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International audience; The thermal behaviours of racemic sibutramine hydrochloride monohydrate as well as that of the anhydrous state were investigated by DSC. A new anhydrous solid was then put in evidence and its crystal structure was solved from XRPD data. This new phase was found to be the conglomerate. Depending on the heat treatment , the initial anhydrous racemic compound may be completely resolved. DSC analyses have shown that the anhydrous conglomerate melts at a higher temperature than the anhydrous racemic compound.

International audience; The crystal structure of p-synephrine was solved from a high-resolution X-ray powder diffraction pattern optimized by energy-minimization calculations using the Dreiding force field. The title compound crystallizes in a monoclinic system (space group P2 1 /c, Z = 4, with a = 8.8504(11) Å, b = 12.1166(15) Å, c = 9.7820(11) Å, ␤ = 122.551(2) • , V = 884.21(19) Å 3 and d = 1.256 g cm −3). Since p-synephrine degrades upon melting, its melting data were determined from DSC experiments carried out as a function of the heating rate. This method allowed determining a melting temperature and enthalpy equal to 199.8 ± 1.3 • C and 57 ± 3 kJ mol −1 , respectively.

Recent experiments on tunneling systems in insulating glasses showed an unexpected magnetic-field dependence of spontaneous polarization echoes. Besides a strong overall increase with the magnetic field, the echo amplitude slowly oscillates with a frequency that is proportional to B. We develop a theory that couples the tunneling motion to nuclear quadrupoles and obtain the beat frequency and dephasing in terms of the nuclear magnetic and quadrupolar moments, the waiting time, and the magnetic field. Our results compare favorably with measurements on various amorphous solids.

The dynamical behaviour of an entangled network, a melt made of long polymer chains, with an oscillating nanotip, is investigated to provide evidence for a close relationship between the spatial scale and dynamics of the network, from the molecular to the mesoscopic scale. The experimental results show how a dynamic force microscope is able to discriminate between slow and fast relaxation processes related to molecular motion at different scales. In particular, it is shown that within a very small variation of the tip indentation depth, a few angstroms, the relaxation times involved in the process of dissipation vary by more than three orders of magnitude.

We consider a ferromagnetic film, with perpendicular magnetization, only coupled by the magnetic field, to a superconducting film. Our results show how the superconducting film thickness influences the equilibrium domain structure of the ferromagnetic film.

We determine the phase diagram of the ferromagnetic (F)/superconducting (S) atomic superlattice with arbitrary coupling between the layers. The exchange field acting on F layers may induce different kinds of modulated superconducting phases with either in-plane modulation or a phase shift between adjacent S layers. When the exchange field is increased, the re-entrance of the superconductivity emerges and the "π phase" where the superconducting order parameter changes its sign in successive S layers is realized.

This work contains a short overview on two phenomenological models aiming to evaluate the loss of energy when a nanotip oscillates at the proximity of a surface. In one case, the surface is described as a continuous medium with a local stiffness ks and a local damping term γs. The viscoelastic properties allows an easy evaluation of the energy loss as a function of the surface relaxation times τs=γs/ks. The main purpose of the second approach is to evaluate the effect of large fluctuations on the dynamic properties of the oscillator. The surface is built of atoms, each of them lying in a symmetric double-well potential and the oscillating nanotip produces a stochastic resonance process. The linear response theory is applied to evaluate fluctuations of the particle corresponding to interwell motions. Then from the amplitude, fluctuation is calculated the loss of energy. The result shows that a stochastic resonance process can increase the amount of dissipate energy of two orders of magnitude at a forcing frequency much smaller than the usual ones corresponding to atom motions.

