Table of contents

This issue is a collection of papers presented at the 2nd International Conference on Photo-Induced Phase Transitions; Cooperative, non-linear and functional properties (PIPT'05), which was held in Rennes (France) on 24–28 May 2005 and chaired by Hervé Cailleau and Tadeusz Luty. The first PIPT conference was organized by Professor Keiichiro Nasu in Tsukuba, Japan, in 2001. During 5 days, PIPT'05 provided an interdisciplinary forum for research communications between solid state physicists, photophysicists, photochemists and photobiologists, as well as material scientists. Scientists came from all around the world (Europe, Japan, USA, Canada, ...). The fascinating scientific challenge of the possibility of triggering physical properties of a material by light excitation was at the heart of the exchange of ideas between scientists of the different fields. The topics of the conference were about light-induced phenomena in functional materials, nano-particles and devices, photo-induced structural, magnetic and/or electronic phase transitions, photo-induced gauge type phase transitions, photo-induced cooperative molecular switching and chemical reactions in solids, dynamics of out-of-equilibrium processes, light-driven non-thermal processes such as coherent phonons, shock waves, surface melting and femtomagnetism, precursor phenomena, coherent, co-operative and non-linear processes in excited states and new investigations by light, x-ray and electron probes.

As you will see, the collection of papers presented here cover many of the fields mentioned above. The PIPT conferences, encompassing different areas of light-induced phenomena are also meant to bridge the gap between experimentalists and theoreticians, and to promote communication between scientists working on different continents. The present conference was attended by 148 participants from 17 countries, with the total number of 87 presentations (16 invited, 27 contributed talks and 44 posters). It is encouraging to note that, in addition to the unwavering participation of the established members of the PIPT community, numerous young researchers could also participate in the conference. This has been possible thanks to the generous sponsorship from the University of Rennes 1, Rennes Métropole, Conseil Général d'Ille et Vilaine, Région Bretagne, CNRS (France) and JST/ERATO (Japan) and the organizers would like to express their enormous gratitude to these organizations.

The present conference benefited also from substantial logistic support from the University of Rennes 1, which is gratefully acknowledged here.

The papers published in this issue were refereed and accepted papers, according to the normal rules of the Journal of Physics: Conference Series. This proceedings issue comprises 40 papers. Thanks are due to IOP Publishing Ltd, of Dirac House, Temple Back, Bristol BS1 6BE, UK for their willingness to publish these proceedings.

The Guest Editors express their gratitude to all the authors and the referees, while the organizers of PIPT'05 would like to take this opportunity to thank the members of the International Advisory Committee and of the Local Organization Committee, the invited speakers, the session chairmen and all the attendees (see photo below) for their contribution to the conference. At the website http://www.gmcm.univ-rennes1.fr/pipt additional information and some photographs can be found. It is also a pleasure to announce that the next conference of the PIPT series will be held in 2007 in Japan, and will be chaired by Shin-ya Koshihara.

In connection with the recent experimental discoveries on gigantic photoenhancements of the electronic conductivity and the quasi-static dielectric susceptibility in SrTiO₃, we theoretically study a photo-generation mechanism of a charged ferroelectric domain in this quantum dielectric. The photo-generated electron, being quite itinerant in the 3d band of Ti⁴⁺, is assumed to couple weakly but quadratically with soft-anharmonic T_1u phonons in this quantum dielectric. The photo-generated electron is also assumed to couple strongly but linearly with the breathing type high energy phonons. Using a tight binding model for electron, we will show that these two types of electron-phonon couplings result in two types of polarons, a ''super-para-electric (SPE) large polaron'' with a quasi-global parity violation, and an ''off-centre type self-trapped polaron'' with only a local parity violation. We will also show that this SPE large polaron is nothing else but a singly charged (e⁻) and conductive ferroelectric (or SPE) domain with a quasi macroscopic size. This polaron or domain is also shown to have a high mobility and a large quasi-static dielectric susceptibility.

