田中 義人

Simultaneous measurements of picosecond time-resolved synchrotron-radiation (SR) X-ray diffraction and femtosecond time-resolved visible absorption were performed at BL40XU beamline of SPring-8 on two films of Co-Fe cyanides, Na(0.77)Co[Fe(CN)(6)](0.90)2.9H(2)O (NCF90) and Na(0.16)Co[Fe(CN)(6)](0.71)3.8H(2)O (NCF71). The simultaneous measurements unambiguously revealed that the photoinduced charge transfer between the Co and Fe sites causes uniform volume expansion/contraction within 100 ps. The photoinduced lattice contraction observed in the NCF71 film can be interpreted in terms of the photoinduced formation of a hidden metastable phase. (C) 2010 The Japan Society of Applied Physics

Brilliant pulsed x-ray synchrotron radiation (SR) is useful for pump-probe experiment such as time-resolved x-ray diffraction, x-ray absorption fine structure, and x-ray spectroscopy. For laser pump-SR x-ray probe experiments, short pulsed lasers are generally synchronized to the SR master oscillator controlling the voltage for acceleration of electron bunches in an accelerator, and the interval between the laser and the SR pulses is changed around the time scale of target phenomenon. Ideal delay control produces any time delay as keeping the time-precision and pointing-stability of optical pulses at a sample position. We constructed the time delay control module using a continuous phase shifter of radio frequency signal and a frequency divider, which can produce the delayed trigger pulses to the laser without degradation of the time precision and the pointing stability. A picoseconds time-resolved x-ray diffraction experiment was demonstrated at SPring-8 storage ring for fast lattice response by femtosecond pulsed laser irradiation, and suggested the possibility of accurate sound velocity measurement. A delay control unit operating with subpicosecond precision has also been designed for femtosecond pump-probe experiments using a free electron laser at SPring-8 campus. © 2010 American Institute of Physics.

We have developed a pump-probe X-ray diffraction measurement system for a sample with irreversible reaction at BL40XU in the SPring-8. The system mainly consists of a time-resolved measurement system, a sample disk rotation system, and an X-ray microbeam system. The time-resolved measurement system gives time resolution of 50 Ps in laser-pump and X-ray probe method. A sample disk rotation system for repetitive measurements was made to give a virgin sample for every measurement. The number of repetitions for one sample disk was increased by using the X-ray microbeam technique. To keep the overlap of the X-ray microbeam and the laser beam on the sample surface during the disk rotation, the sample disk rotation system was constructed by a low-eccentric spindle motor. By using this system, the pump-probe X-ray diffraction measurement was demonstrated for a crystallization process of a DVD material.

Emerging free-electron lasers (FELs) at short wavelengths open a new possibility for ultrafast microscopy of transient phenomena at high spatial resolution due to the ultrashort pulse duration. As a first step, we performed sub-picosecond snapshot imaging with extreme ultraviolet FEL by adopting a recently proposed holographic technique, holography with extended reference by autocorrelation linear differential operation (HERALDO). We point out that HERALDO enables efficient single-shot phase-contrast imaging by carefully designing the aperture size of the reference without sacrificing spatial resolution. The successful reconstruction of a test pattern from a single-shot hologram demonstrates an effective method of taking phase-contrast sub-picosecond snapshots. (C) 2010 The Japan Society of Applied Physics

The temporal coherence of Free-Electron Laser (FEL) sources, which exhibit, in the self-amplified spontaneous-emission mode, spiking spectral and temporal distributions, can be drastically improved by seeding with an external laser or high-order harmonics. Here, experiments at 160 nm put in evidence that the improvement of spectral properties (and thus temporal coherence) of the FEL radiation takes place for a larger seed intensity than the one required to overcome the shot noise. Copyright (C) EPLA, 2009

We achieved stable operation of a free-electron laser (FEL) based on the self-amplified spontaneous-emission (SASE) scheme at the SPring-8 Compact SASE Source (SCSS) test accelerator in the extremely ultraviolet region. Saturation of the SASE FEL power has been achieved at wavelengths ranging from 50 to 60 nm. The pulse energy has reached similar to 30 mu J at 60 nm. The observed fluctuation of the pulse energy is about 10% (standard deviation) for several hours, which agrees with the expectation from the SASE theory showing the stable operation of the accelerator. The SASE FEL has been routinely operated to provide photon beams for user experiments over a period of a few weeks. Analysis on the experimental data gave the normalized-slice emittance at the lasing part is around 0.7 pi mm mrad. This result indicates that the normalized-slice emittance of the initial electron beam, 0.6 pi mm mrad in a 90% core part, is kept almost unchanged after the bunch compression process with a compression factor of approximately 300. The success of the SCSS test accelerator strongly encourages the realization of a compact XFEL source.

