Jean-François Gouyet

Two-dimensional pitting corrosion of aluminium thin layers

The influence of Cl− and Fe3+ ions on the morphology of pits grown in aluminium thin films was investigated. Under open circuit conditions various morphologies ranging from percolation-cluster-like patterns to dense holes with smooth perimeters were observed. Temporal fluctuations in the free corrosion potential did not reflect significant differences in pit shapes. The growth of two-dimensional pits is likely to be influenced by two main factors: (a) local fluctuation in the electrolyte aggressiveness due to copious hydrogen bubbling at dissolution sites; (b) global restriction of pit growth imposed by the cathodic reactions occurring on the oxidised surface. A spreading percolation model with feed-back is able to reproduce all the morphologies observed and provides a possible explanation to the 1/f2 character of the potential noise.

Dynamics of the creation of fractal objects by diffusion and 1/f noise

Abstract We present a video picture which shows the dynamics of diffusion or intercalation on a 2D square lattice. The images of that film are obtained from a simulation of diffusion in a simple lattice gas model. The static images show the fractal geometry of the diffusion or intercalation front. Different magnifications of the diffusion front exhibit the self-similarity which is characteristic of the fractal geometry. The visual observation of the dynamics of diffusion exhibits a new effect : the dynamic changes of the topography of the objects are erratic : strong changes in the geometry occur at random times. These dynamical changes are produced on a semi-macroscopic scale. The existence of a new effect, intercalation noise, is predicted. The same physical process could be a source of 1/f α noise in non-homogeneous electronic devices.

SCF CI calculation on HCN: HCN dissociation curves☆

Abstract SCF CI HCN dissociation curves are reported for several singlet excited states. Predissociation seems the main photofragmentation process leading to CN(B 2 Σ + ) and thus it is questionable whether HCN is a proper molecule to test dissociation models based on a single repulsive surface; however, there is also some indication of direct dissociation. Some comments on the process producing CN (A 2 II) are included. A cross section related to the 1 1 A″ predissociation is reported. The exponential fitting to our 2 1 Σ + repulsive stage gave V (eV) = 23 exp [−0.67 d CH (A)]. Large angular effects are found on the 2 1 A′− 1 II state. The calculations suggest the existence of metastable excited HCN ( 1 Σ + ) molecules with d CH ≈ 2.5 A.

Mean-field kinetic lattice gas model of electrochemical cells

We develop electrochemical mean-field kinetic equations to simulate electrochemical cells. We start from a microscopic lattice-gas model with charged particles, and build mean-field kinetic equations following the lines of earlier work for neutral particles. We include the Poisson equation to account for the influence of the electric field on ion migration, and oxido-reduction processes on the electrode surfaces to allow for growth and dissolution. We confirm the viability of our approach by simulating (i) the electrochemical equilibrium at flat electrodes, which displays the correct charged double layer, (ii) the growth kinetics of one-dimensional electrochemical cells during growth and dissolution, and (iii) electrochemical dendrites in two dimensions.

SCF CI calculations on HCN excited states

Abstract SCF CI bending curves for the ground and first four low lying singlet excited states of HCN were calculated using the experimental equilibrium distances. No other 1A″ state appears close to 11A″. We agree with former theoretical studies suggesting that one of the first two low lying excited states is of 1A′ symmetry; however we find them in reversed order. Both observed A and B systems probably arise from the same 11A″ electronic state. Our calculated 31A′ state is in fairly good agreement with the experimental C1A′ system. Some comments on the D system are added.

Diffusion in time dependent potentials: A systematic treatment

The motion of particles in modulated potentials is studied, starting from a Smoluchowski equation for the evolution of the probability distribution function. A general method to solve this explicitely time dependent Smoluchowski equation is presented and applied to the case of particles in time dependent double well potentials.For special kinds of time dependence (modulation of the barrier between the wells, modulation of the energy difference between the bottoms of the wells, modulated dilatation) the probability distribution function as well as the occupation probabilities of the two wells and the related time dependent transition probabilities are explicitely given and discussed.

Invasion noise during drainage in porous media

Fluctuations of invaded volume during an invasion percolation process in presence of a gravity field are examined. Their behaviour is related to the gradient percolation model and to previous studies of fluctuations of diffusion fronts.

Structure of noise generated on diffusion fronts

During a diffusion process the diffusion front fluctuates leading to a “geometrical noise”. We predict a 1/ƒ2 noise at high frequency and a 1/ƒ noise at low frequency separated by a crossover ƒ∗. We give here scaling expressions sustained by numerical calculations for the fluctuations and the distribution of events as a function of time and diffusion length. The close relation of this approach with noise in invasion experiments in presence of a gravity field is also pointed out.

Lattice gas model generalized to kramers regime

Abstract We propose a generalization of lattice gas models which can account for the dynamic of hopping particles in the low damping Kramers regime.

Simulation of fractal objects obtained by intercalation in layered compounds

Abstract A computer simulation of the diffusion of ions on a 2D square lattice is presented. It gives a new model for the geometry of an inhomogenously intercalated layered material. The computation is made in the case of hardcore interaction with first nearest neighbours. After a given time the line separating the diffusion source from isolated intercalated clusters is a fractal object with the fractal dimension value of 1.76±0.02. The geometry of the object is discussed as a function of connection rules between diffused particles. It is shown than the same fractal dimension is obtained in several cases. This sustains the idea that the result is “universal”. Intercalation of interacting particle is briefly discussed.