Coherent soft x-ray generation in the water window with quasi-phase matching

We demonstrate enhanced generation of coherent light in the "water window" region of the soft x-ray spectrum at 4.4 nanometers, using quasi-phase-matched frequency conversion of ultrafast laser pulses. By periodically modulating the diameter of a gas-filled hollow waveguide, the phase mismatch normally present between the laser light and the generated soft x-ray light can be partially compensated. This makes it possible to use neon gas as the nonlinear medium to coherently convert light up to the water window, illustrating that techniques of nonlinear optics can be applied effectively in the soft x-ray region of the spectrum. These results advance the prospects for compact coherent soft x-ray sources for applications in biomicroscopy and in chemical spectroscopy.     

Determination of macromolecular structures from anomalous diffraction of synchrotron radiation

Resonance between beams of x-ray waves and electronic transitions from bound atomic orbitals leads to a phenomenon known as anomalous scattering. This effect can be exploited in x-ray crystallographic studies on biological macromolecules by making diffraction measurements at selected wavelengths associated with a particular resonant transition. In this manner the problem of determining the three-dimensional structure of thousands of atoms is reduced to that of initially solving for a few anomalous scattering centers that can then be used as a reference for developing the entire structure. This method of multiwavelength anomalous diffraction has now been applied in a number of structure determinations. Optimal experiments require appropriate synchrotron instrumentation, careful experimental design, and sophisticated analytical procedures. There are rich opportunities for future applications.     

Imaging atomic structure and dynamics with ultrafast x-ray scattering

Measuring atomic-resolution images of materials with x-ray photons during chemical reactions or physical transformations resides at the technological forefront of x-ray science. New x-ray-based experimental capabilities have been closely linked with advances in x-ray sources, a trend that will continue with the impending arrival of x-ray-free electron lasers driven by electron accelerators. We discuss recent advances in ultrafast x-ray science and coherent imaging made possible by linear-accelerator-based light sources. These studies highlight the promise of ultrafast x-ray lasers, as well as the technical challenges and potential range of applications that will accompany these transformative x-ray light sources.     

Harnessing attosecond science in the quest for coherent X-rays

Modern laser technology has revolutionized the sensitivity and precision of spectroscopy by providing coherent light in a spectrum spanning the infrared, visible, and ultraviolet wavelength regimes. However, the generation of shorter-wavelength coherent pulses in the x-ray region has proven much more challenging. The recent emergence of high harmonic generation techniques opens the door to this possibility. Here we review the new science that is enabled by an ability to manipulate and control electrons on attosecond time scales, ranging from new tabletop sources of coherent x-rays to an ability to follow complex electron dynamics in molecules and materials. We also explore the implications of these advances for the future of molecular structural characterization schemes that currently rely so heavily on scattering from incoherent x-ray sources.     

Exponential gain and saturation of a self-amplified spontaneous emission free-electron laser

Self-amplified spontaneous emission in a free-electron laser has been proposed for the generation of very high brightness coherent x-rays. This process involves passing a high-energy, high-charge, short-pulse, low-energy-spread, and low-emittance electron beam through the periodic magnetic field of a long series of high-quality undulator magnets. The radiation produced grows exponentially in intensity until it reaches a saturation point. We report on the demonstration of self-amplified spontaneous emission gain, exponential growth, and saturation at visible (530 nanometers) and ultraviolet (385 nanometers) wavelengths. Good agreement between theory and simulation indicates that scaling to much shorter wavelengths may be possible. These results confirm the physics behind the self-amplified spontaneous emission process and forward the development of an operational x-ray free-electron laser.     

Origin of the hard x-ray emission from the Galactic plane

The Galactic plane is a strong emitter of hard x-rays (2 to 10 kiloelectron volts), and the emission forms a narrow continuous ridge. The currently known hard x-ray sources are far too few to explain the ridge x-ray emission, and the fundamental question of whether the ridge emission is ultimately resolved into numerous dimmer discrete sources or truly diffuse emission has not yet been settled. In order to obtain a decisive answer, using the Chandra X-ray Observatory, we carried out the deepest hard x-ray survey of a Galactic plane region that is devoid of known x-ray point sources. We detected at least 36 new hard x-ray point sources in addition to strong diffuse emission within a 17' by 17' field of view. The surface density of the point sources is comparable to that at high Galactic latitudes after the effects of Galactic absorption are considered. Therefore, most of these point sources are probably extragalactic, presumably active galaxies seen through the Galactic disk. The Galactic ridge hard x-ray emission is diffuse, which indicates omnipresence within the Galactic plane of a hot plasma, the energy density of which is more than one order of magnitude higher than any other substance in the interstellar space.     