Understanding the mechanism of rapid phase change process in DVD (digital versatile disc) materials is one of the important topics in materials science, and hence numerous studies investigating the phase change process as well as structural analysis of the crystal and amorphous phases have been reported. Nevertheless, the mechanism of rapid phase change is still unclear, owing to the lack of detailed structure analysis, in particular on the amorphous phase and its crystallisation process. We have studied the amorphous structure of Ge 2Sb2Te5 and the crystallisation process by high energy synchrotron x-ray diffraction with the aid of structure modelling and time-resolved synchrotron x-ray diffraction, in order to obtain key information for revealing the rapid phase change mechanism. We found a large fraction of 4-fold and 6-fold rings in the atomic configuration of the amorphous phase. Intriguingly the bond angle distributions of rings exhibit a peak at approximately 90°, which corresponds to that in the crystal phase. Therefore, it is suggested that the 4-fold and 6-fold rings are nuclei for crystallisation. Furthermore, time-resolved x-ray diffraction measurements combined with photoreflectivity measurements during the crystallisation of the amorphous phase suggest that the crystallisation of Ge2Sb 2Te5 can be explained by a nucleation driven process. We conclude that a large amount of nuclei is the reason for rapid crystallisation in Ge2Sb2Te5 with a nucleation driven process.

Understanding the mechanism of rapid phase change process in DVD (digital versatile disc) materials is one of the important topics in materials science, and hence numerous studies investigating the phase change process as well as structural analysis of the crystal and amorphous phases have been reported. Nevertheless, the mechanism of rapid phase change is still unclear, owing to the lack of detailed structure analysis, in particular oil the amorphous phase and its crystallisation process. We have studied the amorphous structure of Ge2Sb2Te5 and the crystallisation process by high energy synchrotron x-ray diffraction with the aid of structure modelling and time-resolved synchrotron x-ray diffraction, in order to obtain key information for revealing the rapid phase change mechanism. We found a large fraction of 4-fold and 6-fold rings in the atomic configuration of the amorphous phase. Intriguingly the bond angle distributions of rings exhibit a peak at approximately 90 degrees, which corresponds to that in the crystal phase. Therefore, it is suggested that the 4-fold and 6-fold rings are nuclei for crystallisation. Furthermore, time-resolved x-ray diffraction measurements combined with photoreflectivity measurements during the crystallisation of the amorphous phase suggest that the crystallisation of Ge2Sb2Te5 can be explained by a nucleation driven process. We conclude that a large amount Of nuclei is the reason for rapid crystallisation in Ge2Sb2Te5 with a nucleation driven process.

A time-resolved structural investigation of the data storage process in a digital versatile disk (DVD) or Blu-ray disk media is desired for better understanding of the fast phase-change mechanism for designing a higher performance optical recording material. Thus, the development of an in situ structural observation technique at the picosecond level has been carried out for the investigation of the fast phase-change phenomena using synchrotron radiation pulsed X-rays and synchronized femtosecond laser irradiation. Then, for Ge(2)Sb(2)Te(5) (GST) and Ag(3.5)In(3.8)Sb(75.5)Te(17.7) (AIST), the technique has been applied to a snapshot of X-ray diffraction pattern in the nanosecond crystallization process from the amorphous to the crystal phase, which corresponds to the erasing process of the DVD recording system. The key to success of the time-resolved experiment was to optimize the measurement timing system efficiency for both sufficient counting statistics, i.e., counting quantity and precision of time resolution, i.e., counting quality. Our time-resolved experimental system has sufficient accuracy, so that our previous study showed a significant difference in the crystal growth process between GST and AIST, indicating that the crystal growth process is key to fast phase change. To increase the precision of data in terms of signal-to-noise ratio and counting statistics, we developed a highly repetitive pump-probe measurement system combined with an X-ray microbeam technique providing high counting quantity, and used it to demonstrate a snapshot of an X-ray diffraction profile of GST. The measurement system has a time resolution of 50 ps and a 1 kHz repetition rate. Our development of the technique to take a snapshot for the structural visualization of phase-change phenomena with the time-resolved microbeam X-ray diffraction is described. (C) 2009 The Japan Society of Applied Physics

