佐藤 文菜

Photoinduced phase transitions are of special interest in condensed matter physics(1,2) because they can be used to change complex macroscopic material properties on the ultrafast timescale. Cooperative interactions between microscopic degrees of freedom greatly enhance the number and nature of accessible states, making it possible to switch electronic, magnetic or structural properties in new ways(2-9). Photons with high energies, of the order of electron volts, in particular are able to access electronic states that may differ greatly from states produced with stimuli close to equilibrium(10). In this study we report the photoinduced change in the lattice structure of a charge and orbitally ordered Nd(0.5)Sr(0.5)MnO(3) thin film using picosecond time-resolved X-ray diffraction. The photoinduced state is structurally ordered, homogeneous, metastable and has crystallographic parameters different from any thermodynamically accessible state. A femtosecond time-resolved spectroscopic study shows the formation of an electronic gap in this state. In addition, the threshold-like behaviour and high efficiency in photo-generation yield of this gapped state highlight the important role of cooperative interactions in the formation process. These combined observations point towards a 'hidden insulating phase' distinct from that found in the hitherto known phase diagram.

Despite a century of research, the cellular function of myoglobin (Mb), the mechanism regulating oxygen (O(2)) transport in the cell and the structure-function relationship of Mb remain incompletely understood. In particular, the presence and function of pores within Mb have attracted much recent attention. These pores can bind to Xe as well as to other ligands. Indeed, recent cryogenic X-ray crystallographic studies using novel techniques have captured snapshots of carbon monoxide (CO) migrating through these pores. The observed movement of the CO molecule from the heme iron site to the internal cavities and the associated structural changes of the amino acid residues around the cavities confirm the integral role of the pores in forming a ligand migration pathway from the protein surface to the heme. These observations resolve a long-standing controversy - but how these pores affect the physiological function of Mb poses a striking question at the frontier of biology.

In order to explore the ligand-migration dynamics in myoglobin induced by photodissociation, cryogenic X-ray crystallographic investigations of carbonmonoxy myoglobin crystals illuminated by continuous wave and pulsed lasers at 1-15 kHz repetition rate have been carried out. Here it is shown that this novel method, extended pulsed-laser pumping of carbonmonoxy myoglobin, promotes ligand migration in the protein matrix by crossing the glass transition temperature repeatedly, and enables the visualization of the migration pathway of the photodissociated ligands in native Mb at cryogenic temperatures. It has revealed that the migration of the CO molecule into each cavity induces structural changes of the amino-acid residues around the cavity which result in the expansion of the cavity. The sequential motion of the ligand and the cavity suggests a self-opening mechanism of the ligand-migration channel arising by induced fit.

The first direct observation of the transient spin-state in a disordered magnetic system with time-resolved XAFS is reported. By observing the evolution of the Fe(II) 1s -3d transition, the spin crossover transition from the (1)A(1) low spin state to (5)T(2) high spin state has been directly observed on a picosecond time scale. Moreover, observation of the transient spin state with time-resolved XAFS allows for the investigation of the variations in the electronic state and molecular structure. This unique experimental technique probes the excited states involved in the ultrafast photoinduced reactions in disordered magnetic systems.

Experimental strategy and setup for the 100 ps time-resolved X-ray absorption spectroscopy (TR-XAS) is presented. X-ray positional active feedback combined with a figure-of-merit scan of the laser beam position was employed as a key technique. It is shown that the pump-probe TR-XAS technique using the X-ray pulse structure of synchrotron radiation sources is a powerful tool to investigate dynamics of the electronic state and molecular structure of non-crystalline samples. A TR-XAS measurement of a photo-induced spin crossover reaction of the tris(1,10-phenanthrorine)iron(II) complex in water is presented.

100 ps time-resolved X-ray solution-scattering capabilities have been developed using multilayer optics at the beamline NW14A, Photon Factory Advanced Ring, KEK. X-ray pulses with an energy bandwidth of Delta E/E = 1-5% are generated by reflecting X-ray pulses (Delta E/E = 15%) through multilayer optics, made of W/B(4)C or depth-graded Ru/C on silicon substrate. This tailor-made wide-bandwidth X-ray pulse provides high-quality solution-scattering data for obtaining photo-induced molecular reaction dynamics. The time-resolved solution scattering of CH(2)I(2) in methanol is demonstrated as a typical example.

Proteins harbor a number of cavities of relatively small volume. Although these packing defects are associated with the thermodynamic instability of the proteins, the cavities also play specific roles in controlling protein functions, e. g., ligand migration and binding. This issue has been extensively studied in a well-known protein, myoglobin (Mb). Mb reversibly binds gas ligands at the heme site buried in the protein matrix and possesses several internal cavities in which ligand molecules can reside. It is still an open question as to how a ligand finds its migration pathways between the internal cavities. Here, we report on the dynamic and sequential structural deformation of internal cavities during the ligand migration process in Mb. Our method, the continuous illumination of native carbonmonoxy Mb crystals with pulsed laser at cryogenic temperatures, has revealed that the migration of the CO molecule into each cavity induces structural changes of the amino acid residues around the cavity, which results in the expansion of the cavity with a breathing motion. The sequential motion of the ligand and the cavity suggests a self-opening mechanism of the ligand migration channel arising by induced fit, which is further supported by computational geometry analysis by the Delaunay tessellation method. This result suggests a crucial role of the breathing motion of internal cavities as a general mechanism of ligand migration in a protein matrix.

