Photochemical behavior of some substituted benzophenoxazinones

Irradiation of some substituted benzophenoxazinones in methanol solution by visible light yields dimerization and substitution products. A possible radical mechanism is proposed and validated by quantum mechanical computations.     

Surface electromagnetic radiation from a spatially modulated oscillating dipole layer

We present the calculation of surface electromagnetic radiation to all orders from a spatially modulated, oscillating dipole layer sandwiched between two dielectric media. The results properly include the macroscopic local field factors. In specular reflection direction, we show that the total radiation is partitioned into a term which is the zeroth-order counterpart to the higher-order diffraction and a term related only to the properties of two adjoining bulk media. The results unify the calculations by Shen and Heinz and McIntyre [1, 6, 7]     

Convergence of a relaxation scheme for hyperbolic systems of conservation laws

Summary. This paper concerns the study of a relaxation scheme for hyperbolic systems of conservation laws. In particular, with the compensated compactness techniques, we prove a rigorous result of convergence of the approximate solutions toward an entropy solution of the equilibrium system, as the relaxation time and the mesh size tend to zero. Received September 29, 1998 / Revised version received December 20, 1999 / Published online August 24, 2000     

Recherches générales sur les courbes et les surfaces réglées algébriques.

Sans résumé     

Macroscopic quantum tunneling in "small" Josephson junctions in a magnetic field

We study the phenomenon of macroscopic quantum tunneling (MQT) in small Josephson junctions (JJ) with an externally applied magnetic field. The latter results in the appearance of the Fraunhofer type modulation of the current density along the barrier. The problem of MQT for a pointlike JJ is reduced to the motion of the quantum particle in the washboard potential. In the case of a finite size JJ under consideration, this problem corresponds to a MQT in a potential which itself, besides the phase, depends on space variables. The general expression for the crossover temperature T0 between thermally activated and macroscopic quantum tunneling regimes and the escaping time tau(esc) have been calculated.     

Effect of the π Bridge and Acceptor on Intramolecular Charge Transfer in Push–Pull Cationic Chromophores: An Ultrafast Spectroscopic and TD‐DFT Computational Study

Three (donor–π–acceptor)⁺ systems with a methyl pyridinium or quinolinium as the electron‐deficient group, a dimethyl amino as the electron‐donor group, and an ethylene or butadiene group as the spacer have been investigated in a joint spectroscopic and TD‐DFT computational study. A negative solvatochromism has been revealed in the absorption spectra, which implies a solution color change, and interpreted by considering the variation in the permanent dipole moment modulus and orientation upon photoexcitation. The fluorescence efficiency decreases upon increasing solvent polarity, in agreement with the excited‐state optimized geometries (planar in low‐polarity media and twisted in high‐polarity media). Femtosecond transient absorption has revealed the occurrence of a fast photoinduced intramolecular charge transfer (ICT) and the molecular factors that determine an efficient ICT. Considering the crucial role of the ICT in tuning the nonlinear optical (NLO) properties, these compounds can be considered promising NLO materials.     

Similar Structural Dynamics for the Degradation of CH3NH3PbI3 in Air and in Vacuum

We investigate the degradation path of MAPbI₃ (MA=methylammonium) films over flat TiO₂ substrates at room temperature by means of X‐ray diffraction, spectroscopic ellipsometry, X‐ray photoelectron spectroscopy, and high‐resolution transmission electron microscopy. The degradation dynamics is found to be similar in air and under vacuum conditions, which leads to the conclusion that the occurrence of intrinsic thermodynamic mechanisms is not necessarily linked to humidity. The process has an early stage, which drives the starting tetragonal lattice in the direction of a cubic atomic arrangement. This early stage is followed by a phase change towards PbI₂. We describe how this degradation product is structurally coupled with the original MAPbI₃ lattice through the orientation of its constituent PbI₆ octahedra. Our results suggest a slight octahedral rearrangement after volatilization of HI+CH₃NH₂ or MAI, with a relatively low energy cost. Our experiments also clarify why reducing the interfaces and internal defects in the perovskite lattice enhances the stability of the material.