A time-resolved structural investigation of the data storage process in a digital versatile disk (DVD) media has been desired for better understanding of the fast phase-change mechanism to get a clue in designing of higher performance optical recording material. Thus, the development of an in situ structural observation technique in the level of picosecond has been carried out for the investigation of the fast phase-change phenomena, by using synchrotron radiation pulsed X-rays and synchronized femtosecond laser irradiation. Then, for Ge₂Sb₂Te₅ and Ag_3.5In_3.8Sb_75.0Te_17.7, the technique has been applied to a snapshot of X-ray diffraction observation of nanosecond crystallization process from amorphous to crystal phase which corresponds to erasing process of DVD recording system. The details of the time-resolved X-ray diffraction apparatus coupled with in situ photoreflectivity measurement and its performance are described.

Charge and lattice dynamics has been investigated for Na0.79Co[Fe(CN)(6)](0.9)2.9H(2)O film at 300 K by means of the time-resolved x-ray diffraction apparatus coupled with the in situ time-resolved absorption measurement. We observed an uniform lattice expansion whose relaxation time is similar to 40 ns. Based on the close correlation between the charge and lattice dynamics, the lattice expansion is ascribed to the photo-created charge-transferred Co2+ - Fe3+ state.

Scanning tunneling microscopy (STM) combined with synchrotron radiation (SR) enabled elemental analysis on solid surfaces at an anometer scale. The principle of analysis is based on the inner-shell excitation of a specific energy level under STM observation. After first demonstrative results on a semiconductor heterointerface (Ge nanoisland on an Si(111) 7 x 7 surface), second trial of nanoscale elemental analysis by the SR-STM system was accomplished for Cu nanodomains on a Ge(111) 2 x 8 surface. A key to achieve successful results is to effectively increase the signal to noise ISM) ratio and to extract the element-specific modulation of the tunneling current, which is derived from the inner-shell excitation. An essential point for signal detection with a high S/N ratio is to construct a total system including lock-in amplifier with an optical chopper for high accuracy, and a real-time control system to check the incident-beam condition by observing the beam-induced current profiles. Using these techniques, it has been shown that the SR-STM system provides new possibilities forn anoscale surface analysis. Copyright (C) 2008 John Wiley & Sons, Ltd.

Conventional synchrotron radiation sources enable the structure of matter to be studied at near-atomic spatial resolution and picosecond temporal resolution. Free-electron lasers promise to extend this down to femtosecond timescales. The process by which free-electron lasers amplify synchrotron light-known as self-amplified spontaneous emission(1-3) - is only partially temporally coherent, but this can be improved by seeding it with an external laser(4,5). Here we explore the use of seed light produced by high-order harmonic generation in a gas(6-9), covering wavelengths from the ultraviolet to soft X-rays. Using the SPring-8 Compact SASE Source test accelerator(10-12), we demonstrate an increase of three orders of magnitude in the intensity of the fundamental radiation at 160 nm, halving of the free-electron laser saturation length, and the generation of nonlinear harmonics(13) at 54 nm and 32 nm. The low seed level used in this demonstration suggests that nonlinear harmonic schemes should enable the generation of fully coherent soft X-rays at wavelengths down to the so-called 'water window', vital for the study of biological samples.

The crystallization process in digital versatile disc (DVD) media was investigated using a time-resolved X-ray diffraction apparatus coupled with in situ photoreflectivity measurement. The time profiles of crystallization were found to be consistent with the changes in photoreflectivity. The phase changes were characterized by the start and end time; 90 +/- 1 and 273 +/- 1 ns for Ge(2)Sb(2)Te(5), and 85 +/- 1 and 206 +/- 1 ns for Ag(3.5)In(3.8)Sb(75.0)Te(17.7), respectively. The faster crystallization time in Ag(3.5)In(3.8)Sb(75.0)Te(17.7) is ascribed to its characteristic crystallization process; its X-ray diffraction profile shows a significant sharpening during the crystallization process, whereas the peak width of Ge(2)Sb(2)Te(5) remained unchanged. The present findings suggest that crystal growth control is another key for designing faster phase-change materials. (C) 2008 The Japan Society of Applied Physics.

