Advanced Physical Chemistry Problems (II), Hydrogen Atom and Hyrdrogen Molecular Ion

Slightly advanced problems in Physical Chemistry, herein concerning the H-atom and the Hydrogen Molecular Cation, are presented and discussed.

Parameterizing a Simple Potential Energy Function Appropriate for Vibrational Frequencies Using Molecular Dynamics

A Maple scheme for quickly parameterizing vibrational potential energy functions is presented. As an example, the potential energy function's parameters for the vibrational motions in H_2O_2 are obtained assuming the simplest potential energy function. This paper was originally written as a research paper, but rejected by the referees. It is therefore being edited into an ``educational'' paper for student usage.

Molecular Mechanics of Water Monomer

Simulation of the classical molecular dynamics of a water molecule can be useful in explaining normal modes of motion, Fourier Transforms, and fundamental frequencies of vibration, as illustrated herein.

Triatomic Molecular Orbitals

If elementary Quantum Chemistry stops at diatomic molecules, some students may be left with false impressions concerning how one builds polyatomic molecule's LCAO-MOs. This reading discusses building such molecular orbitals from atomic orbitals centered at different spatial coordinates.

Variational Calculation of the Hydrogen Molecular Cation Using Maple

The variational calculation of the energy of the Hydrogen Molecular (H2+) Cation's LCAO-MO for the sigma and sigma* states as functions of the AO's screening constant and the internuclear distance is carried out explicitly in great detail.

Variational Calculation of the Hydrogen Molecular Cation using Maple (II). The pi and pi-star Orbitals

The pi and pi-star orbitals of the hydrogen molecular cation are obtained using Maple in the same manner as the sigma and sigma-star orbitals were obtained in paper-36.

Molecular Dynamics and the Diatomic Molecule as Harmonic Oscillator and Rigid Rotor

The HCl molecule is simulated (using Maple) in its dynamics, for both vibrational (and implied) rotational motions. A discussion of the center of mass transformations involved is part of the total presentation.

Caracterización molecular de variedades de vid ( Vitis vinifera L.) de calidad enológica por marcadores microsatelitales

Siete variedades de vid empleadas en Argentina para la elaboración de vinos de alta gama fueron caracterizadas molecularmente a través del empleo de microsatélites. Seis de los 8 pares de cebadores usados amplificaron patrones de bandas reproducibles. El número de alelos detectados por locus varió entre 5 y 10, con un total de 42 alelos, registrándose desde 1 a 7 alelos únicos. El número de genotipos microsatélites encontrados para cada locus osciló entre 3 y 7, con un total de 28. Las variedades Tempranillo y Chenin mostraron 6 y 5 genotipos «únicos», respectivamente, con los 6 loci analizados. La heterocigosidad observada varió entre 42,9 y 100 %, mientras que la heterocigosidad esperada osciló entre 64,3 y 87,8 %. El locus más informativo fue VrZAG79 (con un contenido de información polimórfica de 84,9 % y un número de alelos efectivos de 8,17) mientras que el menos informativo fue VrZAG62. La probabilidad acumulada de obtener genotipos idénticos fue de 7,68 x 10-05 indicando que, mediante el uso combinado de estos 6 cebadores microsatélites se pueden discriminar todas las variedades ensayadas, con alto grado de certeza. El análisis de agrupamiento por el método UPGMA separó claramente las variedades francesas (Merlot, Pinot Noir y Cabernet Sauvignon) y Syrah de los cepajes Tempranillo y Bonarda. Esta técnica podría ofrecerse como servicio a viveristas y productores vitícolas interesados en garantizar la identidad genética de sus materiales comercializados.

Liquid lead and Lithium. A Comprehensive molecular dynamics study

Pb17Li is today a reference breeder material in diverse fusion R&D programs worldwide. One of the main issues in these programs is the problem of liquid metals breeder blanket behavior. Structural material of the blanket should meet high requirements because of extreme operating conditions. Therefore the knowledge of eutectic properties like optimal composition, physical and thermodynamic behavior or diffusion coefficients of Tritium are extremely necessary for current designs. In particular, the knowledge of the function linking the tritium concentration dissolved in liquid materials with the tritium partial pressure at a liquid/gas interface in equilibrium, CT=f(PT), is of basic importance because it directly impacts all functional properties of a blanket determining: tritium inventory, tritium permeation rate and tritium extraction efficiency. Nowadays, understanding the structure and behavior of this compound is a real goal in fusion engineering and materials science. Simulations of liquids can provide much information to the community; not only supplementing experimental data, but providing new tests of theories and ideas, making specific predictions that require experimental tests, and ultimately helping to lead to the deeper understanding and better predictive behavior.

Amorphous piezoresistive and piezoelectric sensors for robotics applications

Ultrasonic transducers have often been used in the development of sensory systems for robotics applications. In most cases, these sensory systems are based on the determination of times of flight for signals from every transducer. In this work we have used piezoresistive and piezoelectric materials to measure the instant and position collision in metallic structures by using the difference of the times of propagation of an acoustic wave when it is produced over a ferromagnetic (iron, steel or another material) based structure. An immediate application of the proposed method is the detection and location of impacts over the metallic links of an industrial robot or the collision position in a metallic structure for an automated inspection

PWM Control of a Buck Converter with an Amorphous Core Coil

Pulse-width modulation is widely used to control electronic converters. One of the most topologies used for high DC voltage/low DC voltage conversion is the Buck converter. It is obtained as a second order system with a LC filter between the switching subsystem and the load. The use of a coil with an amorphous magnetic material core instead of air core lets design converters with smaller size. If high switching frequencies are used for obtaining high quality voltage output, the value of the auto inductance L is reduced throughout the time. Then, robust controllers are needed if the accuracy of the converter response must not be affected by auto inductance and load variations. This paper presents a robust controller for a Buck converter based on a state space feedback control system combined with an additional virtual space variable which minimizes the effects of the inductance and load variations when a not-toohigh switching frequency is applied. The system exhibits a null steady-state average error response for the entire range of parameter variations. Simulation results are presented.