It is an attractive target for materials science to find a system which shows the phase transition triggered by external stimulation of light. The purpose of our study is to review experimental evidences indicating that the photo-injected local excitation can really trigger the cooperative phenomena in solids. In this sense, this unique photo-induced effect can be named as photo-induced phase transition (PIPT). Here, I will also make brief review on the experimental research on PIPT combining with a development of ultra-fast quantum electronics technology.

Recent developments in the study of ultrafast dynamics are reviewed concerning the spin ladder system, α'-NaV₂O₅, which is known to have a charge order phase transition accompanying spin dimerization at 34 K. The low energy excitations specific to the low temperature phase is observed by coherent excitation method and their origins are discussed, comparing with the Raman and infrared absorption data. Photoinduced phenomena are studied by transient reflectance and time-resolved Raman spectroscopy and the thermal and nonthermal effects are distinguished by their response time.

We have first recalled the T(LIESST) procedure which consists to determine the temperature above which the photo-magnetic effect is erased. In addition we have selected to series of iron(II) spin crossover complexes, the [Fe(PM-L)₂(NCS)₂] and [Fe(bpp)₂]X₂·nH₂O families, to analyse the influence of the cooperativity on the stability of the photo-induced HS state. Some of these complexes exhibit gradual thermal spin crossover behaviours while some others undergo an abrupt thermal transition, with and without hysteresis. Interestingly, whatever the cooperativity effect on the thermal spin crossover transition, the lifetime of the metastable state of all these derivates remains governed by the T(LIESST) = T₀ - 0.31 T_1/2 relation. Finally, we have investigated the magnetic and the photomagnetic properties of a [Fe(bpp)₂]-Nafion film. Once more the role of the cooperativity on the stability of the photoinduced HS state appears minor. Conversely, the influence of the nature and the geometry of the inner coordination sphere appears preponderant.

Effectron-effectron interactions play an important role in nonequilibrium properties of molecular materials. First, we show differences between photoinduced ionic-to-neutral and neutral-to-ionic transitions in quasi-one-dimensional extended Peierls Hubbard models with alternating potentials. Cooperative dynamics lead to nonlinear ionicity in the former, while uncooperative dynamics lead to quite linear ionicity in the latter, as a function of the energy supplied from the oscillating effectric field. Interchain effectron-effectron interactions bring about initial competition among metastable and stable domains in neighboring chains, slowing down the phase transition. Interchain elastic couplings are necessary to form a ferroeffectric long-range order. Second, we show differences between field-effect characteristics of Mott insulators and those of band insulators in one-dimensional Hubbard models, to which tight-binding models are attached for metallic effectrodes and scalar potentials are added for interfacial barriers. Ambipolar characteristics are found in the former, while unipolar characteristics generally appear in the latter. In the former, charge transport is cooperative so that the drain current is insensitive to the difference between the work function of the channel and that of the effectrodes, and thus insensitive to the polarity of the gate bias.

A quasi-one-dimensional halogen-bridged Pd complex, of which the chemical formula is [Pd(chxn)₂][Pd(chxn)₂Br₂]Br₄, is explored theoretically to demonstrate its possibility for a ultrafast photoinduced phase transition (PIPT) from a charge-density-wave (CDW) state to a Mott-insulator (MI) state. We use a dp model of which the parameter values are chosen to reproduce the absorption peak energies. First, we calculate adiabatic potential energy curves along possible relaxation channels from CDW to MI. The results show that there is no energy barrier in between. Exciton excitations in the former smoothly lead to a spin excitation plus a fractional charge pair excitation of ± 1/2e (e = |e|) in the latter. Second, assuming a highenergy photoexcitation, an extra effectron or a hole is injected in the system. The result is that a PIPT can occur at least for a hole carrier case, again yielding a spin excitation and a fractional charge pair of +1/2e and +1/2e.