Time-resolved X-ray techniques utilizing pulsed nature of synchrotron radiation are becoming general and powerful tools to explore structural dynamics in materials and biological systems. The time-resolved technique enables to produce structural movies at 100-picosecond temporal and sub-nanometer spatial resolution. It will be fascinating to apply such capabilities to capture ultrafast cooperative phenomena in strongly-correlated electron systems, photochemical catalytic reaction dynamics in liquid or on solid surface, light-induced response of photosensitive protein molecules, etc. The time-resolved X-ray studies conducted at NW14, PF-AR, KEK will be presented.

We report the lattice dynamics in the photoinduced phase of Nd0.5Sr0.5MnO3 thin film by means of time-resolved x-ray diffraction measurements at the NW14A beamline in the Photon Factory Advanced Ring. We observed the decrease of intensity after the photoirradiation for a superlattice reflection which is associated with Jahn-Teller distortion.

Studying photo-induced molecular dynamics in liquid with sub-nanosecond time-resolution and sub-Angstrom spatial resolution gives information for understanding fundamental chemical process in the photo-induced cooperative phenomena of molecular systems and also for developing new materials and devices. Here, we present time-resolved Xray absorption fine structure on the spin-crossover complex Fe-II tris-(1,10-phenanthroline) dissolved in aqueous solution. We utilized femtosecond laser at 400nm pulse for excitation and 100ps X-ray pulse for probe.

The study of photo-controled nature of materials, including their optical, magnetic, and conducting properties, is a fascinating research field. The finding of photo-induced phase transition (PIPT) has triggered the search for inorganic and organic systems with highly efficient and ultrafast photo-responses. As a result of the recent progress in quantum-beam technologies, the time-resolved study of PIPT dynamics on the femto-second time scale, which is comparable with the single-cycle of phonon vibration, has become feasible. In contrast, ultra-slow dynamics on the time scales of a few seconds to several minutes play an important role in the cooperative phenomena in complex systems. Here, we review both the ultra-fast and ultra-slow dynamics of the photo-induced cooperative effects in a typical organic CT crystal (EDO-TTF)(2)PF6 and a protein molecule, myoglobin (Mb). In the case of Mb, we discuss the results from the viewpoint of a unique photo-functionality, i.e., the photo-induced transportation of a small molecule in the "super-structure" of a protein molecule.

An experimental set-up for time-resolved X-ray absorption spectroscopy with 100 ps time resolution at beamline NW14A at the Photon Factory Advanced Ring is presented. The X-ray positional active feedback to crystals in a monochromator combined with a figure-of-merit scan of the laser beam position has been utilized as an essential tool to stabilize the spatial overlap of the X-ray and laser beams at the sample position. As a typical example, a time-resolved XAFS measurement of a photo-induced spin crossover reaction of the tris(1,10-phenanthrorine)iron(II) complex in water is presented.

We report a single-shot nanosecond time-resolved Laue diffraction measurement of cadmium sulfide (CdS) single crystal under laser-induced shock compression. The observed Laue diffraction pattern maintains sixfolding axis of the wurtzite structure for 10 ns at a shock pressure of 3.92 GPa, which is above the threshold pressure of phase transition to a rocksalt structure. This result shows that a transient wurtzite structure is observed above its threshold pressure to a rocksalt structure on a nanosecond time scale. Uniaxial compression was confirmed by the c/a value of the transient structure obtained from the (201) and (302) peaks. (c) 2007 American Institute of Physics.

NW14A is a newly constructed undulator beamline for 100 ps time-resolved X-ray experiments at the Photon Factory Advanced Ring. This beamline was designed to conduct a wide variety of time- esolved X-ray measurements, such as time-resolved diffraction, scattering and X-ray absorption fine structure. Its versatility is allowed by various instruments, including two undulators, three diffractometers, two pulse laser systems and an X-ray chopper. The potential for the detection of structural changes on the 100 ps time scale at NW14A is demonstrated by two examples of photo-induced structural changes in an organic crystal and photodissociation in solution.

Ultra-fast time-resolved X-ray crystallography using electron-accelerator-based Xray sources has been becoming a general and powerful tool to explore structural dynamics of crystalline state in material and biological sciences. Photon Factory Advanced Ring (PF-AR) is a full-time single-bunch synchrotron radiation source operated for such time-resolved X-ray studies using pulsed X-rays. Here we report instrumentation, feasibility, and applications of time-resolved X-ray diffraction at PF-AR. The time-resolved X-ray diffraction equipment consists of an X-ray pulse selector, an X-ray diffractometer, a femtosecond laser system and their timing modules which provide synchronized X-ray pulses with 50-picosecond duration (rms) and 150-femtosecond laser pulses for laser-pump-X-ray-probe experiment. The current status of the beam lines NW2 and NW14, and their applications are described.

Two independent luminescence channels are observed from manganese-doped spinel Mn:MgAl2O4. The luminescence around 520 nm is assigned to transition from the lowest electronic excited state T-4(1) to the ground state (6)A(1) of Mn2+ (3d)(5) ion by analyzing the excitation spectrum and electron spin resonance measurement. The emission at 650 nm is triggered by the band edge excitation and is assigned similarly to the charge-transfer process associated with the manganese ion. (C) 2004 Elsevier B.V. All rights reserved.

Strong blue emission is observed from Ti-doped MgAl2O4, yellow emission from a Mn-doped one and white light emission from a V-doped one. The optimum condition to obtain the strongest emission was studied by changing the doping density and growing atmosphere. The microscopic models for these emission processes are proposed from observation of other optical responses and electron spin resonance spectra both in the electronic ground and excited states.