We study the dynamic response of a [111] quantum impurity, such as lithium or cyanide in alkali halides, with respect to an external field coupling to the elastic quadrupole moment. Because of the particular level structure of a eight-state system on a cubic site, the elastic response function shows a biexponential relaxation feature and a van Vleck type contribution with a resonance frequency that is twice the tunnel frequency $\Delta/\hbar$. This basically differs from the dielectric response that does not show relaxation. Moreover, we show that the elastic response of a [111] impurity cannot be reduced to that of a two-level system. In the experimental part, we report on recent sound velocity and internal friction measurements on KCl doped with cyanide at various concentrations. At low doping (45 ppm) we find the dynamics of a single [111] impurity, whereas at higher concentrations (4700 ppm) the elastic response rather indicates strongly correlated defects. Our theoretical model provides a good description of the temperature dependence of $\delta v/v$ and $Q^{-1}$ at low doping, in particular the relaxation peaks, the absolute values of the amplitude, and the resonant contributions. From our fits we obtain the value of the elastic deformation potential $\gamma_t=0.192$ eV., Comment: 19 pages, 5 figures

We present a lattice model based on two n→0 spin vectors, capable of treating the thermodynamics of living networks in micellar solutions at any surfactant concentration. We establish an isomorphism between the coupling constants in the two spin vector Hamiltonian and the surfactant energies involved in the micellar situation. Solving this Hamiltonian in the mean-field approximation allows one to calculate osmotic pressure, aggregation number, free end and cross-link densities at any surfactant concentration. We derive a phase diagram, including changes in topology such as the transition between spheres and rods and between saturated and unsaturated networks. A phase separation can be found between a saturated network and a dilute solution composed of long flexible micelles or a saturated network and a solution of spherical micelles.

Quasi-two-dimensional decaying turbulence is studied in a flowing soap film by measuring the moments of the probability density function P(δv(r)) for the longitudinal velocity differences δv(r) on a scale r. As in three-dimensional (3-D) turbulence, P becomes non-Gaussian with decreasing r. The third moment S3(r)≡〈(δv(r))3〉 is small and negative at small scales, but becomes positive at larger scales. The exact calculation of S3(r) for 2-D homogeneous isotropic turbulence relates this change in sign to the development of the velocity correlation function as the turbulence decays.

Efficient collection and detection of fluorescence coupled with careful minimization of background from impurities and Raman scattering now enable routine optical microscopy and study of single molecules in complex condensed matter environments. This ultimate method for unraveling ensemble averages leads to the observation of new effects and to direct measurements of stochastic fluctuations. Experiments at cryogenic temperatures open new directions in molecular spectroscopy, quantum optics, and solid-state dynamics. Room-temperature investigations apply several techniques (polarization microscopy, single-molecule imaging, emission time dependence, energy transfer, lifetime studies, and the like) to a growing array of biophysical problems where new insight may be gained from direct observations of hidden static and dynamic inhomogeneity.

The relaxation dynamics of nanoscale molecules such as arises from spin-lattice coupling and interaction with nuclear spins. Using a resolvent method in terms of the energy eigenstates and the first Born approximation with respect to phonon scattering, and averaging over the hyperfine field, we obtain a controlled approximation for the non-equilibrium magnetic relaxation behaviour and, in particular, for the corresponding rate. The rate is finite at T = 0, then increases linearly with T, and shows Arrhenius behaviour at higher temperature; for zero magnetic field B there are two different activation energies. The resonances as a function of B are shown to be slightly asymmetric about B = 0. Taking account of a quartic crystal field gives rise to a temperature-dependent shift of the resonant values of B. We find that, contrary to previous results, the rate is independent of the magnetic field at low but finite temperatures; for it is linear in B. Finally we compare our findings with various experimental data.

We discuss extensive sets of experimental data including static and dynamic light scattering, ultrasonic velocity measurements and high and low frequency dielectric relaxation, taken in the vicinity the critical point of a ternary microemulsion system. Upon approaching the critical point we observed a slowing down of the dielectric relaxation time and of the first cumulant of the time-dependent droplet density correlation function C2(t) which shows a non-exponential behavior at long time. These features can be well acccounted for by assuming that the microemulsion system is made of polydispersed transient fractal aggregates having a fractal dimension df=2.5 and a polydispersity index τ=2.2.

We provide experimental evidence of hysteretic behavior of a vibrating microlever that arises when a surface approaches the microlever. An analytical solution is derived from a variational method that shows that the width of the hysteresis depends on the strength of the attractive interaction at a given driving amplitude. For a large attractive interaction, the hysteretic behavior disappears, and the oscillating amplitude of the microlever decreases as the tip is close to the surface without needing to touch the surface. As a direct consequence, the way the cycle of the hysteresis varies close to the surface can be used to measure the change in attractive interaction without disturbing the surface.