Ultrafast light-induced insulator-metal phase transitions (IM PT) in VO₂ thin films was studied with use of a pump-probe technique. These studies show that the IM PT could be realized via an intermediate state. The relaxation processes after optical pumping are dependent on pump energy. The excitonic controlled model for such type of IM PT is proposed on this basis. The main channel for the ultrafast light-induced IM PT is the resonant transition between excited states of correlated vibronic Wannier-Mott excitons (WME) in insulator phase and the unoccupied excited states in metallic phase. The experiment observation, such as pump power drastic dependences of relaxation could be interpreted in the framework of the model.

We have investigated the structural dynamics of gold nanoparticles induced by femtosecond light excitation. Structure evolution in both embedded particles (glass matrix or liquid water suspension) and quasi-free particles adsorbed on a solid surface is analyzed. By use of stroboscopic laser pump- x-ray probe techniques the structural relaxations have been resolved on the 100 ps time scale at the European Synchrotron Radiation Facility. Several methods including powder scattering, liquid scattering and small angle scattering serve to resolve microscopic and mesoscopic length scales of the composite system. The thermal response includes the heating, lattice melting, explosive solvent evaporation and solvent cooling subsequent to the laser flash excitation. Nonthermal effects are observed with femtosecond excitation. They are attributed to ablation from the particle and particle explosion at strong nonequilibrium conditions. The observations can form a complete picture of the energy dissipation and phase transitions involved in nanoscale composites.

We investigated the switching behavior of small particles of an Ising-like model under constant excitation by means of Monte Carlo simulations to study photoinduced spinstate switching in nanoparticles of transition metal complexes. The threshold intensity required for that switching becomes drastically small in small particles with diameter of less than 10 pseudospins. This lower intensity results enhancement of the pseudospin fluctuation at the surface in the small particles. Our result might originate the increase of the photoinduced magnetization in nanoparticles of a Mo-Cu cyanide.

We investigate time evolution of the magnetization and high spin fraction of the system in which spin-crossover phenomena and magnetic transitions are involved, proposing a new model which allows us to describe both spin-crossover (SC) transition and magnetic ordering of Prussian blue analogs under photoirradiation. Both the intermolecular lattice interaction and the intermolecular magnetic interaction are considered in this model, which enables unified descriptions of the photoinduced SC transition and magnetic transitions. For the dynamics of the system, we take into account the multi-time scales in a master equation approach for the magnetic relaxation, the lattice (electronic) relaxation, and the photoexcitation process. We observe non-linear dependence of the growth of the magnetization and high spin fraction on irradiation time and that on the strength of irradiation, which are typical properties of SC and PBA compounds. We also observe irreversible magnetization process in a temperature cycle, which reproduces qualitatively well the novel behavior observed in the corresponding experiments.

Today, it is known that the [Fe(L₂₂₂(N₃O₂))(CN)₂]·H₂O complex exhibits an atypical thermal spin-crossover behaviour probably connected to an unusual change of the iron(II) environment from a six to a seven coordination sphere. Here, we show that the photomagnetic properties of this derivative are also atypical. When the sample is initially prepared in the LS form, by a cooling process of 3 K/min, the light irradiation gives access to a long-lived metastable HS state with a T(LIESST) value of 132 K. This photoinduced HS state is moreover characterised by a strong cooperative lattice, as reflected by the kinetic relaxations which follow a sigmoidal model and the existence of a Light-Induced Thermal Hysteresis (LITH) loop. At the opposite, when the sample is carefully placed into the LS/HS mixture state, by using a slow cooling process of less than 1 K/min, the light irradiation generates a metastable state with a T(LIESST) value of only 73 K and the influence of the cooperativity is weak. The relaxation curves are indeed satisfactory fitted by using a stretched exponential model and the LITH phenomenon is not present. All these data will be discussed in term of multimetastability.