An ultra-high-precision clock system for long time delay has been developed for picosecond time-resolved x-ray diffraction measurements using synchrotron radiation (SR) pulses and synchronized femtosecond laser pulses. The time delay control between pump laser pulse and the probe SR pulse was achieved by combining an in-phase quadrature modulator and a synchronous counter. This method allowed us to change the delay time by a nearly infinite amount while maintaining the precision of +/- 8.40 ps. Time-resolved diffraction measurements using the delay control system were demonstrated for precise measurement of an acoustic velocity in a single crystal of gallium arsenide. (C) 2008 American Institute of Physics.

Today, single-pass Free-Electron Lasers (FEL) produce a highly bright radiation, the Self Amplified Spontaneous Emission (SASE), which spectral and temporal profiles are composed of a series of spikes. We demonstrate here the strong and coherent amplification of the 5(th) harmonic of a Ti: Sa laser (800 nm, 10 Hz, 100 fs) generated in a gas cell, i.e. 160 nm, by the SCSS (SPring-8 Compact SASE Source, Japan) Test Accelerator FEL. This is obtained by overlapping transversally, spectrally and temporally the external harmonic source in the in-vacuum undulator with the electron beam (150 MeV, 10 Hz, 1 ps). With only one undulator section, the 160 run seeded emission achieves three orders of magnitude higher intensity than the unseeded one, and presents a quasi perfect Gaussian shape in the spectral distribution. With two undulator sections, the seeded FEL spectrum reveals first effects of saturation. Those spectacular phenomena are associated to the generation of intense and coherent Non Linear Harmonics (NLH) at 54 nm and 32 nm. Finally, in view of the low seed level required, such amplification associated to NLH schemes would allow the generation of fully coherent soft X-ray radiations down to the water window.

To solve difficulties of instability and inaccuracy in synchrotron radiation based scanning tunneling microscopy, a method to reduce noise was investigated. New insulator-coat tips were developed to shut out electrons coming from a wide area that damage the spatial resolution. By changing the exposed conductive area at the end of the insulator-coat tips, the effect of noise reduction was estimated. The tip with an exposure area of 50 nm in diameter was found to reduce noise effectively. Also a key discriminating condition was found to obtain the local signal, which is based on the modulation of the X-ray-induced tip current caused by excitation of the specific element. (c) 2007 Elsevier B.V. All rights reserved.

We investigate light-induced crystallization mechanism of hen egg-white lysozyme. Photochemical intermediate radical has been expected to form cluster which grows to nucleus. To confirm the nucleation process that the radical grows to the crystal, SDS-PAGE, crystallization experiments were carried out. Covalent-bonded dimer was produced by photochemical reaction, and it grows to the crystal. We conclude that the dimer plays the role of the smallest stable cluster in early stage of the crystallization process.

Intense synchrotron radiation sources have enabled us to combine time-resolved measurements and triple-crystal diffractometry. The time-resolved triple-crystal diffractometry (TRTCD) determines the time-dependent dilational and shear components of deformation tensor, separately. The TRTCD experiments have been performed at a long undulator beamline of SPring 8. The time-resolved measurement system using pump-probe technique and a fast multi-channel scaler covers a full range of milliseconds with a time-resolution of several tens of picoseconds. The TRTCD with wide time range was applied to the dynamic strain measurement for semiconductor wafers irradiated by a femtosecond pulse laser. We observed a dilational component of acoustic echo pulses to analyze the time-varying pulse shape due to propagation. The lattice motion in the successively induced flexural standing wave has also been observed through a shear component.

The pulse characteristic and high coherent x-ray beam of SPring-8 allow us to investigate dynamics of chemical reactions and phase transition of materials caused by applied field. In order to realize such direct investigation,,,x-ray pinpoint structural measurement", which is the advanced x-ray measurement technique in nanometer spatial scale and/or pico-second time scale, is being developed at SPring-8. The features of "x-ray pinpoint structural measurement" technique are, 1) spatial resolution: similar to 100 mn, 2) time resolution: similar to 40 ps, and 3) measurement under the photo-irradiation, electric field, magnetic field, high pressure and active devices. Using this technique, we will explore the novel concept and new phenomena for nanomaterials and/or devices, and also demonstrate their validity.

