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Synchrotron radiation news 33(6), 24 - 30 (2020). doi:10.1080/08940886.2020.1841493, Published by Taylor & Francis, Philadelphia, Pa.

Physics reports 808, 1 - 74 (2019). doi:10.1016/j.physrep.2019.02.002, 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$_2$H$_2$ 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 systemsFrom 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-TaS$_2$ 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., Published by Elsevier Science, North-Holland, Amsterdam [u.a.]

Accelerator-based light sources stimulated progress in photon science in a truly extraordinary manner. The spectral brightness of storage-ring-based facilities increased by three orders of magnitude every 10 years since the 1960s. The extreme peak brightness at single-pass free electron X-ray lasers with pulse durations variable between about 1 and 300 femtoseconds will allow transformative experiments in many areas of science. This article is an attempt to show how progress in accelerator science and technology stimulated advancement in photon science, by discussing a limited number of examples of work at third generation storage ring facilities and free electron lasers. Hopes for further improvements in specific beam properties are expressed.

X-ray free-electron lasers produce pulses of coherent X-rays that are up to nine orders of magnitude higher in peak brightness than the brightest synchrotron sources. These pulses vaporize any object placed in the focused beam, yet are brief enough to diffract from the object before significant radiation damage occurs. This process of “diffraction before destruction” overcomes previous exposure and dose limitations when imaging biological structures, which allow atomic-resolution structures to be determined from macromolecules without the need for large, strongly diffracting crystals that are difficult or impossible to grow. The extreme pulse intensity has allowed protein crystal sizes to be shrunk down to dimensions of hundreds of nanometers, expanding the range of structures that can be studied, potentially increasing the rate at which new structures can be determined, and allowing the tracking of conformational dynamics down to femtosecond timescales. Efforts are ongoing to reduce this all the way to the single molecule, opening up possibilities for robust phasing procedures to acquire model-free structures directly from the measurements. The new science of coherent diffractive imaging would be well understood by the Braggs and Laue but makes use of recent theoretical insights, modern computational capabilities, and the laser-like X-ray sources of the twenty-first century.

Self-seeding is an active filtering method for Free-Electron Lasers (FELs) enabling the production of nearly Fourier-limited pulses in the X-ray frequency range where external seeding is not available. Schematically, it is composed by three parts: a Self-Amplified Spontaneous Emission (SASE) FEL working in the linear regime, a monochromator, and an FEL amplifier. Active filtering is achieved by letting the FEL pulse produced in the SASE FEL through the monochromator, while the electron beam is sent through a bypass, and its microbunching is destroyed due to dispersion. The filtered SASE pulse, serving as a seed, is recombined with the electron beam at the entrance of the FEL amplifier part. It is then amplified up to saturation and possibly beyond via post-saturation tapering. This allows for the production of high-brightness, nearly single-mode FEL pulses. The technique has been or will be implemented in a number of X-ray FEL (XFEL) facilities under operation or in the construction phase. In this chapter, we review the principle of self-seeding, its practical realizations, and related techniques.

Coherence and interference phenomena are the fundamental properties of light. Synchrotrons and x-ray free-electron lasers (XFEL) are nowadays the principal sources of high-brilliance hard x-ray radiation. In this chapter we present an overview of theoretical developments and experimental results on measurements of coherence properties of synchrotron radiation and XFEL sources. We start with an overview of the basic principles of the theory of optical coherence and apply this general theory to the description of coherence properties of third-generation synchrotron sources as well as XFELs. We then describe propagation of partially coherent x-ray radiation through the beamline optics, especially focusing optics. We next proceed with the experimental methods of coherence measurements based on amplitude correlation measurements (e.g. Young’s doublepinhole experiments). Then we focus on the intensity correlation measurements (Hanbury Brown and Twiss experiments) demonstrating their advantages and complications. Finally, we give a short overview of the coherence properties of the new-generation x-ray sources that are in the planning stage. With the construction of the next-generation x-ray facilities aiming to reach the diffraction limit, the necessity to study and understand the coherence properties of the x-ray sources will become even more important. This knowledge will pave the way for new applications of coherent x-rays to study structure and dynamics of matter at various conditions.

