A Direct S0→Tn Transition in the Photoreaction of Heavy‐Atom‐Containing Molecules

According to the Grotthuss–Draper law, light must be absorbed by a substrate to initiate a photoreaction. There have been several reports, however, on the promotion of photoreactions using hypervalent iodine during irradiation with light from a non‐absorbing region. This contradiction gave rise to a mystery regarding photoreactions involving hypervalent iodine. We demonstrated that the photoactivation of hypervalent iodine with light from the apparently non‐absorbing region proceeds via a direct S₀→T_n transition, which has been considered a forbidden process. Spectroscopic, computational, and synthetic experimental results support this conclusion. Moreover, the photoactivation mode could be extended to monovalent iodine and bromine, as well as bismuth(III)‐containing molecules, providing new possibilities for studying photoreactions that involve heavy‐atom‐containing molecules.     

Contribution to 3D impact problems: collisions between two slender steel barsUne contribution aux problèmes d'impact en trois dimensions : collisions entre deux barres élancées

This Note deals with the three-dimensional phenomenon of collision between two slender steel bars. The problem posed is whether the restitution concept developed in rigid-body theory is relevant in the case of such slender contactors. Some elements of an answer are provided through the use of two complementary approaches of collision, a theoretical one based on coefficients of restitution and series of experiments. Our main conclusion is that the alleged Newton coefficient of restitution varies according to the impact location on the rods. To cite this article: C. Le Saux et al., C. R. Mecanique 332 (2004).     

Heavy ion collisions in AdS5

We study heavy ion collisions at strong ?t Hooft coupling using AdS/CFT correspondence. Heavy ion collisions correspond to gravitational shock wave collisions in AdS₅. We construct the metric in the forward light cone after the collision perturbatively through expansion of Einstein equations in graviton exchanges. We obtain an analytic expression for the metric including all-order graviton exchanges with one shock wave, while keeping the exchanges with another shock wave at the lowest order. We read off the corresponding energy-momentum tensor of the produced medium. Unfortunately this energy-momentum tensor does not correspond to ideal hydrodynamics, indicating that higher order graviton exchanges are needed to construct the full solution of the problem. We also show that shock waves must completely stop almost immediately after the collision in AdS₅, which, on the field theory side, corresponds to complete nuclear stopping due to strong coupling effects, likely leading to Landau hydrodynamics. Finally, we perform trapped surface analysis of the shock wave collisions demonstrating that a bulk black hole, corresponding to ideal hydrodynamics on the boundary, has to be created in such collisions, thus constructing a proof of thermalization in heavy ion collisions at strong coupling.     

On logical gates in precipitating medium: Cellular automaton model

We study a two-dimensional semi-totalistic binary cell-state cellular automaton, which imitates a reversible precipitation in an abstract chemical medium. The systems exhibits a non-trivial growth and nucleation. We demonstrate how basic computational operation can be realized in the system when the propagation of the growing patterns is self-restricted by stationary localizations. We show that precipitating patterns of different morphology compete between each other and thus implement basic logical gates.     

Potential of Si1−xGex alloys for Auger generation in highly efficient solar cells

Recent investigation on Si solar cells demonstrated the utility of Auger generation for the creation of more than merely one electron/hole pair per absorbed photon. The semiconductor Si requires a minimum photon energy of about 3.4 eV for this internal carrier multiplication. The current of a Si cell is therefore not significantly increased by Auger generation when the cell is illuminated by an air mass 1.5 spectrum, which contains only few photons with energies above 3.4 eV. Use of Si_1–x Ge _x alloys promises a lower onset energy. Unfortunately, incomplete data on band structures ofrandom Si_1–x Ge _x alloys preclude a detailed quantitative discussion of the full potential for these materials. Nevertheless, (i) analogies to our own quantum efficiency data from pure Si, (ii) the calculated band structure of the hypothetical,ordered zincblende type Si_0.5Ge_0.5 crystal, and (iii) optical data for Si_1–x Ge _x alloys indicate an optimum Ge content ofx=0.6 tox=0.7.     

Optical-Phonon Deformation Potentials in Diamond-structure crystals: ab initio LMTO-ASA Calculation

The optical-phonon deformation potentials of C, Si, Ge and Sn are calculated by LMTO-ASA approach within the frozen-phonon approximation. The comparisons between our results and those of several present theoretical calculations and experiments demonstrate that our calculation model is reasonable     

Geminate recombination processes induced by rare gas collisions with predissociating NaI molecules prepared by femtosecond excitation

NaI molecules predissociate after excitation with ultrashort pulses at a peak wavelength of 320 nm. In rare gas environments at higher pressures the process of geminate recombination is likely to occur. We studied the dynamics of these processes under various pressure conditions and for different collision partners. The calculations were performed within a three-dimensional statistical model which simulates the NaI wave-packet dynamics with classical trajectories and the collisions via an instantaneous hard-sphere scattering. Received: 4 May 1998 / Accepted: 27 July 1998 / Published online: 28 September 1998     

Contribution ofd-like states to magnesium,aluminium and phosphorusL-emission bands of MgO,Al2O3 and AlPO4

We have measured the MgL-, AlL- and PL-emission bands of MgO, -Al₂O₃ and AlPO₄, respectively. For MgO and Al₂O₃ the electronic structure and the X-ray emission bands have been calculated. In spite of different crystal structure and chemical composition of these compounds the cationL-emission bands are very similar. We have found that for the interpretation of theL-emission bands of these compounds the cation 3d-like electrons are crucial.     

Dynamics of hard rods in one dimension

We examine a system consisting ofN classical, Newtonian, perfectly elastic hard rods constrained to move on a line. The mass and length of each rod are arbitrary. We develop an algorithm which gives, after any given possible sequence of collisions, the new velocities of theN rods and a necessary condition for any given pair of rods to be involved in the next collision, all in terms of the initial velocities of the rods. These results are then used to prove that for the case where there are exactly three rods on the line, the maximum possible number of collisions among them is the largest integern such that , wherem ₂ is the mass of the central particle and ₁₂ and ₂₃ are the reduced masses of the left and right particle pairs. We further derive for this three-particle case a condition on the initial velocities which is necessary and sufficient fork collisions, 1<kn, to occur, as well as explicit expressions for the velocities after each collision in terms of the initial velocities.     

Formation of sub-micron size carbon structures by plasma jets emitted from a pulsed capillary discharge

We have performed an experimental investigation of the potential use of intense plasma jets produced in a repetitive pulsed capillary discharge (PCD) operating in methane gas, to irradiate Si (1 0 0) substrates. The surface modifications induced by the plasma jet using two different material inserts at the capillary end, graphite and titanium, are characterized using standard surface science diagnostic tools, such as scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis and Raman spectroscopy (RS). It has been found that the application of methane plasma jet results in the formation of sub-micron size carbon structures. It is observed that the resulting plasma irradiated surface morphologies are different, depending on the different material inserts used at the capillary end, at otherwise identical operational conditions. To investigate the species responsible for the observed surface changes in different material inserts to the capillary, optical-emission spectroscopy (OES) was recorded using a 300-1000 nm spectrometer. The OES results show the presence of H, CH and C₂ Swan band in the discharge plasma, which play a significant role in the formation of the carbon structures.