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.     

Solid-state photoelectron spectroscopy with synchrotron radiation

Synchrotron radiation sources, providing intense, tunable, polarized, and stable beams of ultraviolet and x-ray photons, are having a great impact on biology, physics, chemistry, materials science, and other areas of research. Synchrotron radiation has revolutionized solid-state photoelectron spectroscopy by enhancing its capabilities for investigating the electronic behavior of solids and solid surfaces. Several fundamental photoemission techniques that rely on synchrotron radiation are discussed in this article. These techniques have an adjustable tunable surface sensitivity and provide the first direct mapping of the electronic band structure. Recent applications of photoelectron spectroscopy for studies of chemisorption geometries and surface structures are discussed.     

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.     

A 6-GeV Storage Ring: An Advanced Photon Research Facility

The high brightness of synchrotron radiation produced by storage rings has enhanced research capability by factors of 10(6) to 10(8), from the ultraviolet to the x-ray region. There has been a steady growth in synchrotron radiation research of interest in physics, chemistry, biology, geology, and the other physical sciences and their associated technologies. A large number of university, government, and industrial research organizations are utilizing this new capability at existing facilities. In the process, new partnerships have been created among these communities that have enabled each to contribute its expertise and to obtain its needed capability. A 6-gigaelectron volt storage ring, designed for use with periodic magnetic insertion devices called wigglers and undulators, has been proposed; such a facility could provide further enhancements of 10(4) to 10(6) in capability and could double the synchrotron radiation research capacity in the United States. This enhanced capability and capacity will provide new opportunities for a broad range of scientific and technological interests.     

Radiation belts of jupiter

Predictions of Jupiter's electron and proton radiation belts are based mainly on decimeter observations of 1966 and 1968. Extensive calculations modeling radial diffusion of particles inward from the solar wind and electron synchrotron radiation are used to relate the predictions and observations.     

Jupiter's Decametric Radio Emission and the Radiation Belts of Its Galilean Satellites

Many of the observed properties of Jupiter's decametric radiation may be explained by postulation that the inner Galilean satellites of Jupiter have magnetic properties that strongly distort Jupiter's magnetic field in the region of each satellite. Charged particles from Jupiter's radiation belts are trapped by these distorted fields and emit synchrotron radiation.     

LIII-Edge Anomalous X-ray Scattering by Cesium Measured with Synchrotron Radiation

Diffraction of monochromatized synchrotron radiation by crystals of cesium hydrogen tartrate has been used to measure the magnitude and phase of x-ray scattering for cesium near the LIII absorption edge. In this wavelength region the scattering amplitude of cesium is reduced by as much as 25 electrons per atom, compared to scattering of copper Kalpha x-rays. This change, which varies as a function of wavelength, affects the diffraction intensities in a manner similar to isomorphous substitution, and it is large enough to have promise for phase determination in the study of macromolecular structures. This experiment also demonstrates that accurate diffractometer measurements are possible with synchrotron radiation produced by an electron storage ring.     

Possibility of maser action in cosmic radio sources

A mechanism is described for maser action in synchrotron sources. The process may contribute to the lowfrequency radiation from 3C273B.     

A scanning x-ray microscope using synchrotron radiation

Focused synchrotron radiation collimated by means of a pinhole has been used to construct a scanning x-ray microscope capable of making stereoscopic element-discriminating pictures of relatively thick specimens in an atmospheric environment.     

Inverse compton effect: some consequences for quasars

The inverse Compton effect can transform enough energy of relativistic electrons into radiation so that an upper limit to the mean energy of the electrons is set. In quasars, the limit is too small to allow the production of any appreciable amount of synchrotron or inverse Compton radiation, unless either the distances are not cosmological or the lifetimes of the relativistic electrons are extremely short, of the order of hours.     

Seeing with a new light: synchrotron radiation

The special properties of synchrotron light are leading to a rapid increase in its utilization for both research and technology. At wavelength in the ultraviolet region of the broad spectrum in these beams a number of atomic, molecular, and solid-state spectroscopies are being pursued; soft x-rays are being used for spectroscopy, lithography, microscopy, and topography; at still shorter wavelengths, advantage is taken of scattering properties to probe the structure of matter. Characteristics of synchrotron radiation and of the sources which produce it are described, and some typical investigations and applications are presented to suggest the versatility of these sources.     

UV-B增加对几种不同作物影响程度的种间比较

采用大田群体试验方法,研究了UV-B增加对小麦、大豆、棉花及玉米生长和产量的影响。结果表明,UV-B强度增加1.00W·m-2,不同作物对紫外线UV-B增加的敏感性存在很大的差异。作物的产量和反应指数均表明,作物对紫外线增加的敏感性为棉花>大豆>小麦>玉米。初步分析了不同作物对紫外线敏感差异的区别。     

X-ray Laue Diffraction from Protein Crystals

In conventional x-ray diffraction experiments on single crystals, essentially monochromatic x-rays are used. If polychromatic x-rays derived from a synchrotron radiation spectrum are used, they generate a Laue diffraction pattern. Laue patterns from single crystals of macromolecules can be obtained in less-than 1 second, and significant radiation damage does not occur over the course of an exposure. Integrated intensities are obtained without rotation of the crystal, and individual structure factors may be extracted for most reflections. The Laue technique thus offers advantages for the recording of diffraction patterns from short-lived structural intermediates; that is, for time-resolved crystallography.     