High-Resolution Imaging by Fourier Transform X-ray Holography

Fourier transform x-ray holography has been used to image gold test objects with submicrometer structure, resolving features as small as 60 nanometers. The hologram-recording instrument uses coherent 3.4-nanometer radiation from the soft x-ray undulator beamline X1A at the National Synchrotron Light Source. The specimen to be imaged is placed near the first-order focal spot produced by a Fresnel zone plate; the other orders, chiefly the zeroth, illuminate the specimen. The wave scattered by the specimen interferes with the spherical reference wave from the focal spot, forming a hologram with fringes of low spatial frequency. The hologram is recorded in digital form by a charge-coupled device camera, and the specimen image is obtained by numerical reconstruction.     

Multiphoton ionization of atoms

Studies of multiphoton ionization of atoms have revealed several unexpected characteristics. The confluence of the experimental evidence leads to the hypothesis that the basic character of the atomic response involves highly organized, coherent motions of entire atomic shells. The important regime, for which the radiative field strength is greater than an atomic unit (e/a(2)(0)), can be viewed in approximate correspondence with the physics of fast (approximately 10 MeV per atomic mass unit) atom-atom scattering. This physical picture provides a basis for the expectation that stimulated emission in the x-ray range can be produced by direct, highly nonlinear coupling of ultraviolet radiation to atoms.     

Ultrafast X-ray Pulses from Laser-Produced Plasmas

A high-temperature plasma is created when an intense laser pulse is focused onto the surface of a solid. An ultrafast pulse of x-ray radiation is emitted from such a plasma when the laser pulse length is less than a picosecond. A high-speed streak camera detector was used to determine the duration of these x-ray pulses, and computer simulations of the plasmas agree with the experimental results. Scaling laws predict that brighter and more efficient x-ray sources will be obtained by the use of more intense laser pulses. These sources can be used for time-resolved x-ray scattering studies and for the development of x-ray lasers.     

Electronic structure of mott insulators studied by inelastic X-ray scattering

The electronic structure of Mott insulators continues to be a major unsolved problem in physics despite more than 50 years of research. Well-developed momentum-resolved spectroscopies such as photoemission or neutron scattering cannot probe the full Mott gap. High-resolution resonant inelastic x-ray scattering revealed dispersive charge excitations across the Mott gap in a high-critical temperature parent cuprate (Ca(2)CuO(2)Cl(2)), shedding light on the anisotropy of the Mott gap. These charge excitations across the Mott gap can be described within the framework of the Hubbard model.     

New Opportunities in Synchrotron X-ray Crystallography

Several high-intensity synchrotron x-ray sources have been constructed over the past few years in the United States, West Germany, Great Britain, Japan, France, Italy, and the Soviet Union. Crystallographers have begun to use these facilities for experiments that take advantage of the characteristics of synchrotron radiation, namely, a broad distribution of wavelengths, high intensity, low divergence, strong polarization, and a pulsed time structure. In addition to more familiar diffraction experiments on single crystals and powdered samples, new types of crystallographic studies, for example, energy-dispersive and surface diffraction studies, have progressed rapidly with more general accessibility of synchrotron sources. These high-intensity sources allow diffraction experiments to be performed on very small crystals or on large biological molecules, and permit weak magnetic scattering to be detected Anomalous dispersion experiments can exploit he ability to vary the wavelength of the radiation, and the pulsed time structure of the beam makes possible fast time-resolved experiments. Because of the availability of synchrotron x-radiation, these and other kinds of experiments will be in the forefront of crystallographic research for the next several years.     

Observation of orbital currents in CuO

Orbital currents are proposed to be the order parameter of the pseudo-gap phase of cuprate high-temperature superconductors. We used resonant x-ray diffraction to observe orbital currents in a copper-oxygen plaquette, the basic building block of cuprate superconductors. The confirmation of the existence of orbital currents is an important step toward the understanding of the cuprates as well as materials lacking inversion symmetry, such as magnetically induced multiferroics. Although observed in the antiferromagnetic state of cupric oxide, we show that orbital currents can occur even in the absence of long-range magnetic moment ordering.     

Synchrotron X-ray Diffraction from a Microscopic Single Crystal Under Pressure

Metallic filaments with submicrometer diametere have been fabricated. Standard diffraction techniques with conventional x-ray sources were unsuccessful in identifying the structure of these materials. However, with the use of synchrotron radiation produced on a wiggler beam line, diffraction data were obtained in measurement periods as short as 10 milliseconds. Two cylindrical single crystals of bismuth were studied, each with a diameter of 0.22 +/- 0.02 micrometer. The volume of sample illuminated for these measurements was 0.38 cubic micrometer, less than 0.5 femtoliter. The crystals are grown in glass capillaries, and, because bismuth expands on solidification, they are under a residual hoop stress. The crystallographic data indicate the presence of a linear compressive strain of about 2 percent, which is assumed to be the result of a residual stress of about 2 gigapascals.     