Hardly any other discovery of the nineteenth century did have such an impact on science and technology as Wilhelm Conrad Rontgen s seminal find of the X-rays. X-ray tubes soon made their way as excellent instruments for numerous applications in medicine, biology, materials science and testing, chemistry and public security. Developing new radiation sources with higher brilliance and much extended spectral range resulted in stunning developments like the electron synchrotron and electron storage ring and the freeelectron laser. This handbook highlights these developments in fifty chapters. The reader is given not only an inside view of exciting science areas but also of design concepts for the most advanced light sources. The theory of synchrotron radiation and of the freeelectron laser, design examples and the technology basis are presented. The handbook presents advanced concepts like seeding and harmonic generation, the booming field of Terahertz radiation sources and upcoming brilliant light sources driven by laser-plasma accelerators. The applications of the most advanced light sources and the advent of nanobeams and fully coherent x-rays allow experiments from which scientists in the past could not even dream. Examples are the diffraction with nanometer resolution, imaging with a full 3D reconstruction of the object from a diffraction pattern, measuring the disorder in liquids with high spatial and temporal resolution. The 20th century was dedicated to the development and improvement of synchrotron light sources with an ever ongoing increase of brilliance. With ultrahigh brilliance sources, the 21st century will be the century of x-ray lasers and their applications. Thus, we are already close to the dream of condensed matter and biophysics: imaging single (macro)molecules and measuring their dynamics on the femtosecond timescale to produce movies with atomic resolution

Linear accelerator driven free-electron lasers based on the principle of self-amplified spontaneous emission represent the most promising approach to the next generation of synchrotron radiation sources going beyond currently operating 3rd generation, storage ring based source. Improvements in time-averaged brilliance by more than 5 orders of magnitude are expected. The overall pulse length of a laser flash is of the order of 100 femtoseconds, the intrinsic length of the individual spikes of the flash is of the order of 1 femtosecond. The peak brilliance of such a laser is expected to be about 10 orders of magnitude higher than the values obtained at the best storage ring based synchrotron radiation sources today. In particular, the short pulse length and the extremely high peak brilliance open up completely new areas in a large number of different fields of science in the wavelength range of the VUV, as well as soft and hard X-rays. Examples are presented in the text.

After a short description of the operation principle of linac driven free-electron lasers based on Self Amplified Spontaneous Emission (SASE) the performance of the world's first X-ray FELs, FLASH at DESY in Hamburg and LCLS at SLAC in Stanford, will be presented. Research at FELs is currently focused on understanding the interaction of extremely intense X-ray pulses with matter. Examples of early science include studies of high field physics in atoms, photo-dissociation of molecules and photo-ionization of clusters, as well as investigation of damage issues and materials in extreme conditions. First successful steps towards single particle imaging with atomic resolution have been performed. For an overview see [1].

We have investigated the nonlinear response of helium to intense extreme-ultraviolet radiation from the free-electron laser in Hamburg (FLASH). We observe a spectral feature between 24 and $26\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ electron kinetic energy in photoemission which shows a quadratic fluence dependence. The feature is explained as a result of subsequent processes involving a resonant two-photon absorption process into doubly excited levels of even parity ($N=5$ and 6), radiative decay to the doubly excited states in the vicinity of the ${\mathrm{He}}^{+}$ $(N=2)$ ionization threshold and finally the photoionization of the inner electron by the radiation of the next microbunches. This observation suggests that even-parity states, which have been elusive to be measured with the low pulse energy of synchrotron radiation sources, can be investigated with the intense radiation of FLASH. This also demonstrates a first step to bring nonlinear spectroscopy into the xuv and soft-x-ray regime.