The three-dimensional atomic configuration of amorphous Ge2Sb2Te5 and GeTe were derived by reverse Monte Carlo simulation with synchrotron-radiation x-ray diffraction data. The authors found that amorphous Ge2Sb2Te5 can be regarded as "even-numbered ring structure," because the ring statistics is dominated by four- and six-fold rings analogous to the crystal phase. On the other hand, the formation of Ge-Ge homopolar bonds in amorphous GeTe constructs both odd- and even-numbered rings. They believe that the unusual ring statistics of amorphous Ge2Sb2Te5 is the key for the fast crystallization speed of the material. (c) 2006 American Institute of Physics.

Top-up operation allows SPring-8 to provide highly stable X-ray beams with arbitrary filling patterns. The implementation of top-up operation is described, with a focus on the simultaneous achievement of stability of stored current, beam orbit, purity of an isolated single bunch, and beam injection efficiency. Stored-current fluctuations have been routinely reduced to a level of 10(-3). Stored-beam oscillation on frequent beam injection, which was originally regarded as the most serious problem, has been successfully suppressed to a sufficiently low level that it never perturbs imaging experiments. Current impurities in nominally empty buckets have been reduced to a level of 10(-9) over more than one week of operation, making possible the measurement of time-resolved spectra using high-current bunches. Finally, excellent injection efficiency, higher than 80%, is routinely obtained, even for small undulator gaps, which is critical for preventing radiation damage to insertion-device magnets and to reduce leakage radiation. The process of achieving highly stabilized top-up operation at SPring-8 and its utility for user experiments are described.

Thermal desorption at a chlorine-adsorbed Si(111) surface was measured with high precision. High-sensitivity measurements of the temperature dependence of the isothermal process, and thermal desorption spectra (TDS) with various parameters, heating rates and levels of surface coverage, indicated that the desorption is a second-order reaction with an activation energy of 2.2 eV. The wide dynamic-range data throw light on the ability of various methods of thermal desorption measurement to describe quantitatively the surface reaction. It is important to obtain a precise energy value, which can be done by considering the whole TDS shape, as well as isothermal data, in order to distinguish various reaction processes. Our results are consistent with model calculations. (c) 2006 Elsevier B.V. All rights reserved.

In situ scanning tunneling microscopy (STM) with highly brilliant hard X-ray irradiation was enabled at SPring-8. To obtain a good signal-to-noise ratio for elemental analysis, an X-ray beam with a limited size of phi 10 mu m was aligned to a specially designed STM stage in ultrahigh vacuum. Despite various types of noises and a large radiation load around the STM probe, STM images Were Successfully observed with atomic resolution. The use of a new system for elemental analysis was also attempted, which was based oil the modulation of tunneling signals rather than emitted electrons. Among tunneling signals, tunneling current was found to be better than tip height as a signal to be recorded, because the former reduces markedly the error of measurement. Oil a Ge nanoisland oil a clean Si(111) surface, the modulation Of tunneling current was achieved by changing the incident photon energy across the Ge absorption edge.

A scanning tunneling microscope dedicated to in situ experiments under the irradiation of highly brilliant hard-X-rays of synchrotron radiation has been developed. In situ scanning tunneling microscopy ( STM) observation was enabled by developing an accurate alignment system in ultrahigh vacuum. Despite the noisy conditions of the synchrotron radiation facility and the radiation load around the probe tip, STM images were successfully obtained at atomic resolution. Tip-current spectra were obtained for Ge nano-islands on a clean Si( 111) surface by changing the incident photon energy across the Ge absorption edge. A current modification was detected at the absorption edge with a spatial resolution of the order of 10 nm.

Acoustic pulse echoes generated by femtosecond laser irradiation were detected using time-resolved x-ray triple-crystal diffractometry. The determined time-dependent longitudinal strain component for pulse echoes in silicon and gallium arsenide plates showed that the polarity of the strain pulse was dependent on the optically induced initial stress, and that the bipolar pulse waveform was gradually deformed and broadened in the course of propagation. The three-dimensional wave front distortion of pulse echoes was shown simply as the pulse duration broadening, which was consistent with a boundary roughness for an unpolished plate.

