A. Dal Pino

Theory of adsorption of atoms and molecules on Si(111)-(7×7)

Abstract Ab initio density functional theory calculations are performed to analyze a regional reactivity index based on chemical concepts of local softness and electronegativity. This leads to the prediction of a general qualitative pattern for the interaction of the Si(111)-(7 × 7) surface with atoms and molecules that preserve the reconstruction. We successfully compare this general pattern with experimental information available. Based on this approach, we predict the reactivity behavior of the reconstruction with a wide range of chemical reactants.

Combining genetic algorithm and simulated annealing: A molecular geometry optimization study

We introduce a new hybrid approach to determine the ground state geometry of molecular systems. Firstly, we compared the ability of genetic algorithm (GA) and simulated annealing (SA) to find the lowest energy geometry of silicon clusters with six and 10 atoms. This comparison showed that GA exhibits fast initial convergence, but its performance deteriorates as it approaches the desired global extreme. Interestingly, SA showed a complementary convergence pattern, in addition to high accuracy. Our new procedure combines selected features from GA and SA to achieve weak dependence on initial parameters, parallel search strategy, fast convergence and high accuracy. This hybrid algorithm outperforms GA and SA by one order of magnitude for small silicon clusters (Si6 and Si10). Next, we applied the hybrid method to study the geometry of a 20-atom silicon cluster. It was able to find an original geometry, apparently lower in energy than those previously described in literature. In principle, our procedure can be applied successfully to any molecular system.

Chemical softness and impurity segregation at grain boundaries

We analyze the process of impurity segregation at grain boundaries as a chemical reaction between the impurity and the interface. From this point of view, we test the ability of the concepts of local softness and hardness to predict the most probable sites for impurity accumulation. As a test, an ab initio investigation of the ∑=5 tilt [310] grain boundary in germanium is presented and the energetics of the accumulation of dopant atoms in this interface are studied. Our results support the utility of an analysis in terms of softness for impurity segregation problems.

Optical and electrical diagnostics of a spark-plug discharge in air

Optical and electrical diagnostics were used to investigate a spark discharge using a commercial spark-plug operating in the glow phase regime. Voltage and current were measured in order to characterize the discharge. The gas temperature was estimated as a function of time and duty cycle using ro-vibrational spectra of the second positive system of nitrogen by comparison between experimental and simulated spectra. It was found that 1600 K ≤ Tg ≤ 2800 K. The reduced electric field varied between 1 and 1000 Td after the end of the current pulse. Using the line intensity method between one line from the ionic argon and the other from the neutral atom, the electronic temperature was measured and found to be between 17 000 and 20 000 K. The electron density was determined from the broadening of Hα line and was found to be 4.0 × 1014 cm−3 ≤ ne ≤ 1.7 × 1015 cm−3.

Self-consistent-field CNDO/BW cluster model of covalent solids

Abstract We have suggested a parametrization scheme for the self -consistent CNDO/BW method suitable to model the electronic structure of perfect and locally perturbed covalent solids. By working on small clusters we have carried out calculations for diamond perfect crystal and for nitrogen replacing a carbon atom in the lattice. The results show that the methos is accurate to calculate one electron energy-eigenvalues and total energies.

Simulating nitrogen molecular plasma spectra using artificial neural networks

We introduce a method based on artificial neural networks (ANNs) to simulate molecular spectra under experimental conditions. The spectra of a nitrogen discharge were measured under variable pressure ranging from 0.3 to 2.0 Torr and discharge current varying from 5.0 to 50.0 mA and they have been used as a training set. Our procedure uses a system of two ANNs. The first one extracts the physical characteristics of the training set. The second one uses the information collected from the first to simulate the spectra under conditions that have not been presented in the training sample. The parameters used to assure the quality of the simulated spectra were the temperatures. Our simulated spectra have agreed with those measured experimentally. These results have indicated the robustness of the ANN in simulating molecular spectra. These facts stimulated us to investigate the ability of our system in generating spectra under different conditions of experimental resolution. The results show that our method could be used to improve the accuracy of experimental temperature whenever the experimental spectrum has a poor resolution.