Theory predicts that with an ultrashort and extremely bright coherent X-ray pulse, a single diffraction pattern may be recorded from a large macromolecule, a virus, or a cell before the sample explodes and turns into a plasma. Here we report the first experimental demonstration of this principle using the FLASH soft X-ray free-electron laser. An intense 25 fs, 4 10^13 W/cm^2 pulse, containing 10^12 photons at 32 nm wavelength, produced a coherent diffraction pattern from a nano-structured non-periodic object, before destroying it at 60,000 K. A novel X-ray camera assured single photon detection sensitivity by filtering out parasitic scattering and plasma radiation. The reconstructed image, obtained directly from the coherent pattern by phase retrieval through oversampling, shows no measurable damage, and extends to diffraction-limited resolution. A three-dimensional data set may be assembled from such images when copies of a reproducible sample are exposed to the beam one by one., revtex, 6 pages, 4 figures

The natural time structure of a monochromatic X-ray beam from the DORIS II storage ring with a repetition rate of 1.04 MHz has been altered by means of a rotating crystal chopper. Using heat treated Si a time interval of 8.52 ms between X-ray pulses of 150 ps length was achieved with a rotational speed of the crystal of 1760 rpm.

We present the first observation of Self-Amplified Spontaneous Emission (SASE) in a free-electron laser (FEL) in the Vacuum Ultraviolet regime at 109 nm wavelength (11 eV). The observed free-electron laser gain (approx. 3000) and the radiation characteristics, such as dependency on bunch charge, angular distribution, spectral width and intensity fluctuations all corroborate the existing models for SASE FELs., 6 pages including 6 figures; e-mail: joerg.rossbach@desy.de

The use of high energy synchrotron radiation (above 80 keV) for diffraction experiments offers many advantages resulting from the high penetration depth of the high energy photons and the small Bragg angles. The main features are: the possibility for the study of large sample crystals in transmission geometry, simple sample environments, high instrumental resolution in reciprocal space, the ability to utilize high momentum transfers and small correction factors for scattered intensities. The experiments performed at this kind of diffractometer are driven by the photon flux, in which the only requirement is a relatively small angular divergence for the incident beam in the scattering plane. The new triple crystal diffractometer introduced here will be installed at the elliptical multipole wiggler beamline at the Advanced Photon Source (APS). Because of the high critical energy of this device, 32 keV, the wiggler will produce high intensities at very high photon energies. To collect up to 1 mrad of the horizontal divergence of the beam, a bent annealed silicon monochromator will scatter and focus in the horizontal scattering plane. The diffractometer will be operated in the vertical scattering plane taking advantage of the small vertical beam divergence.

Single-pass free electron lasers (FEL) based on the self- amplification of spontaneous emission (SASE) do not require optical resonators and may therefore be used in the vacuum ultraviolet (VUV) and X-ray region down to wavelengths around 0.1 nm. Due to the start-up from noise, however, there is only little longitudinal coherence, and the photon pulses consist of many uncorrelated narrow lines. In this paper we present the conceptual design of two different optical schemes for the VUV SASE-FEL at DESY, aiming at the production of fully coherent, tunable VUV and soft X-ray radiation with good pulse-to-pulse stability. At long wavelengths it is planned to install a narrow-band optical feedback using a blazed grating in Littrow mounting. This scheme is based on the high repetition rate of the accelerator and combines the light pulse coming from the first electron bunch with the fifth electron bunch which amplifies it to saturation. At shorter wavelengths it is proposed to divide the undulator into two parts and put a high-resolution grazing-incidence monochromator in between. This scheme works for any pulse timing because the seeding pulse is combined with the same electron bunch at the entrance of the second undulator driving it to saturation.