We discuss the static and dynamic critical behavior of binary solutions of water with some alkyl-oxyethylene-glycol monoether (CiEj) and of di 2 ethylhexyl sodium sulfosuccinate (AOT)-water-decane ternary microemulsion systems. Extensive sets of measurements of the scattered light intensity, of the order-parameter fluctuations time correlation function, and of the shear viscosity, have been performed for different systems. In most of the cases our data sets show systematic deviations from the behavior usually observed for molecular binary fluids. We show that generalized hydrodynamics cannot account for these deviations. A modified version of the Sorensen et al. (1976) dynamical droplet model has been worked out. This model which assumes that the physical clusters formed close to the critical point can be treated like percolating aggregates, and which includes the finite size of the monomers, very well accounts for all our experimental results.

Impurities found in polyethylene and n-hexadecane give rise to intense and narrow single molecule lines at liquid helium temperatures. The fluorescence spectra of several impurity molecules in both matrices are nearly identical, suggesting that a single well-defined compound is involved. This paper summarizes all the photo-physical properties that may help identify the chemical structure of these impurities.

Solutions of the synthetic clay Laponite are strongly viscoelastic, even at very low particle concentrations. The formation of a gel, evidenced by the existence of a fractal network, has been invok...

The electronic and nuclear contributions to nonlinear optical properties of niobium oxide-based glasses have been measured using third-harmonic generation and pump-probe techniques. The results have been found to be in agreement with Hellwarth's model for niobium oxide $({\mathrm{Nb}}_{2}{\mathrm{O}}_{5})$ concentrations below $30\phantom{\rule{0.3em}{0ex}}\mathrm{mol}\phantom{\rule{0.2em}{0ex}}%$. Above $30\phantom{\rule{0.3em}{0ex}}\mathrm{mol}\phantom{\rule{0.2em}{0ex}}%$ ${\mathrm{Nb}}_{2}{\mathrm{O}}_{5}$, discrepancies with the theoretical model have been observed. It has been found that high niobium ion content induces a change of the local structure within the glass and favors the formation of three-dimensional blocks of corner-sharing ${\mathrm{NbO}}_{6}$ octahedra. Since similarities with crystalline structures are present, we propose that some specific collective vibrational modes could be at the origin of the extra nuclear contributions to the optical Kerr effect.

We study the Josephson effect in clean heterojunctions consisting of ferromagnetic bilayer between superconductors. We solve the scattering problem based on the Bogoliubov‐de Gennes equation for any relative orientation of in‐plane magnetizations, arbitrary transparency of interfaces, and mismatch of Fermi wave vectors. Both spin singlet and triplet superconducting correlations are taken into account, and the Josephson current is calculated as a function of the ferromagnetic layers thicknesses and of the angle α between their magnetizations. Dramatic impact of spin triplet superconducting correlations on the Josephson effect is not found in the clean limit.

International audience; no abstract

(2005). Thermal Protection Using Molecular Alloys As Phase Change Materials. Materials Research Innovations: Vol. 9, No. 3, pp. 76-77.

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We study the magnetic-field dependence of the zero-frequency electric susceptibility in glasses. Starting from the two-level tunneling systems and the nuclear spin and quadrupolar potential, we evaluate the free energy perturbatively, in terms of the quadrupolar and Zeeman energies. We find that the static susceptibility varies weakly with an external magnetic field; this effect is by six orders of magnitudes smaller than the observed dependence. One may conclude that the observed magnetic-field dependence is not a static phenomenon but of dynamic origin.

We propose an exactly solvable model to describe the properties of atomic thickness hybrid ferromagnet-superconductor-ferromagnet (F/S/F) structures. We show that the superconducting critical temperature is always higher for antiparallel orientation of the ferromagnetic moments. However, at low temperature the superconducting gap occurs to be larger for parallel orientation of the ferromagnetic moments. This leads to a peculiar temperature dependence of the proximity effect in (F/S/F) structures.

