的时间尺度对物质进行成像。在大约 50 飞秒持续时间的脉冲中，它们提供的光子与现代存储环同步加速器辐射设施在 1 秒内获得的光子一样多。位于汉堡 DESY 的自由电子激光器 FLASH 是 XUV/软 X 射线光谱范围内的第一个 FEL，于 2005 年夏天作为用户设施开始运行，并且是近 5 年来全球唯一的短波长 FEL 设施。因此，用户社区进行的大多数技术发展和科学实验都是新的和独特的，如下所述。FLASH 正在推动 FEL 科学和技术，并为许多新想法铺平了道路。由于在超导 RF 技术中使用线性加速器，FLASH 将 FEL 的极端峰值亮度特性与非常高的平均亮度结合在一起。它也是位于汉堡大都市区的欧洲 XFEL 的原型，于 2017 年夏季开始用户运行。本综述概述了 FLASH 加速器科学和技术在生产具有良好特征的稳定光束方面取得的进展。电子脉冲、将脉冲抖动降低到飞秒级、超短光子脉冲的生成、机器参数与实验的充分同步，以及使用可变间隙波荡器展示先进的 FEL 方案。其中大部分是在 FEL 科学非常令人兴奋的早期完成的，当时甚至不清楚 FEL 概念是否可以用于 X 射线。描述了 FLASH 用户设施的开发和操作，以及为利用新型 X 射线束而开发的技术，包括光子束诊断和光学元件的损坏研究。该评论强调了突破性实验，这些实验表明，在世界范围内对自由电子激光器科学案例的讨论中收集的许多想法确实可以实现，并且它们经常产生意想不到的结果。2002 年进行的第一个氙簇库仑爆炸实验清楚地证明了自由电子激光束实验的可行性，并在原子中展开了热烈的讨论，分子和光学物理社区 (AMO)。时间分辨单次单粒子成像，概括为“用电影代替照片”的口号，是构建自由电子 X 射线激光器最流行的科学驱动因素之一。作为该方向的第一步，使用高度聚焦的 FLASH 光束的实验表明，可以从单次单粒子衍射图案重建二维物体的图片。研究了被强烈 FEL 脉冲击中的纳米尺寸粒子的爆炸动力学。这种称为“破坏前衍射”的方法现在非常成功地应用于硬 X 射线，并在很大程度上解决了结构生物学中的辐射损伤问题。长期目标是从单次衍射图案确定大分子的 3 维结构。沿着这些思路，仅使用来自 FLASH 的软 X 射线，就可以从一种衍射图案中确定游离 Ag 纳米粒子的 3D 结构。为了理解这个新参数空间中的光-物质相互作用，已经进行了许多开创性的 AMO 实验，包括原子、分子和簇中的非线性相互作用。已经研究了在强光场存在下的多光子光电离过程，以及对天体物理学很重要的分子离子上的 XUV 光子能量的光吸收。使用反应显微镜和特定延迟线在 VUV 泵-VUV 探针实验中研究了分子键的形成和断裂的性质。例如，研究了由单光子激发触发的乙炔分子 C 2 H2 的超快异构化过程。异构化过程中的结构变化可视化，发现异构化时间为 52 +/- 15 fs。可以常规产生的可变大小的簇可以区分原子间和原子内效应，并且被认为是研究多原子物体中光-物质相互作用的模型系统。例如，此类实验研究为描述强短波长激光脉冲中的团簇电离的理论模型的基准测试提供了指导性数据。用于确定簇大小的单次单粒子成像与光谱学的结合对于这些实验的成功至关重要。这些研究后来可以扩展到非常大的 Xe 簇，为这种大型系统的纳米等离子体形成和爆炸动力学提供新的见解从早期开始，对高能量密度等离子体和热致密物质的研究一直是建筑领域最突出的研究领域之一。 X 射线自由电子激光器的科学案例。充分了解冷固体和热稀等离子体之间的这种复杂状态对于高压研究、应用材料研究、惯性聚变和行星内部非常重要。随着对铝中 L 壳跃迁的饱和吸收的首次观察以及对暖致密氢的开创性研究，FLASH 开启了对极端条件下自由电子激光器的物质研究。在凝聚态实验中，重点不是 FEL 光束的峰值功率和极端聚焦，而是光束特性，如偏振和脉冲持续时间。样品必须在数小时内在光束中保持完整，并且必须限制撞击样品的每个脉冲的光子数量以避免空间电荷效应。在证明了用 FEL 记录单次共振磁散射图像的可能性之后，第一次时间分辨退磁研究使用泵-探针方法与 IR 泵脉冲和 XUV 探针脉冲来记录作为函数的共振磁散射图案泵-探针延迟也在 FLASH 中进行。自由电子激光器提供了将用于研究物质静态电子结构的成熟 X 射线光谱技术扩展到探测受控激发后时域中电子结构演变的可能性。在 FLASH 中，首次进行了解析核心级光电发射 (TR-XPS) 实验，这些实验是元素特定的，并提供有关特定中心周围局部电荷状态动态的信息。在 Ir 单晶的表面研究中使用 198 eV 光子，可以以足够的仪器分辨率分离 Ir 4f 能级中的表面和体相贡献。时间和角分辨光电子能谱 (TR-ARPES) 是研究凝聚态系统非平衡电子动力学的非常强大的工具，因为它提供了跟踪材料全能带结构动态的可能性。在另一个开创性实验中，通过研究 Ta 4f 光电发射的动力学，在 FLASH 研究了莫特绝缘体 1T-TaS2 的光致动力学。相应电荷密度波 (CCDW) 的形成导致 Ta 4f 能级的分裂，由于 CCDW 的电子熔化，该分裂首先在亚皮秒时间尺度上下降，然后由于电子-声子耦合而在皮秒寿命上下降。这导致能量从电子系统转移到晶格，以及伴随 CCDW 相中周期性电荷排列的周期性晶格畸变部分熔化。在材料科学中，X 射线吸收和发射光谱是研究物质电子结构的最强大的光谱之一。辐射波长在某些元素特定共振上扫描，在 FLASH 1 只能通过扫描电子能量来完成。这是耗时的并且使实验变得困难。尽管如此，第一次时间分辨 X 射线发射光谱 (XES) 实验是在 FLASH 1 进行的，以研究硅样品的非热熔化。从光激发后不同时间观察到的价电子结构的比较可以清楚地看出，在前几个 ps 的熔化过程中，达到了非平衡低密度液态。例如，许多在晶相中显示四方键合的系统（例如水）都假设存在这种亚稳态低密度液态，但在光谱上，FLASH 获得的时间分辨硅 XES 数据首次证实了它的存在。FLASH 2 具有可调谐波荡器，并且证明在那里扫描波长非常容易。