A new design for a single pass X-ray SASE FEL is proposed. The scheme consists of two undulators and an X-ray monochromator located between them. The first stage of the FEL amplifier operates in the SASE linear regime. After the exit of the first undulator, the electron bunch is guided through a non-isochronous bypass and the X-ray beam enters the monchromator. The mail function of the bypass is to suppress the modulation of the electron beam induced in the first undulator. This is possible because of the finite value of the natural energy spread in the bem. At the entrance to the second undulator, the radiation power from the monochromator dominates significantly over the shot noise and the residual electron bunching. As a result, the second stage of the FEL amplifier operates in the steady-state regime. The proposed scheme is illustrated for the example of the 6nm option SASE FEL at the TESLA TEst Facility under construction at DESY. The spectral bandwidth of such a two-stage SASE FEL (Δλ/λ≈ 5 x 10-5) is close to the limit defined by the finite duration of the radiation pulse. The average spectral brilliance is equal to 7 x 1024 photons/(secxmrad2 x mm2 x 0.1 % bandw.) which is by two orders of magnitude higher than the value which could be reached by the conventional SASE FEL.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Physical review / B 48(23), 16965 - 16973 (1993). doi:10.1103/PhysRevB.48.16965, The angular correlation between Compton scattered photons and the recoil electrons has been measured in thin self-supporting carbon and aluminum foils using 100-keV synchrotron radiation from a wiggler at the DORIS III storage ring at Deutsches Elektronen-Synchrotron (DESY). The coincidence count rate is proportional to the three-dimensional electron momentum density (3D EMD) of the target, after integration over the energy distribution of the scattered photons and electrons it is proportional to the 2D EMD. The effect of multiple scattering of the recoil electrons within the target on the measured angular correlation is studied both by Monte Carlo calculations and by experiment., Published by Inst., Woodbury, NY

The development from a test facility for the TESLA project of a linear collider with an integrated x-ray free-electron laser (FEL) to the world's first soft x-ray FEL user facility is described. In the wavelength range from 6.5 to 60 nm, FLASH (Free-Electron Laser in Hamburg) provides short pulses (~25 fs) containing ~1012 photons in laterally coherent beams with brilliance of about nine orders of magnitude higher than achieved at the best synchrotron radiation storage ring facilities today. FLASH has opened up a new area in photon science where matter in non-equilibrium states is studied with atomic resolution in space and time.

In vast regions of the universe highly charged ions (HCI [1, 2]) are the predominant form of visible matter. Their importance extends to high-temperature terrestrial plasmas, such as those used in fusion research. Yet, accurate prediction of their electronic structure remains a challenge for theory due to the strong electromagnetic field in which the remaining bound electrons dwell. Experimental accuracy has now reached the performance limits of conventional photon spectroscopy in the soft and hard x-ray regions. In this work [3], we report on the resonant laser excitation of the 22S1/2—22P1/2 transition of the Li-like Fe22P1/223+ ion at 48.6 eV, an energy range hitherto unattainable with powerful lasers. The HCI stored in an electron beam ion trap (EBIT [4]) were resonantly excited by ultra-brilliant radiation generated at the Free electron LASer in Hamburg (FLASH [5]). While yielding a relative statistical error of only 2.2·10−5, and extending laser spectroscopy on HCI from the near ultraviolet [6] to the soft X-ray region, this novel experiment demonstrates immediate potential to push the current limits of precision by orders of magnitude. Such experiments allow to verify predictions of quantum electrodynamics (QED) in a strong field environment where perturbation theory [7, 8] fails. Future EBIT experiments at upcoming x-ray free electron lasers (X-FEL) like the Stanford Linear Coherent Light Source (LCLS) or the European X-FEL will pave the way for laser spectroscopy into the hard x-ray region.

The wide matrix rocking curves of the in situ eutectic composite TaSi2‐Si make it attractive as a wide‐bandpass monochromator for synchrotron radiation. Wafers with Si[111], Si[110], or Si[100] orientation were studied to determine the origin of the wide rocking curves. The high degree of preferred orientation of the TaSi2 rods relative to the Si matrix was examined using synchrotron Laue patterns and the TaSi2 [100], TaSi2 [003], and TaSi2 [102] reflections. Double and triple axis diffractometry were used to show that the large widths were due to strain and mosaic and not long‐range bending; copper radiation (for some double axis results) and 120 and 160 keV synchrotron radiation were used. At 8 keV, rocking curve widths were about twenty times broader than those from perfect Si, and peak reflectivities approached 20%. Rocking curves from Si[333] and Si[444] (120 and 160 keV, respectively) had identical profiles and reflectivities of about 25%. The triple axis results show compressive strains in the Si m...