Cl etching of silicon surfaces assisted with short pulsed laser irradiation is analyzed to find out chloride removal processes without using thermal reaction. When a Cl-saturated Si(111) surface was irradiated by a picosecond pulsed laser at 400 nm, desorptions of SiCl and SiCl2 molecules were observed with a quadrupole mass spectrometer with a pulse-counting system. The presence of the SiCl desorption shows previously unfound etching process without thermally induced recombinative reaction. Time-of-flight and signal-ratio of the SiCl and SiCl2 molecules indicated that they were assigned to products from restatom monochlorides and adatom polychlorides, respectively. High desorption rates of the chlorides which depend on laser fluence nonlinearly suggest that multiple excitation by substrate hot carriers is essential to the etching.

Triple-crystal X-ray diffractometry has been combined with time-resolved measurement techniques. A simple time-resolved technique was employed by using a digital storage oscilloscope and a fast X-ray detector. The time dependence of the deformation tensor was determined for a gallium arsenide wafer after a flash of a 130 fs laser pulse. The laser pulse produced instantaneous expansion resulting in a localized convex surface. Time-resolved measurements showed that a flexural standing wave, explained well by the classical elasticity theory for thin plates, appeared in the relaxation process.

To furnish semiconductor surfaces at device fabrication, control of micro-structures is expected of the most importance in the near-future. Highly sensitive measurement of desorption from Cl-adsorbed Si(111) surface indicated that a diffusive process plays an important role on the surface. The desorption barrier was estimated to be 2.2 eV from temperature-dependence of time-course in isothermal desorption, suggesting the desorption site contains distortion. Defects, which may be steps or clusters, are the dominant sites where the associative reaction occurs. In STM images, clusters were observed at step edges, as well as growth of pits and defects. From comparison of the desorption with high temperature STM images, a cluster pins the step movement to form step bunches. Interaction between clusters through the step movements makes clusters aligned. Ordered structures can be obtained when velocity of the surface diffusion matches the rate of the chloride desorption. (C) 2004 Elsevier B.V. All rights reserved.

As the first step to modify a semiconductor surface with light, we have studied the effect of infrared (IR) pulsed laser irradiation on Cl-saturated Si(1 1 1)-7 x 7 surface. After the surface was irradiated by a femto-second IR pulsed laser, we measured temperature programmed desorption (TPD) to observe the change of surface chemical component. Thermal desorption of SiCl2 molecules was reduced and that of SiCl4 molecules was enhanced by the IR irradiation. This result can be interpreted that polychloride radicals made of silicon adatoms are removed from the rest atom and form silicon polychloride clusters on the surface. The change was found to occur in the time scale less than 1s, so that it may be via an electronic excitation process. Desorption of chlorides under the IR irradiation was detected at room temperature, but not detected at low temperature (150 K). The desorption at room temperature increased during the first 60 s of the IR irradiation, which implies that the desorption is due to a temperature rise of the substrate. It is concluded that the IR irradiation effect on the surface is classified into two processes dominated by the electronic excitation and the thermal effect. (C) 2004 Elsevier B.V. All rights reserved.

A time-to-space conversion scheme has been investigated to determine the time interval between laser and x-ray pulses. In future pump-probe experiments using ultrashort x-ray pulses, the precise register of the interval of arrival times of the laser and x-ray pulses can make obtained data with a non-negligible jitter be sorted, providing a precision higher than the jitter. This scheme was tested using the SPring-8 synchrotron radiation pulses with a duration of a few tens ps. A femtosecond pulsed laser beam was guided to a GaAs crystal surface with a shallow glancing angle, while an x-ray beam onto the surface from the opposite direction of the laser beam with the similar glancing angle, giving rise to enlarging asymmetric diffraction. The position where the both pulses meet on the surface was identified by observing the change in the diffraction intensity due to the transient expansion of lattice spacing. Since the position is a function of the interval of arrival times of the pulses, the relative arrival time has been determined with a precision of less than the SR pulse duration from the observed spatial distribution of the diffraction intensity.