Tunable Coherent X-rays

A modern 1- to 2-billion-electron-volt synchrotron radiation facility (based on high-brightness electron beams and magnetic undulators) would generate coherent (laser-like) soft x-rays of wavelengths as short as 10 angstroms. The radiation would also be broadly tunable and subject to full polarization control. Radiation with these properties could be used for phase- and element-sensitive microprobing of biological assemblies and material interfaces as well as reserch on the production of electronic microstructures with features smaller than 1000 angstroms. These short wavelength capabilities, which extend to the K-absorption edges of carbon, nitrogen, and oxygen, are neither available nor projected for laboratory XUV lasers. Higher energy storage rings (5 to 6 billion electron volts) would generate significantly less coherent radiation and would be further compromised by additional x-ray thermal loading of optical components.     

UV-B增强对作物生产影响的研究回顾与展望

大气平流层臭氧耗损引起的地表紫外辐射(UV-B)增强是气候变化问题之一。UV-B辐射增强对作物生长、生理代谢、产量及品质的影响受到人们普遍关注。通常借助两种方法开展模拟试验研究,即平方波模型法和太阳追踪模型法。UV-B辐射增强引起作物生长受阻、分蘖数减少、株高下降、叶面积和叶绿素含量下降、光系统Ⅱ受抑、光合效率降低;UV-B辐射增强导致活性氧代谢失衡、叶片气孔器受到破坏、叶绿体结构变形、基粒片层排列紊乱;UV-B辐射增强使作物有效穗数、穗粒数、千粒重下降,导致产量下降;UV-B辐射增强对籽粒蛋白质影响因作物、品种而异。未来应加强UV-B辐射增强影响作物内源激素代谢分子机制研究、区域和全球模拟及其应对措施研究。     

Transient interface sharpening in miscible alloys

We observed that diffuse interfaces sharpen rather than broaden in completely miscible ideal binary systems. This is shown in situ during heat treatments at gradually increasing temperatures by scattering of synchrotron radiation in coherent Mo/V multilayers containing initially diffuse interfaces. This effect provides a useful tool for the improvement of interfaces and offers a way to fabricate better x-ray or neutron mirrors, microelectronic devices, or multilayers with giant magnetic resistance.     

太阳辐射减弱对冬小麦生长发育及产量的影响

[目的]研究太阳辐射减弱对冬小麦生长发育及产量的影响。[方法]通过盆栽模拟试验,设置不同的太阳辐射减弱梯度(自然光、70%太阳辐射、50%太阳辐射),研究太阳辐射减弱对冬小麦干物质积累及产量的影响。[结果]太阳辐射减弱使冬小麦生育期延长,穗、叶和茎等干物质积累减少。太阳辐射减弱下,有效穗数、穗粒数、穗重、千粒重等产量构成指标下降。[结论]太阳辐射减弱对冬小麦的生长发育和产量存在明显的抑制效应。     

Desorption of H from Si(111) by resonant excitation of the Si-H vibrational stretch mode

Past efforts to achieve selective bond scission by vibrational excitation have been thwarted by energy thermalization. Here we report resonant photodesorption of hydrogen from a Si(111) surface using tunable infrared radiation. The wavelength dependence of the desorption yield peaks at 0.26 electron volt: the energy of the Si-H vibrational stretch mode. The desorption yield is quadratic in the infrared intensity. A strong H/D isotope effect rules out thermal desorption mechanisms, and electronic effects are not applicable in this low-energy regime. A molecular mechanism accounting for the desorption event remains elusive.     

Amorphization of Serpentine at High Pressure and High Temperature

Pressure-induced amorphization of serpentine was observed at temperatures of 200° to 300°C and pressures of 14 to 27 gigapascals with a combination of a multianvil apparatus and synchrotron radiation. High-pressure phases then crystallized rapidly when the temperature was increased to 400°C. These results suggest that amorphization of serpentine is an unlikely mechanism for generating deep-focus earthquakes, as the temperatures of subducting slabs are significantly higher than those of the rapid crystallization regime.     

Density of phonon states in iron at high pressure

The lattice dynamics of the hexagonal close-packed (hcp) phase of iron was studied with nuclear inelastic absorption of synchrotron radiation at pressures from 20 to 42 gigapascals. A variety of thermodynamic parameters were derived from the measured density of phonon states for hcp iron, such as Debye temperatures, Gruneisen parameter, mean sound velocities, and the lattice contribution to entropy and specific heat. The results are of geophysical interest, because hcp iron is considered to be a major component of Earth's inner core.