Band structure calculations of the Compton profiles of copper are compared with the experiment in the light of a rigorous interpretation of momentum densities in the Hohenberg-Kohn-Sham ground state density-functional formalism. There are strong evidences that the significant discrepancies for the Compton profile anisotropies do not originate from inaccurate solutions of the Kohn-Sham self-consistent equations, but from a non-local momentum density correlation correction functional.

The energy degradation of various monochromatic gamma ray sources due to self-scattering has been studied by Monte Carlo simulation techniques. A direct measurement of the energy spectrum of the 198 Au source used for Compton scattering experiments at the Hahn-Meitner Institute (HMI) confirms the predictions of the emission of a low energy tail. The effect of the tail on Compton profiles is examined and a method of correction is proposed and applied to measurements made at the HMI.

A combined qualitative description of the band structure, Compton profiles and charge density structure factors of copper is possible by a tight-binding model with a small number of parameters. The response of the calculated properties to modifications of the model is examined in order to retrieve the complementary information about the electronic structure as obtained by Bragg diffraction and Compton scattering experiments.

γ-Ray diffractometry is a convenient method for studying the degree of perfection of large “as-grown” single crystals without the need of precedent cutting or surface treatments. By means of a double crystal arrangement the angular resolution in γ-ray diffractometry has been improved by at least one order of magnitude to 1 second of arc. The high resolution studies of crystal mosaicity on niobium, vanadium, copper and FeS2 (pyrite) single crystals as well as the measurement of the diffraction pattern of a curvature distorted Si wafer, which are presented in the paper, demonstrate the power of the technique and its enlarged range of application due to the gain in instrumental resolution.

After a brief description of the characteristic features of neutron diffraction experiments, the emphasis is on the presentation of γ-ray diffractometry as a convenient method for studying the degree of perfection of large “as-grown” single crystals without the need for cutting or surface treatments. Commercially available 192 Ir sources of 200 Ci activity and a half-life of 74.2 day provide sufficiently strong γ-lines at 316.4, 468.0 and 604.4 keV to measure a series of rocking curves in a reasonable amount of time. Because of the weak interaction of this γ-radiation with matter, its mean free path is of the order of 1 cm. Single crystals contained in a large variety of furnaces, cryostats or high pressure cells can be studied easily. In general, the kinematical diffraction theory applies in γ-ray diffractometry so that the measured rocking curves are directly proportional to the mosaic distribution function which generally describes the degree of perfection of imperfect single crystals.

The Compton profiles of aluminium and germanium have been measured with 412 keV gamma-radiation from a 198Au source. The resolution obtained in this experiment improves on previous Compton scattering studies of electron momentum distributions made with lower energy gamma-ray sources. The results for aluminium are in good agreement with other Compton profile measurements, and with theory, thus establishing the validity of the technique. The advantages and disadvantages of using this source are discussed, particularly for heavier materials such as germanium.

The total Compton profile for the 〈110〉 direction of crystalline copper has been measured with high statistical accuracy by a \ensuremath{\gamma}-ray scattering experiment. Special attention was paid to the correction of multiple photon scattering in the sample and the slight spectral contamination of the incident \ensuremath{\gamma} radiation. The results are discussed in conjunction with earlier measurements of the Compton profile anisotropies and existing band-structure calculations. The effect of the electron correlation on the Compton profile and the momentum density is found to be significantly larger than in the homogeneous electron gas. A correlation correction functional required by the Hohenberg-Kohn-Sham density-functional theory is calculated in the local density approximation and is shown to improve the agreement between density-functional band-structure calculations and experiment. The remaining discrepancies can be characterized by a redistribution of momentum density in reciprocal space as compared to the model of noninteracting electrons. The appearance of this nonlocal correlation effect is specific for the momentum density and does not contradict the generally good agreement between one-electron theory and other experimental results for copper. Possible origins of the effect are discussed tentatively.