We present subnanosecond-resolved X-ray diffraction experiments at the SPring-8 high flux beamline BL40XU by use of time structure of synchrotron radiation. A phase-locked mechanical chopper (x-ray pulse selector), which is made by Forschungszentrum Julich, has minimum opening time of 400 nanoseconds and generates single-bunch x-ray pulse trains with variable intervals of 1-2 millisecond (0.5-1 kHz) depending on the rotation speed and with the x-ray pulse duration of similar to 30 picoseconds from the hybrid bunch mode (D-mode) of the SPring-8 ring operation. The interval of pulse trains is further expanded to more than I second by another slower shutter, if necessary. The pulse train is applicable to time-resolved diffraction experiments with both monochromatic and pink beams. An observed Laue data set from lysozyme crystal showed sufficient quality to refine the protein structure at 1.9 Angstrom resolution. A 10-nanosecond pulse from a Nd:YAG laser initiating photoreaction in a crystal can be synchronized with the X-ray pulse. Installation of a mode-locked Ti:sapphire laser/regenerative amplifier system which has a pulse duration of similar to 2 picoseconds for further pump-probe experiments is also planed.

The effect of 800 nm picosecond-pulsed laser irradiation on the local adsorbate structure of the Cl/Si(111) surface was investigated using a highly sensitive mass spectrum measurement system. It was found from the desorption profile obtained after laser irradiation at room temperature that the concentration of polychlorides and silicon chloride clusters were increased on the surface. Irradiation to the surface at about 150 K causes decomposition of tetrachloride, fragments of which are trapped by defects. As desorptions exhibit strong anisotropy to laser polarization and cooling the substrate suppresses desorption, it is suggested that the process is inequilibriumly coupled with the substrate lattice due to very short pulse excitation.

Thermal desorption kinetics of bromine adsorbed onto Si(111) was studied using a highly sensitive mass spectrum measurement system. Temperature programmed desorption measurements simultaneously monitored SiBr+, SiBr+ and SiBr3+. Low-energy electron diffraction and an isothermal desorption method identified desorption spectral features associated with the surface structures. As the temperature increased to similar to600 K, silicon polybromides composed of Si adatoms desorbed lwith desorption barriers of 0.8 eV for SiBrn (n greater than or equal to 3) and 1.0 eV for SiBr2. The desorption of SiBrn (n greater than or equal to 3) originated from silicon polybromide clusters, which emerged at similar to550 K. When the transition of surface structure occurred from 7 x 7 into 1 x 1 at similar to800 K, different SiBr2 desorption barriers were observed for the growing 1 x 1 domain and the disappearing 7 x 7 domain. After reconstruction around 900 K, the recombinative desorption, which is caused by the diffusion of the atoms on the surface, had a desorption barrier of 2:6 eV for SiBr2 from the stable Br-terminated 1 x 1 surface.

The optical switching of X-rays using laser-induced crystal lattice expansion is described. Irradiation of a gallium arsenide (GaAs) crystal using picosecond laser pulses shifts the Bragg angle through the lattice expansion with a response time of a few hundred picoseconds. A single pulse was extracted from the synchrotron radiation pulse train using a double-crystal arrangement of GaAs, in which the two crystals were irradiated by way of two successive laser pulses with an appropriate time delay.

Synchronization system between an intense picosecond laser and the target X-ray pulses has been developed at SPring-8. The intense laser pulses were obtained by amplification of the pulses picked up from a mode-locked Ti:sapphire laser synchronized with the radio frequency of the storage ring. The repetition rate of amplified laser pulses was controlled to be I In of the RF, where n is a multiple of the number of RF buckets in the ring, so that the laser pulses meet the SR pulses originated from a particular electron bunch in partial filling patterns. The temporal overlap of the laser and the target X-ray pulses was achieved as monitored with a streak camera in synchroscan and repetitive single shot operation modes, and was stable with a precision of a few ps for several hours. (C) 2001 Elsevier Science B.V. All rights reserved.

We have developed a highly sensitive X-ray streak camera system, which synchronously operates with the RF signal of the SPring-8 storage ring. The streak camera was installed at an undulator beamline of SPring-8, and the beam loading effect for various electron bunch structures (filling pattern) has been observed. The camera has also been operated as a timing monitor for a synchronization system of synchrotron radiation and a Ti:sapphire laser. The highly sensitive X-ray streak camera system can be used not only as an accelerator diagnosis, but also as a fast temporal detector for beamline applications, such as the observation of fast temporal transitions of diffraction images and relaxation process. (C) 2001 Elsevier Science B.V. All rights reserved.