A photo-crystallographic study of Fe(btr)₂(NCS)₂·H₂O was performed in order to describe the modification of structures and charge densities on going from the ground low spin (LS) state to the metastable high spin (HS) state during the LIESST phenomenon at 15 K. Related photo-magnetic and spectroscopic measurements are also described. We show that at 15 K, the thermally quenched and photo-induced structures of the metastable HS state are identical. For comparison, we also derived the structure of the HS and LS states at 130 K in the hysteresis loop; the thermal spin transition and the LIESST spin transition exhibit similar structural behaviours.

A microstructural mechanism of photoinduced transformations in amorphous arsenic selenide films was studied with IR Fourier-spectroscopy technique in 300-100 cm⁻¹ region. It was shown that stage of irreversible photostructural changes was connected with cooperative process of coordination defect formation accompanied by homopolar chemical bonds switching in heteropolar ones. On the contrary, reversible photoinduced effects were caused by heteropolar chemical bonds switching in homopolar ones, as well as additional channel of bridge heteropolar bonds switching in short-layer ones. The both processes were associated with formation of anomalously coordinated defect pairs and accompanying atomic displacements at the level of medium-range ordering. The developed mathematical simulation procedure testified in a favour of defect-related origin of the reversible photo-thermallyinduced transformations, since their kinetics corresponded to known stretched-exponential dependence, tending to bimolecular behaviour rather then to single-exponential one.

Free-electron lasers will change the way we carry out time-resolved x-ray experiments. At present date, we use laser-produced plasma sources or synchrotron radiation. Laser-produced plasma sources have short pulses, but unfortunately large pulse-to-pulse fluctuations and large divergence. Synchrotron radiation from third generation source provide collimated and stable beams, but unfortunately long pulses. This means that either the timeresolution is limited to 100 ps or rather complex set-ups involving slicing or streak cameras are needed. Hard x-ray free-electron lasers will combine the best properties of present-day sources and increase the number of photons by many orders of magnitude. Already today, a precursor to the free-electron lasers has been built at Stanford Linear Accelerator Centre (SLAC). The Sub-Picosecond Photon Source (SPPS) has already shown the opportunities and challenges of using short-pulse x-ray sources.

By solving analytically and numerically dynamical diffraction equations, we study the modulation of x-rays in ideal crystals by a co-propagating wave of impulsively excited optical phonons. Our results for a diamond crystal suggest that the output diffracted x-ray field can have a time structure much shorter than the period of the excited vibrational mode (T_v = 25 fs). We explain this dynamics in terms of high-order Raman scattering of x-rays by optical phonons. The potential of this ultrafast modulation technique for controlling x-rays from synchrotron and FEL sources is discussed.

Ultra-fast time-resolved x-ray crystallography using electron-accelerator-based x-ray 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.

Parent insulating cuprates appear to be unstable with regard to a self-trapping of the low-energy one-and two-center CT excitons with a nucleation of electron-hole (EH) droplets being actually the system of coupled electron CuO₄^7- and hole CuO₄^5- centers having been glued in lattice due to strong electron-lattice polarization effects. Such a system can be regarded as an EH Bose liquid described by the generalized Bose-Hubbard Hamiltonian. The scenario can readily explain photo-induced effects in underdoped cuprates. Indeed, illumination of a material with light leads to the generation of EH pairs that will proliferate and grow up to be a novel nonequilibrium EH droplet or simply to be trapped in either EH droplet with the rise in its volume fraction. During the illumination of underdoped cuprate near the insulator-metal transition, the material may even become superconducting.

In electron–hole (e–h) systems in photoexcited insulators, the exciton Mott transition may take place, which is a typical photoinduced phase transition. To understand how the exciton Mott transition depends on the e–h carrier density and the Coulomb correlation, we study an e–h two-band Hubbard model by means of the dynamical mean-field theory assuming that electron–hole pairs do not condense. The phase diagram on the plane of interactions at zero temperature is obtained. When both electron and hole bands are half-filled, two types of insulating states appear: the Mott–Hubbard insulator and the biexciton-like insulator. Away from half-filling we find another insulating phase, the exciton-like insulator, which is followed by the formation of incoherent (not condensed) e–h pairs, while the Mott–Hubbard insulator phase disappears. The exciton Mott transition is found to be the first-order transition. Linear optical susceptibilities are also discussed.

We calculated the dynamics of photoinduced domain growth in two-dimensional models. When a single molecular cell is excited by a photon, vibronic interaction induces energy transfer to the vibration mode in the cell and subsequently the inter-cell interaction helps energy distribution to the adjacent cells. Since nonadiabtic transition between ground and the excited electronic states within a cell is important to consider the dynamics, we fully quantized the vibrational degree of freedom to solve the time-dependent Schrödinger equation numerically. We show that both the interaction between the relevant vibration modes and the electronic states of different cells is necessary to the domain growth in higher dimensional systems.

The valence tautomerism of low-spin Co^III(Cat-N-BQ)(Cat-N-SQ) was investigated by means of UV-vis pump-probe transient absorption spectroscopy in chloroform. By exciting the CT transition of the complex at 480 nm, an intramolecular electron transfer process is selectively triggered. The photo-induced charge transfer is pursued by a cascade of two main molecular events characterized by the ultrafast transient absorption spectroscopy: the first gives rise to the metastable high-spin Co^II(Cat-N-BQ)₂ that, secondly, reaches the chemical equilibrium with the reactant species.

We report that the 1/4 filled A₂B salt (EDO-TTF)₂PF₆ developed as a candidate for organic superconductor unexpectedly shows highly sensitive and ultra-fast photo-induced phase transition (PIPT) from insulator (I) to metal (M). We show the results of time resolved experiments performed on this crystal. A large reflectivity change in the near infrared region (1.2–2 eV), which occurs within 2 ps for weak excitation intensity, has been observed. We also observed coherent vibrations in the dynamical process of this transition.

We present the detailed results of the investigation by x-ray diffraction of the photoinduced phase transition in the tris(α-picolylamine)iron(II) chloride–ethanol complex. This system undergoes a two-steps spin transition between a high spin state at room temperature and a low spin state at low temperature. The intermediate phase consists of stripes of high spin and low spin molecules. The x-ray diffraction data evidence the structural rearrangement at the molecular scale accompanying the photoinduced change of electronic state between the low-spin phase and the photo-steady high-spin one, which is similar to the stable one in the high temperature phase. The dynamical mechanism of the transformation is discussed with the observation of a domain nucleation process, associated with the coexistence of the stable and photoinduced phases.

When different SrTiO₃ single crystals are irradiated at room temperature with a continuous wave 325 nm laser light in an evacuated specimen chamber, their luminescence increase in intensity, creating a broad visible luminescence band centred at about 2.4 eV. Then, introducing oxygen gas into the specimen chamber, the photoluminescence spectra return reversibly to the original weak luminescence with the same laser light irradiation. After removing the laser light irradiation, each photoluminescent state is stored for a long time at room temperature under room light, regardless of any changes of atmosphere. Such photoinduced spectral change has been also observed at different temperatures from 6 K to room temperature. The observed phenomenon is explained by the photo-induced oxygen defect at the surfaces of SrTiO₃ crystal. The observed photo-induced optical phenomena are discussed in the light of both the crystal defect chemistry of SrTiO₃ and the exciton theory.

The first order phase transition between the metal (M) and insulating (I) phases of the molecular compound (EDO-TTF)₂PF₆ is investigated by single crystal x-ray diffraction. The coexistence of the insulating and metallic phases and the growth of the domains in the thermal hysteresis are clearly observed during the phase transition. We also present ultra-fast optical experiments using a nitrogen gas flow cryostat. A change of the reflectivity in the photoinduced phase was observed just after excitation. We will also discuss the influence of the excitation light polarisation on the efficiency of the photo-induced I–M phase transition.

We consider the two-photon emission by a dielectric where refractive index n oscillates in time. The oscillations of n cause analogous oscillations of the optical length. The velocity of the latter oscillations, in principle, may be arbitrarily large. If the maximal velocity of these oscillations V is much less than the light velocity c then the intensity of the emission will increase with V. However, if it strongly exceeds c then the opposite dependence takes place as the zero-point fluctuations cannot appreciably react on such a fast oscillations. Therefore, the crossover for the dependence of the emission on V exists at V_r ∼ c and a resonant enhancement of it is observed at this value of V. Our calculation gives V_r ≈ 3c.

Anomalous optical spectra of the centres with strongly-reduced local elastic springs in the final electronic states are studied. In this case the transitions take place to the vicinity of a flat potential curve in the configurational coordinate space which corresponds to a very strong mode mixing at the electronic transition. To take this mixing into account we apply the operator transform method. We have found that the mixing results in a strong enhancement of the effect of low-frequency vibrations, causing an intensive low-energy phonon sideband. In this case the spectrum (or its envelope) has the shape of an asymmetrical lambda-letter. Suchtype spectra have been observed experimentally. The spectra may also reveal the Airy-type oscillations if the potential of the final state has a finite slope. This-type oscillations have been observed in the hot luminescence; the oscillating structure of zero-point lines in the absorption spectra of super-fluid ⁴He droplets probably has an analogous origin.

In this work we investigate the coupling of the F_1u vibrations with the actual electronic states of BO₆ⁿ⁻ cluster in ABO₃ ferrorelectric-oxides. This coupling leads to the dynamical covalency hybridization of B(Ti,Ta,Nb) and oxygen electronic states. It is shown that at fulfilment of definite criteria the free energy at T = 0, the adiabatic potential of BO₆ⁿ⁻ cluster have the following configurations: (1) one maximum at x₀ = y₀ = z₀ = 0 (ferroelectric instability); (2) eight minima in the points |x₀| = |y₀| = |z₀| = y₀; (3) twelve saddle points at |p| = |q| ≠ 0, r = 0 (p, q, r = x, y, z) with a maximum in the r cross-section and minima along p and q. We show that the photo-induced changes of local ferroelectric distortions can take place. A gigantic enhancement of the dielectric constant by UV-light illumination is calculated in the electron-lattice theory in quantum paraelectrics of perovskite oxides, such as SrTiO₃ and KTaO₃, under a weak DC electric field. The temperature dependence of the gigantic real part of the dielectric constant _UVDC of SrTi¹⁶O₃ under both UV-light and DC electric fields is calculated in satisfactory agreement with the experiment.

Charge Transfer Vibronic Excitons (CTVEs) are bi-polaronic excitons characterized by specific recombination luminescence. The latter has been investigated with respect to the influence of IR pumping and led to the experimental evidence of an non-linear CTVE. Such an essentially anharmonic CTVE with respect to the charge transfer and lattice displacements was predicted recently [Vikhnin, 2003 Solid State Commun.127 2832], and should manifest a well-defined metastable behavior. It is shown that an IR pumping recalls practically the same CTVE-recombination emission band as that induced initially by a UVpumping. Such an effect is characterized by relaxation time τ after the UV-pumping (for example, τ ∼ 60 sec. at 3 K ). This behavior is explained as a result of occupation of a metastable anharmonic CTVE minima. An additional IR-pumping could transfer the occupation from the anharmonic CTVE state to the harmonic CTVE which is active for recombination luminescence. The temperature dependence of τ for the anharmonic CTVEs with metastable behavior shows that the relaxation time is controlled by resonance as well as by phonon-induced tunneling.

We measured third-order nonlinear susceptibility (χ⁽³⁾) spectra in semiconducting single-walled carbon nanotubes (SWNTs) by the Z-scan method. |Im χ⁽³⁾| is remarkably enhanced under resonant excitation to the lowest interband transition, reaching 4.2 × 10⁻⁶ esu. A comparison of the transient absorption changes evaluated by degenerate and nondegenerate pump-probe measurements suggests that the resonant enhancement of |Im χ⁽³⁾| is dominated by a coherent process rather than by incoherent one such as saturation of absorption.