Isotropic Compton profiles of the metals Cr and Fe 0.5 Ti 0.5 have been measured by 412 keV γ radiation. In contrast to the predictions of the rigid band model the difference is significant and well described by the renormalized free atom model for the configurations Fe(3d 6 4s 2 ), Ti(3d 2 4s 3 ) and Cr(3d 4 4s 2 ).

Bragg diffraction experiments with γ-radiation of energies of the order of 400 keV allow for high resolution studies of bulk properties of large single crystals which are relevance for the characterization of as grown single crystals as well as for the investigation of structural phase transitions. The absorption of this radiation in matter is very weak, as an example, the mean free path in copper is μ o −1 ⋍ 1 cm. Therefore samples can be mounted in any cryostat, furnace or high pressure device without causing window problems. The Bragg angles are only of the order of 1° and thus the shape of the measured diffraction profile is mainly affected by lattice tilts. γ-ray diffractometry is complementary to back scattering techniques which are most suitable for lattice parameter measurements. Using a double crystal setting diffraction patterns are recorded with an angular resolution of 1 second of arc. The integrated reflecting power is measured absolutely with an accuracy of 1% or better and by using various wave lengths in the range between 0.02 and 0.04 A, highly accurate, model independent structure factors can be determined from imperfect single crystals by means of wave length extrapolation techniques. Because of the short wave length polarization effects are neglibible. The full width at half maximum of the diffraction pattern of a perfect crystal as predicted by dynamical diffraction theory is generally less than 0.5 seconds of arc and the extinction length is of the order of 0.5 mm. Therefore small distortions from a perfect lattice cause rather large changes in the measured integrated reflecting power. Recently Pendellosung intensity beats could be measured in Si with 316 and 468 keV γ-radiation allowing to determine the 220 structure factor with an accuracy of ±- 0.1 %. In addition a surprisingly anisotropic strain field has been observed in floating-zone grown Si crystals.

High energy photon or gamma-ray scattering (E > 100 keV) is a widely recognised technique in condensed matter physics which has specific advantages. For instance, when performing diffraction experiments the low absorption, the small Bragg angles and the small wavelengths substantially reduce or eliminate corrections for absorption, polarisation and extinction, and facilitate sample environment control. A major shortcoming of both elastic and Compton scattering using high energy photons is the relatively weak intensity produced by the nuclear gamma-ray sources which have mainly been used until now. The availability of bright high energy synchrotron radiation sources and the development of adequate X-ray optics will permit a gain of several orders of magnitude in flux and open up new experimental possibilities, for instance high Q scattering and studies of anharmonicities. While designed mainly for diffraction experiments, the instrument recently proposed for the European Synchrotron Radiation Facility in Grenoble also provides the possibility of Compton scattering studies [1].

A new accurate technique, $\ensuremath{\gamma}$-ray diffractometry, has been used to study the order parameter in K${\mathrm{H}}_{2}$P${\mathrm{O}}_{4}$ near the transition point. Results are compared to birefringence measurements. The transition is of first order with a thermal hysteresis of about 0.07\ifmmode^\circ\else\textdegree\fi{}K. Unusual behavior of the order parameter has been observed for decreasing or increasing temperature near the transition.

Because phonon energies \(\hbar \omega \sim kT \sim 5x{10^{ - 4}}\) to 0.5 eV cannot beresolved in x-ray scattering experiments until now, inelastic scattering experiments are best performed with neutrons and are not discussed in this lecture. For the application of neutron diffraction techniques to the study of disordered crystals we would like to refer to a recent review article by Schmatz [1], which includes a presentation of recent results from neutron small angle scattering, and diffuse elastic scattering by nonmagnetic crystals as well as scattering by disordered magnetic systems.

The ultrafast, ultrabright X-ray pulses offered by a new generation of free-electron lasers is ushering in extraordinary new capabilities in X-ray science, with a wide range of applications in fundamental atomic-physics, ultrafast-chemistry and materials science.