An X-ray undulator beamline with a 1 km long transport channel was constructed. High quality beam with more than 6 x 10(13) photons/s is delivered by an in-vacuum undulator and a cryogenic cooling system adapted to the monochromator. Experimental stations contain versatile multi-axis precision diffractometers, various detectors and a synchronized picosecond laser system. (C) 2001 Elsevier Science B.V. All rights reserved.

The first beamline for a long X-ray undulator source is under construction at SPring-8 for development of coherence-related X-ray applications. The structure of the beamline, with four experimental hutches, is based on those of SPring-8 standard X-ray undulator beamlines. The station equipment prepared for the initial operation are a multi-axis precision diffractometer, a femto-second laser system and a multi-circle diffractometer mounting a 15 T super-conducting magnet. Commissioning of the beamline will start in October 2000. (C) 2001 Elsevier Science B.V. All rights reserved.

We observed the x-ray-excited optical luminescence (XEOL) of erbium-doped silicon (Si:Er) thin films to make a site-selective x-ray absorption fine structure (XAFS) measurement of an optically active Er atom. The undulator beam was used for the increment of the electron population in the excited state, and following XEOL at an infrared wavelength of 1.54 mum with minimum absorption loss in the host Si was detected. The edge-jump and XAFS oscillation were successfully obtained at the Er L-III-edge. This spectrum originated from inner-shell excitation and relaxation of only the optically active Er atom, indicating that site-selectivity at an atomic level was achieved.

The X-ray-excited optical luminescence (XEOL) of erbium-doped silicon (Si:Er) thin film is observed for the site-selective X-ray absorption spectroscopy (XAS) of an optically active Er atom. In order to achieve the site-selectivity at the atomic level, intense X-ray from an undulator is used for the excitation of Er. Indirect excitation caused by X-ray absorption at Si around Er is investigated, and X-ray induced luminescence independent of the inner-shell excitation of Er is observed. A possible mechanism is proposed, in which the formation of an electron-hole pair in Si and the subsequent transfer of recombination energy excite the valence electron of Er. According to this model, energy back-transfer with phonon absorption is used to suppress the indirect excitation, resulting in the site-selective XAS of Er atom. (C) 2001 Elsevier Science B.V. All rights reserved.

A one-kilometer long synchrotron radiation beamline with an x-ray undulator source was completed at SPring-8. The beamline was planned to facilitate various applications of a wide-area coherent x-ray beam, development of bi-crystal x-ray interferometers for gravitational red-shift measurement and development of highly sensitive diagnostic methods of accelerator dynamics. This paper reports the structure of the long beamline as well as some selected first results including phase contrast imaging and diffraction imaging applications.

In order to discuss the local structure of an optically active center in Er-doped Si thin film, site-selective x-ray absorption fine structure (XAFS) analysis using x-ray-excited optical luminescence was performed. The XAFS spectrum at the Er L-III edge was obtained from the x-ray photon energy dependence of the peak intensity of infrared luminescence due to Er intra-4f transition. Although conventional XAFS measurement analyzes the average structure of all of the Er, this method intrinsically provides structural information for only optically active Er. A broad 2p-5d resonant peak in the site-selective XAFS spectrum is reproduced by a density-of-state calculation of a distorted ErO6 cluster, assuming an Er transformation from an octahedral center of 0.25 Angstrom. (C) 2001 American Institute of Physics.

The performance of a CsI photocathode for hard x rays (8-46 keV) was compared to a Au photocathode using synchrotron radiation. We found that the sensitivity of CsI (100 nm thick) is 20-100 times higher than that of Au (30 nm thick), depending on the photon energy. The time response for 40 ps pulses is also improved slightly using CsI due to its narrower secondary electron energy spread. The study was performed in a streak camera environment; thus the results obtained include the instrumental effects of the camera and directly show the real performance of the cathodes for streak cameras. Although the question of material stiffness still remains for CsI, no degradation of cathode performance was observed during the present study. (C) 2000 American Institute of Physics. [S0034-6748(00)01001-7].