Photoinduced optical responses are studied in the one-dimensional ionic Hubbard model with the nearest-neighbor repulsion V. For V = 0, carriers introduced by photoirradiation move freely both in the Mott insulator phase and in the band insulator phase, giving rise to a Drude peak in the optical conductivity spectrum. The carriers in the Mott insulator phase remain conducting for 0 < V < 2t because their kinetic energy overcomes the binding energy. By contrast, the electrons and the holes in the band insulator phase are bound to form excitons for V > 0, which do not contribute to the charge transport unless the excitation energy allows them to be separated. The dependence of the Drude weight in the lowest-energy photoexcited state on V and the system size is investigated for both phases. Implications to experimental studies of halogen-bridged metal-complex chains are discussed.

Different Eu₂O₃ fine particle films were prepared by the laser ablation method. Under a continuous wave (CW) 325 nm laser light irradiation, their photoluminescence spectra are measured both in vacuum and in O₂ gas atmosphere. All the specimens display reversible spectral change and clear luminescence colour changes (red luminescence ↔ white luminescence) for changing specimen atmosphere. Moreover, the white-luminescent state is stored for more than several years at room temperature in air under room light. The obtained results suggest that the spectral changes arise from both the photo-induced valence (Eu³⁺ → Eu²⁺) change of europium ions and the photo-generated oxygen defect structure at Eu₂O₃ particle surfaces in vacuum, which is expressed by the following reaction, Eu₂O₃ in lattice + CW 325 nm photon → (Eu²⁺-oyygen vacancy-Eu²⁺) complex +½O₂. The obtained results are discussed in the light of the exciton theory.

Theory of photo-induced phase transitions has been adapted to describe the cooperative dynamics of the lattice of ryanodine receptors/channels (RyR) in cardiac muscle which regulate the release of the intracellular activator calcium from calcium stores in the sarcoplasmic reticulum (SR) by a process of Ca²⁺-induced Ca²⁺ release (CICR). We introduce two main degrees of freedom for RyR channel, fast electronic and slow conformational ones. The RyR lattice response to the L-type channel triggering evolves due to a nucleation process with a step-by-step domino-like opening of RyR channels. Typical mode of RyR lattice functioning in a CICR process implies the fractional release with a robust termination due to the depletion of SR with a respective change in effective conformational strain. The SR overload leads to an unconventional auto-oscillation regime with a spontaneous calcium release. The model is believed to consistently describe the main features of CICR, that is its gradedness, coupled gating, irreversibility, inactivation/adaptation, and spark termination.

Picosecond time resolved Raman scattering measurements were performed in a spin ladder system α'−NaV₂O₅. We observed the decrease and recovery of the superstructure peak at 66 cm⁻¹ in the low temperature phase by photoexcitation. The transient local temperature of the sample by photoexcitation was obtained from the intensity ratio of the anti-Stokes and Stokes scattering. It is found that the highest temperature of 23 K induced by the optical pumping is far below the phase transition temperature of 34 K. The temporal changes of the Raman scattering intensities are ascribed to the decrease of the order parameter in the superlattice structure induced by optical pumping in the low temperature phase. We demonstrate a high potentiality of time-resolved Raman scattering spectroscopy for investigating the photoinduced phenomena.

In the last few years, bent crystal x-ray mirrors have played an important role in time-resolved x-ray diffraction experiments when x-ray pulses from femtosecond laserproduced plasmas were used. Improvements in manufacturing techniques have significantly increased the quality of this type of mirror.

