3D problem of heat and mass transfer at the ignition of a combustible liquid by a heated metal particle

A nonlinear nonstationary 3D problem of heat and mass transfer at gas phase ignition of a combustible liquid spread on the surface of a solid body by a metal particle heated to a high temperature is solved. This is done within the framework of a model taking into account the heat conduction and evaporation of the liquid, the diffusion and convection of the combustible vapors in the oxidizer medium, the crystallization of the ignition source, the kinetics of the processes of evaporation and ignition of liquids, the dependence of the thermophysical characteristics of the interacting substances on the temperature, and the moisture content of the oxidizer—air. The dependences of the ignition delay time of the liquid on the temperature and sizes of the heating source are established. