From substrate disorder to contact angle hysteresis, and back

Based on Monte Carlo simulation of the contact line as a long-range elastic model, we develop tools relating substrate traps, trapping time and trapping length. We demonstrate the possibility of retrieving some information on the substrate topography from measurements of contact line motion, near the threshold in forced spreading or near the advancing angle in spontaneous spreading.     

Product measures and dynamics for solid-on-solid interfaces

A two-species asymmetric exclusion process is considered with general transition rates subject only to the constraint of charge conservation. Conditions for the existence of a stationary product measure are found in both the cases of odd-even parallel dynamics and continuous-time dynamics. The results are then applied to a one-dimensional restricted solid-on-solid model, considered as a model of driven interfacial growth, showing a nontrivial dependence of the stationary measure on the external driving field. The dependence of the growth velocity on the slope of the interface is given and interface shapes in finite volume with opposite boundary conditions are investigated numerically     

Conservative Langevin dynamics of solid-on-solid interfaces

We study the dynamics of an interface between two phases in interaction with a wall in the case when the evolution is dominated by surface diffusion. For this, we use an SOS model governed by a conservative Langevin equation and suitable boundary conditions. In the partial wetting case, we study various scaling regimes and show oscillatory behavior in the relaxation of the interface toward its equilibrium shape. We also consider complete wetting and the structure of the precursor film.     

Time resolved luminescence of CdS at high excitation

We report a study at low temperature of the time resolved luminescence of CdS, excited by two photon absorption. Concerning the so called EHP-LO and P bands, we confirm our results previously obtained on CdSe[1, 2]. (a) At high excitation a broad band (peak position at λ > 495 nm) occurs due to radiative recombination in an electron hole plasma, assisted by the emission of one LO phonon. (b) The simultaneous kinetics of the P line (λ ? 490.5 nm) and A-LO line (λ ? 492.5 nm) are conflicting with the interpretation of the P line as resulting of radiative exciton-exciton collisions. We interpret the P line as due to biexciton recombination.We have studied the luminescence in the (I₂-I₁) region (486nm < λ < 490nm) at low excitation. We observe clearly the following, (a) A broad band (488 nm < λ < 490.5 nm) which corresponds to the gain observed in previous experiments of pulse and probe spectroscopy and interpreted as direct recombination in an electron hole liquid (EHL). (b) After the disappearance of the EHL band, one single line (M_D), which shifts continuously towards the I₂ position during the time resolved kinetics. We suggest it to be connected with the high excitation effect on donor impurities (bound polyexcitons).     

Catalytic C-H amination: the stereoselectivity issue

Catalytic C-H amination has recently emerged as a unique tool for the synthesis of amines. This tutorial review highlights the existing protocols catalyzed by metal complexes (rhodium, copper, ruthenium, manganese and palladium) allowing diastereo- and enantioselective C-H amination. Substrate-, catalyst- and reagent-controlled methodologies are detailed. They involve either catalytic nitrene C-H insertion or C-H activation.     

Impacting materials by light and seeing their structural dynamics

The emergence of the field of photo-induced transformation triggered by intense and ultra-short light pulse in a material has opened a new route to acting on matter. It combines many of today’s issues such as far away from equilibrium phenomena, coherence and quantum control, complex correlations and non linear responsiveness, inter-conversion of energy and information. The advent of ultrafast structural dynamics motivated developing new tools to directly watch how matter works, by probing the time- and length-scales on which photo-induced processes take place. Different points will be briefly overviewed: (i) the development of laser pump and X-ray probe for watching ultrafast structural dynamics, (ii) the physical features resulting from this way of impacting materials by light, (iii) the presentation of some examples investigated by time-resolved X-ray diffraction and diffuse scattering experiments to illustrate the complex structural dynamics of a photo-induced transformation.     

High resolution infrared study of the 2ν9 and ν4 bands of 10BF2OH and 11BF2OH: evidence of large amplitude effects for the OH-torsion and OH-bending modes in the 92 and 41 excited states

High resolution (2–3?×?10^?3?cm^?1) Fourier transform infrared spectra of gas phase ¹⁰B and ¹¹B enriched and natural samples of BF₂OH (difluoroboric acid) were recorded at Wuppertal and Richland. Starting from the results of previous studies, it was possible to perform the first rovibrational analysis of the 2ν₉ (first overtone of ν₉, the OH torsion) and ν₄ (BOH bending) bands located at about 1043.9 and 961.7?cm^?1 and 1042.9 and 961.5?cm^?1 for the ¹⁰BF₂OH and ¹¹BF₂OH isotopic species, respectively. Numerous ‘classical’ perturbations were observed in the analysis of the 2ν₉ and ν₄ bands. The energy levels of the 9² bright state are indeed involved in a B-type Coriolis resonance with those of the 6¹9¹ dark state. The 4¹ levels are perturbed by a B-type Coriolis resonance and by an anharmonic resonance with the levels of the 7¹9¹ and the 6¹7¹ dark states, respectively. In addition large amplitude effects were observed for the 2ν₉ and also, more surprisingly, the ν₄ bands. This results in splittings of the energy levels of about 0.005 and 0.0035?cm^?1 for the 9² and 4¹ states, respectively, which are easily observable in the P and R branches for both bands. The theoretical model used to reproduce the experimental levels accounts for the classical vibration–rotation resonances. Also the large amplitude torsional (or bending) effects are accounted for within the frame of the IAM (Internal Axis Method)-like approach. The Coriolis resonances between the two torsional (or bending) substates are taken into account by {J_x,?J_z} non orthorhombic terms in the v-diagonal blocks. This means that the z-quantification axis deviates from the a inertial axis by an axis switching effect of ～35° for the {9²,?6¹9¹} system and of ～16.6° for the {4¹,?7¹9¹,?6¹7¹} system of interacting vibrational states. The calculation of the relative line intensities for the 2ν₉ and ν₄ bands accounts for these axis switching effects as well as for the intensity alternation which is due to the nuclear spin statistical weights since the OH large amplitude torsion and/or bending motion results indeed in an exchange of the two fluorine nuclei.     

Photo-Induced Transformations: Three States Model

Here, we propose an extension of the diabatic approach to photo-induced phase transitions and suggest three states model, which mimics a system with three local diabatic states or equivalently two states where one of them is unstable against a nonsymmetric distortion. The model allows to discuss symmetry breaking transformations and is intended to describe photo-induced transformations in molecular systems, where a path of structural relaxation involves two coordinates. The three states are mapped into a minimum version of Ising spin-1 model, which exhibits metastability. Conditions for switching between stable and metastable phases are discussed in terms of a competition between crystal field, changed by illumination, and dipolar coupling. Examples are neutral (N) to ionic (I) transformation in mixed-stack charge-transfer system and low-temperature photo-induced spin-crossover transformation in metal complexes.