We analyze the mechanical response of a surface under the action of an oscillating tip in non-contact situations. The surface behavior is modelled as a simple viscoelastic material. In particular, special attention is paid to the surface displacement due to the attractive interaction between the tip and the surface. Analytical expressions describing the oscillation behavior of the tip at the proximity of the surface are obtained by considering two asymptotic regimes. The two asymptotic regimes are controlled by the characteristic relaxation time of the surface. Fast relaxation times correspond to time shorter than that of the residence time, e.g. the time during which the oscillating tip is at the proximity of the surface. Slow surface relaxation processes correspond to time larger than that of the oscillation period. The results allow the calculation of the non-contact dissipation as a function of the surface mechanical properties. Also, two criterions are derived that give the threshold values of the surface mechanical instability. The threshold values are shown to be strongly dependent of the characteristic time scale of the surface relaxation, thus of its dissipative properties.

We have simulated three possible effects of the driving force fluctuation of the cantilever, fluctuation of the surface location and contribution of an additional dissipation due to the sample, with the hope to identify effective methods of improvement of the achievable resolution during a dynamic force microscopy experiment. This study is performed through numerical simulations of the approach–retract curves. We find that in the case of soft materials, the driving force fluctuation has only a small effect on the amplitude of the cantilever oscillations, while surface fluctuation can markedly decrease the maximum amplitude. We also show that dissipation has a larger effect on the phase of oscillator than on its amplitude.

In this paper, a dynamical force microscopy study of self-assembled monolayers of organosilanes, grafted onto a silica support, is reported. Organosilanes, terminated either with a functional group (ethylene glycol) or with a methyl group, were used. The influence of the reaction time and the solvent composition on the grafting was investigated to improve the homogeneity of the self-assembled monolayers. Numerical simulations of approach−retract curves, obtained in the tapping mode, were performed and compared to experimental ones. Information such as mechanical response and height of the grafted organic layers has been obtained.

A virtual NC − AFM machine has been built using the Matlab language. The virtual NC − AFM is identical to a real hybrid machine built with Digital Instruments and Omicron blocks. The role of every subset is described in detail, and special attention is paid to the parameters of the Automatic Gain Control (AGC) which controls the amplitude of the oscillations. The virtual machine is a powerful tool, that allows to solve, without any approximation, non linear coupled differential equations describing the physics of the tip-surface interaction. The machine is then used to study the frequency shift and damping signal in the approach-retract mode. Two types of situations are analyzed: the first one corresponds to the case where no dissipative force is introduced in the tip-surface interaction, the second one deals with dissipative forces. These last ones are here introduced by assuming a mechanical deformation of the surface under the tip. Interesting results are then obtained, in particular the fact that unstability may occur under some particular conditions. Most of the results obtained by simulation are then compared with those of analytical models already published in the literature.

The abrupt increase of the damping coefficient of an oscillating microlever in the proximity of a surface is discussed as a direct consequence of a local relaxation. The power law found and the dependence of the damping coefficient on the oscillation amplitude are in good agreement with the experimental data recorded on a graphite surface. We show that a combined study of the resonance frequency shift and of the dissipation should bring new information on the intrinsic mechanical properties of a surface at a local scale.

This paper is a theoretical and a numerical investigation of the stability of a tip-cantilever system used in noncontact atomic force microscopy (NC-AFM) when it oscillates close to a surface. No additional dissipative force is considered. The theoretical approach is based on a variational method exploiting a coarse grained operation that gives the temporal dependance of the non-linear coupled equations of motion in amplitude and phase of the oscillator. Stability criterions for the resonance peak are deduced and predict a stable behavior of the oscillator in the vicinity of the resonance. The numerical approach is based on the results obtained with a virtual NC-AFM developed by our group. The effect of the size of the stable domain in phase is investigated. These results are in particularly good agreement with the theoretical predictions. Also they show the influence of the phae shifter in the feedback loop and the way it can affect the damping signal.

A large number of works have been done on double molecules. For 2,2′-Biquinoline (2,2′-BQ) (Fig-1), the studies were investigated essentially for the triplet state by various techniques; steady state ESR[1], raman and ODMR techniques [2]. Some works on the 2,2′-BQ complexes have stressed the close connection between the metal complex formation and the conformation change of the ligand molecule [1,3,4]. Our interest in this molecule arises from our earlier observations on other double molecules such as 1,1′-binaphthyl and 2,2′- binaphthyl [5,6] and 9,9′-bianthryl [7,8]. In the present work, the investigation was carried out experimentally on 2,2′-BQ for which original results are given.

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Optical tweezers have recently been used to locally induce liquid–liquid phase separations and to nucleate a single domain inside the trap [H. Masuhara and co-workers, J. Phys. Chem. B 101 (1997) 5900; Langmuir 13 (1997) 414; Bull. Chem. Soc. Japan 69 (1996) 59]. We investigate theoretically these laser-driven transitions in liquid mixtures in a tightly focused wave and analyze their kinetics. After a description of the different quenching processes (electrostriction, thermodiffusion and thermal heating), the droplet growth rate is derived in each case. To illustrate the generality of the purpose, the model is developed for critical binary fluids and the kinetics are discussed in terms of universal behaviors using a comparison with classical uniform quench situations. We also analyze how finite size effects induced by the beam break this dynamic universality. To validate the model, a comparison of the predicted behaviors with recent experimental results is presented. The good agreement illustrates the potentialities of this new application of optical tweezers as micro-physical chemistry tools.

In this paper, the dynamic behaviour of an oscillating tip-microlever system at the proximity of a surface is discussed. The attractive tip-surface interaction is simply described with a Van der Waals dispersive term and a sphere-plane geometry. We show that the non linear behaviour of the oscillator is able to explain the observed shifts of the resonance frequency as a function of the tip-surface distance without the need of introducing a particular short range force.

We study the aging of a colloidal glass, which is obtained for extremely low volume fractions due to strong electrostatic repulsions, leading to the formation of a "Wigner glass". During the aging, a new crossover between a complete and incomplete decay of the correlation function is observed, accompanied by an increase in the non-ergodicity parameter. The dynamics can be described as a cage-diffusion process. For short times, the escape of the particles from "cages" formed by neighbouring particles dominates; for long times the particles cannot escape anymore and the system becomes strongly non-ergodic.

A new model is proposed in order to account for the superparamagnetic resonance spectra arising from the ferromagnetic particles dispersed in a diamagnetic matrix (e.g., ferrofluids, partially devitrified glasses). The model explicitly takes into account non-sphericity of magnetic nanoparticles in a joint distribution density of sizes and demagnetising factors. The distribution in particle shapes is shown to take an important part in determining the superparamagnetic resonance spectra. By computer simulating the resonance spectra of annealed iron-containing borate and sol–gel silica glasses, characteristics of size and shape distributions of magnetic nanoparticles are evaluated. As the anneal temperature is increased from 456°C to 480°C, the most probable diameter of the magnetic particles formed in the borate glass is increased from 3.4 to 4.7 nm, while their shapes become better ordered. The mean size of particles formed in the sol–gel silica glass is small, ca. 3.0 nm, and their shapes are less ordered in comparison with the borate glass.

cw laser waves can be used to locally induce phase transitions. We investigate experimentally and theoretically a laser-driven liquid-liquid phase separation in a micellar phase of microemulsion and analyze its late stage kinetics. The medium is optically quenched in composition in the metastable region of the phase diagram. Two processes can lead to these concentration variations: electrostriction and thermodiffusion. The first originates from induced dipolar couplings in a field gradient. The second corresponds to a variation in concentration driven by a small thermal gradient. Since the nucleated droplets are optically trapped by the beam, we show that it becomes possible to experimentally analyze an academic situation, i.e., the diffusion-driven growth of a single droplet in compensated gravity. The late stage of this kinetics can be divided into two parts, a bulk behavior and a regime controlled by the finite transverse size of the beam. The bulk regime is totally analogous to that observed in classical situations (R proportional, variant t(1/3), where R is the droplet radius and t the time), and the scaling of the amplitudes in terms of reduced length and time scales is in total agreement with the expected behaviors for fluids belonging to the same Ising universality class. Moreover, the Gaussian beam behaves as an optical bottle with "soft walls" in which the absence of rigid boundaries, and thus of wetting couplings, allows an intrinsic description of the influence of finite-size effects on the kinetics. The beam size breaks the dynamic universality when the growing domains start to feel it. We experimentally investigate the resulting slowing down, and a diffusion-driven model of the growth inside a laser wave is built for comparison. The good agreement observed for the bulk regime and during its modification induced by finite-size effects opens a promising field for the development of this new application of laser waves to control out-of-equilibrium liquid mixtures.

We study the influence of thermal fluctuations on the dynamics of a fine magnetic particle in a uniaxial potential. At low temperature, the transverse component of the magnetization precesses coherently about the easy axis. With rising T, thermal fluctuations reduce the precession frequency and, as the thermal energy approaches the anisotropy potential, they drive a crossover to incoherent motion. Our findings agree with recent neutron scattering data.

We present an original method of laser probing to observe thermomechanical, stresses at interfaces of solder joints. The stress is generated by Peltier heat exchange and observed through thermal expansion in the picometer scale. A simple model allows to extract a parameter measuring the interface bond quality. (C) 1998 Elsevier Science ltd. All lights reserved.

One of the source terms of Z-pinch experiments is the gas puff density profile. In order to characterize the gas puff, we have used two interferometrical methods and performed some numerical simulations. The merits of both optical techniques are presented in terms of sensitivity, accuracy, and full time recording. Hence, one technique has been chosen to characterize the gas puff. The computation fluid dynamics (CFD) code (ARES) has been used to simulate the gas flow with the aim of testing its performances. Comparing experimental and numerical data shows off the taking into account of gas viscosity in computations. Given these consistent results, the nozzle geometries can be optimized in order to obtain specific Z-pinch gas puffs and check the computation with the interferometric method. Results obtained with a cylindrical nozzle are presented herein.

Electron spin resonance in copper-doped polymeric iron(II)-1,2,4-triazole compound [Fe(NH 2 trz] 3 (NO 3 ) 2 is studied at different temperatures below the low spin to high spin transition of Fe 2+ ions. Between liquid helium and liquid nitrogen temperatures typical anisotropic spectra of Cu 2+ in a static Jahn-Teller configuration are observed, while the room temperature spectra undergo dynamic averaging. At intermediate temperatures both the anisotropic and averaged spectra coexist. These results are accounted for assuming a Gaussian distribution of the energy difference Δ between the lowest and the higher valleys of the Jahn-Teller potential energy surfaces, with the mean Δ-value ca. 0.021 eV and the standard deviation ca. 0.0066 eV.

The quasi two-dimensional single crystal [NH3-(CH2)5-NH3]CuCl4 has been studied by EPR measurements at X-band, room temperature andT = 100 K in three perpendicular planes (a, b), (a, c), (b, c*). This compound crystallizes in the monoclinic system with four entities per unit cell. Only one EPR exchange-collapsed line was observed for the four magnetically inequivalent copper ions in the lattice at room temperature. A careful analysis of experimental data permits us to obtain the principal values of theg-factor:g┴=2.052 andg║=2.267. The EPR linewidth exhibits a minimum along the г п ≈/4 direction of (a, c*) plane attributable to spin diffusion. Along the г п ≈/4 direction of (b, c*) plane the linewidth exhibits a maximum attributable to the two magnetically inequivalent sub-systems located in two adjacent mineral sheets. The simulations of EPR spectra obtained along the bisector of (b, c*) angle at room temperature and atT = 100 K show that exchange interaction between these two dissimilar sub-systems is nearly equal to zero.

A lithium borate glass containing a small amount of iron oxide is studied by electron magnetic resonance at room temperature after repeated annealing steps between 460 and . As the anneal temperature increases, the sharp line characteristic of isolated iron ions decreases in intensity and finally disappears. Simultaneously, a narrow line emerges at , superposed with a broader one, the narrow and the broader components predominating respectively after annealing at lower and at higher temperatures. Computer simulations of spectra have been carried out, based on a model of resonance of ferromagnetic single-domain nanoparticles randomly dispersed in the devitrified glass (superparamagnetic resonance). As the anneal temperature increases, the most probable particle diameter obtained assuming a log-normal distribution of diameters increases from 2.9 to 4.7 nm showing a saturation at higher anneal temperatures, whereas the relative number of larger particles grows continuously.

The Kerr effect in a model three-component microemulsion has been investigated using an improved experimental setup. These new measurements, which cover a wide range of droplet volume fractions, $0%l\ensuremath{\varphi}l30%$, suggest that the Kerr coefficient $K$ scales as ${\ensuremath{\varphi}}^{2}$ throughout the entire range of concentrations. This scaling behavior suggests that Kerr effect in the microemulsion is intrinsically a many-body problem rather than a single-body problem as previously suggested. It remains an intriguing possibility that the microemulsion forms clusters comprising many droplets even in dilute concentrations.

A polymer compound has been studied by optical response and electron spin resonance (EPR) using and as dopants and paramagnetic probes. The transition between low-spin (LS) and high-spin (HS) states of ions occurs with a broad hysteresis loop around room temperature and is accompanied by a colour change from bright pink in the LS state to chalky white in the HS state. With increasing doping level the hysteresis loop narrows and shifts to lower temperatures. In the HS state of the EPR spectra of and are severely broadened by spin-spin interactions with paramagnetic HS modulated by a rapid spin-lattice relaxation of the latter ions. The EPR data give evidence of the presence of domains of the LS and HS ions in the transition region.

International audience; The results of transient degenerate four-wave mixing ~DFWM! and ‘‘pump-probe’’ spectroscopy in aggregates of 1,18-diethyl-2,28-cyanine ~pseudoisocyanine, PIC! chloride at 300 K are reported. Spectral dispersion of DFWM efficiency within the J-band and near exciton resonance has been measured. Time response of both transient absorption changes and DFWM signal is found to be strongly dependent on pump photon fluence and wavelength. This behavior is qualitatively explained within a physical model of nonlinear optical dynamics in aggregate domains which accounts for exciton annihilation, and the effect of nonthermal phonons produced as a result of decay of two-exciton states. Intensity-dependent evolution of excess dynamic disorder due to nonthermal phonons manifests itself in pump-probe experiments as dispersion-type differential spectrum appearing at high pump intensity. The third-order nonlinear susceptibility of PIC aggregates has been calculated supposing highest density packing. The nonlinear figure of merit of aggregates was evaluated which is intensity dependent due to exciton annihilation and associated subsequent processes.

Drastic changes are observed in magnetic resonance spectra of borate glasses doped with low concentrations of iron oxide, after heat treatment above the glass transition temperature. A sharp g ef = 4.3 line, characteristic of diluted Fe 3+ ions in the vitreous matrix, progressively disappears in the course of treatment. A new two-line pattern grows in the vicinity of g ef ≈ 2.0, consisting of two superimposed features: a narrow one and a broad one. The narrow feature is dominant at lower treatment temperatures, while the broader one grows rapidly at higher treatment temperatures. Computer simulations succeed in fitting the spectra as being due to superparamagnetic resonance of static monodomain magnetic particles, randomly distributed in the vitreous matrix and characterised by a magnetocrystalline anisotropy constant, a demagnetisation field and a line shape function depending on the particle volume. The two-line pattern observed is shown to result from a unimodal log-normal distribution of particle diameters, with the most probable diameter value increasing with treatment temperature. Such a distribution results from a very inhomogeneous iron distribution in the glass matrix.

We report on pump-probe experiments on single molecules in a solid matrix, with strong laser fields and on a new host-guest system, dibenzanthanthrene in a naphthalene crystal. Besides strong light-shifts of the molecular lines, we observe the hyper-Raman resonance and for strong pump and probe beams, we also observe subharmonic resonances due to a ‘kicked pendulum’ motion of the Bloch vector. The experimental results are in very good agreement with calculations based on Bloch's equations, demonstrating their validity for single molecules in solids under intense resonant fields.

Atomic force microscopy is employed to study the grafting process on a silica surface of several kind of silane molecules in anhydrous and aqueous solutions: mono- and trichlorosilanes and mono- and triethoxysilanes. A systematic study of the adhesive interaction and the friction behavior is performed. With the anhydrous silane solutions, we show that only the trichlorosilane molecules are able to graft the silica surface, while with the aqueous silane solutions, except for the monoethoxysilane, the silica surface was always grafted. This study provides further support of the use of tribological properties to investigate a surface modification. We show that the influence of the tip velocity on the friction behavior is a powerful way to discriminate between the silica surface and the grafted one.