NW14 is a new insertion device beamline at the Photon Factory Advanced Ring (PF-AR), which is a unique ring with full-time single-bunched operation, aiming for timeresolved x-ray diffraction/scattering and XAFS experiments. The primary scientific goal of this beamline is to observe the ultrafast dynamics of condensed matter systems such as organic and inorganic crystals, biological systems and liquids triggered by optical pulses. With the large photon fluxes derived from the undulator, it should become possible to take a snapshoot an atomic-scale image of the electron density distribution. By combining a series of images it is possible to produce a movie of the photo-induced dynamics with 50-ps resolution. The construction of the beamline is being funded by the ERATO Koshihara Non-equilibrium Dynamics Project of the Japan Science and Technology Agency (JST), and the beamline will be operational from autumn 2005.

We have measured ultrafast reflectivity changes of (EDO-TTF)₂PF₆ in the infrared region (0.51 eV – 1.03 eV) after excitation with a 1.58 eV photon in order to reveal the mechanism of ultrafast photo-induced insulator-to-metal phase transition. We found that the temporal profiles of reflectivity change have two components: a faster component within 3 ps associated with a 1-ps or 0.5-ps period oscillation and a slower component lasting over hundreds of a picosecond. In addition, we found that the sign of a reflectivity change is often different from that expected from the thermally induced phase transition. These results indicate that the PIPT takes place via a complicated process due to strong electron and phonon coupling.

{Fe(abpt)₂[N(CN)₂]₂} is an intriguing spin crossover system which crystallizes in two separate phases. Phase A has already been investigated; its crystal structure consists of a single molecule sitting on a center of symmetry. Phase B is the subject of the present structural analysis and presents two independent molecules in the asymmetric unit. We have observed a unique order-disorder phase transition at very low temperature induced either by laser light irradiation or through rapid thermal quenching from room temperature. The corresponding metastable phase exhibits an incommensurate modulation of the crystal structure, disappearing at the HS to LS relaxation temperature (52 K). This modulation is most probably related to a particular ordering of the dicyanamide ligands.

Polaronic model of luminescence processes in nominally pure gallium nitride has been proposed and developed. The model is based on two mechanisms which co-exist and are of a common origin. One of them is recombination luminescence of charge transfer vibronic excitons (bi-polaronic excitons) trapped by charged defects with different effective charge, like Ga- or N- related vacancies with different number of trapped electrons or holes, or interstitial O²⁻ and O⁻ ions, in the visible range (yellow-green luminescence). The other is recombination luminescence (blue band) of non-correlated self-localized electronic and hole polarons in the violet region. The model allows us to explain experimental results including the fine structure of yellow–green luminescence.

The effect of photo-doping on the quantum paraelectric SrTiO₃ is studied by using the one-dimensional quantum Ising model, where the Ising spin describes the effective lattice polarization of an optical phonon. Two types of electron-phonon couplings are introduced through the modulation of transfer integral via lattice deformations. After the exact diagonalization and the perturbation studies, we find that photo-induced low-density carriers can drastically alter quantum fluctuations when the system locates near the quantum critical point between the quantum para- and ferro-electric phases.

The photo-induced magnetic effects (PME) have been observed during the investigation into magnetic properties of [MnR₄TPP][TCNE]*2PhMe (R = OC₁₂H₂₅, TPP tetraphenylporphyrin, TCNE tetracyanoethylene) charge transfer salt molecular magnet. The study of magnetic properties of [MnR₄TPP][TCNE]*2PhMe and PME were performed in both static (DC) and dynamic (AC) modes with Lake Shore 7225 equipment using a sample probe with optical fiber. The Xe lamp with optical cut-off filters or the He–Ne laser was used. Results of DC magnetization relaxation measurements obtained for different magneticfields in He bath temperature (4.2 K) during illumination differ significantly from those obtained before illumination. Both the value and the time dependence of magnetization changed during illumination. Hysteresis loop measured before and during illumination was also found to undergo considerable change. On the other hand the AC measurements revealed only slight alteration in the real and imaginary components of the susceptibility. Simple theoretical description of the magnetization process recorded at 4.2 K, was performed and the relaxation data were found to be consistent with the model.