Refractive index changes in amorphous SiO2 (silica) by swift ion irradiation

The refractive index changes induced by swift ion-beam irradiation in silica have been measured either by spectroscopic ellipsometry or through the effective indices of the optical modes propagating through the irradiated structure. The optical response has been analyzed by considering an effective homogeneous medium to simulate the nanostructured irradiated system consisting of cylindrical tracks, associated to the ion impacts, embedded into a virgin material. The role of both, irradiation fluence and stopping power, has been investigated. Above a certain electronic stopping power threshold (∼2.5 keV/nm), every ion impact creates an axial region around the trajectory with a fixed refractive index (around n = 1.475) corresponding to a certain structural phase that is independent of stopping power. The results have been compared with previous data measured by means of infrared spectroscopy and small-angle X-ray scattering; possible mechanisms and theoretical models are discussed.

Techniques to accelerate convergence of stress-controlled molecular dynamics simulations of dislocation motion

Dislocation mobility —the relation between applied stress and dislocation velocity—is an important property to model the mechanical behavior of structural materials. These mobilities reflect the interaction between the dislocation core and the host lattice and, thus, atomistic resolution is required to capture its details. Because the mobility function is multiparametric, its computation is often highly demanding in terms of computational requirements. Optimizing how tractions are applied can be greatly advantageous in accelerating convergence and reducing the overall computational cost of the simulations. In this paper we perform molecular dynamics simulations of ½ 〈1 1 1〉 screw dislocation motion in tungsten using step and linear time functions for applying external stress. We find that linear functions over time scales of the order of 10–20 ps reduce fluctuations and speed up convergence to the steady-state velocity value by up to a factor of two.

Deep levels in a-plane, high Mg-content MgxZn1-xO epitaxial layers grown by molecular beam epitaxy

Deep level defects in n-type unintentionally doped a-plane MgxZn1−xO, grown by molecular beam epitaxy on r-plane sapphire were fully characterized using deep level optical spectroscopy (DLOS) and related methods. Four compositions of MgxZn1−xO were examined with x = 0.31, 0.44, 0.52, and 0.56 together with a control ZnO sample. DLOS measurements revealed the presence of five deep levels in each Mg-containing sample, having energy levels of Ec − 1.4 eV, 2.1 eV, 2.6 V, and Ev + 0.3 eV and 0.6 eV. For all Mg compositions, the activation energies of the first three states were constant with respect to the conduction band edge, whereas the latter two revealed constant activation energies with respect to the valence band edge. In contrast to the ternary materials, only three levels, at Ec − 2.1 eV, Ev + 0.3 eV, and 0.6 eV, were observed for the ZnO control sample in this systematically grown series of samples. Substantially higher concentrations of the deep levels at Ev + 0.3 eV and Ec − 2.1 eV were observed in ZnO compared to the Mg alloyed samples. Moreover, there is a general invariance of trap concentration of the Ev + 0.3 eV and 0.6 eV levels on Mg content, while at least and order of magnitude dependency of the Ec − 1.4 eV and Ec − 2.6 eV levels in Mg alloyed samples.

Crack mechanical failure in ceramic materials under ion irradiation: case of lithium niobate crystal

Swift heavy ion irradiation (ions with mass heavier than 15 and energy exceeding MeV/amu) transfer their energy mainly to the electronic system with small momentum transfer per collision. Therefore, they produce linear regions (columnar nano-tracks) around the straight ion trajectory, with marked modifications with respect to the virgin material, e.g., phase transition, amorphization, compaction, changes in physical or chemical properties. In the case of crystalline materials the most distinctive feature of swift heavy ion irradiation is the production of amorphous tracks embedded in the crystal. Lithium niobate is a relevant optical material that presents birefringence due to its anysotropic trigonal structure. The amorphous phase is certainly isotropic. In addition, its refractive index exhibits high contrast with those of the crystalline phase. This allows one to fabricate waveguides by swift ion irradiation with important technological relevance. From the mechanical point of view, the inclusion of an amorphous nano-track (with a density 15% lower than that of the crystal) leads to the generation of important stress/strain fields around the track. Eventually these fields are the origin of crack formation with fatal consequences for the integrity of the samples and the viability of the method for nano-track formation. For certain crystal cuts (X and Y), these fields are clearly anisotropic due to the crystal anisotropy. We have used finite element methods to calculate the stress/strain fields that appear around the ion- generated amorphous nano-tracks for a variety of ion energies and doses. A very remarkable feature for X cut-samples is that the maximum shear stress appears on preferential planes that form +/-45º with respect to the crystallographic planes. This leads to the generation of oriented surface cracks when the dose increases. The growth of the cracks along the anisotropic crystal has been studied by means of novel extended finite element methods, which include cracks as discontinuities. In this way we can study how the length and depth of a crack evolves as function of the ion dose. In this work we will show how the simulations compare with experiments and their application in materials modification by ion irradiation.