Time-resolved x-ray diffraction of biological materials

Instrumental and specimen considerations pertinent to performing time-resolved x-ray diffraction on biological materials are discussed. Existing synchrotron x-ray sources, in conjunction with integrating x-ray detectors, have made millisecond diffraction experiments feasible; exposure times several orders of magnitude shorter than this will be possible with synchrotron sources now on the drawing boards. Experience gained from time-resolved studies together with order-of-magnitude estimates of specimen requirements can be used to determine the instrumental capabilities needed for various time-resolved experiments. Existing instrumental capabilities and methods of dealing with time-resolved specimens are reviewed.     

Electron- and photon-stimulated desorption: probes of structure and bonding at surfaces

Techniques for analyzing the structure and composition of solid surfaces with electron and photon beams often cause radiation damage in samples. Damage-producing processes compete with information-producing events during measurements, and beam damage can be a serious perturbation in quantitative surface analysis. There are, however, substantial benefits of electron- and photonstimulated damage processes for studying molecules adsorbed on surfaces. Direct information about the geometric structure of surface molecules can be obtained from measurements of the angular distributions of ions released by electron- or photon-stimulated desorption. The directions of ion emission are determined by the orientation of the surface bonds that are ruptured by beam irradiation. Moreover, photon-stimulated desorption studies that make use of synchrotron radiation reveal the fundamental electronic excitations that lead to bondbreaking processes at surfaces. These measurements provide new insights into radiation-damage processes in areas as diverse as x-ray optics and semiconductor electronics.     

Time scales in atmospheric chemistry: coupled perturbations to N2O, NOy, and O3

Nitrous oxide (N2O) is one of the top three greenhouse gases whose emissions may be brought under control through the Framework Convention on Climate Change. Current understanding of its global budget, including the balance of natural and anthropogenic sources, is largely based on the atmospheric losses calculated with chemical models. A representative one-dimensional model used here describes the photochemical coupling between N2O and stratospheric ozone (O3), which can easily be decomposed into its natural modes. The primary, longest lived mode describes most of the atmospheric perturbation due to anthropogenic N2O sources, and this pattern may be observable. The photolytic link between O3 and N2O is identified as the mechanism causing this mode to decay 10 to 15 percent more rapidly than the N2O mean atmospheric lifetime, affecting the inference of anthropogenic sources.     

Atomic memory for correlated photon states

We experimentally demonstrate emission of two quantum-mechanically correlated light pulses with a time delay that is coherently controlled via temporal storage of photonic states in an ensemble of rubidium atoms. The experiment is based on Raman scattering, which produces correlated pairs of spin-flipped atoms and photons, followed by coherent conversion of the atomic states into a different photon beam after a controllable delay. This resonant nonlinear optical process is a promising technique for potential applications in quantum communication.     

Microholography of living organisms

By using intense pulsed coherent x-ray sources that are currently under development, it will be possible to obtain magnified three-dimensional images of elementary biological structures in the living state at precisely defined instants. For optimum contrast, sensitivity, and resolution, the hologram should be made with x-rays tuned to a resonance of nitrogen near 0.3 nanometer. Resolution will then be limited mainly by the hydrodynamic expansion that occurs while the necessary number of photons is being registered. Problems of technique are also briefly discussed.     

High-Resolution X-ray Observations of the Orion Nebula

Observations of the Trapezium region in the Orion Nebula obtained with the high-resolution x-ray imaging instrument on board the Einstein Observatory reveal at least 58 sources of x-ray emission. All but two of the sources can be identified with visible stars. The strongest x-ray source is the star Theta(1)C, which excites the emission nebula. Its x-ray luminosity is 6 x 10(32) ergs per second. The rest of the x-ray sources may be identified with stars of all spectral types. Strong x-ray emission is not observed from members of the infrared cluster embedded within the Orion molecular cloud.     

Generation of femtosecond pulses of synchrotron radiation

Femtosecond synchrotron pulses were generated directly from an electron storage ring. An ultrashort laser pulse was used to modulate the energy of electrons within a 100-femtosecond slice of the stored 30-picosecond electron bunch. The energy-modulated electrons were spatially separated from the long bunch and used to generate approximately 300-femtosecond synchrotron pulses at a bend-magnet beamline, with a spectral range from infrared to x-ray wavelengths. The same technique can be used to generate approximately 100-femtosecond x-ray pulses of substantially higher flux and brightness with an undulator. Such synchrotron-based femtosecond x-ray sources offer the possibility of applying x-ray techniques on an ultrafast time scale to investigate structural dynamics in condensed matter.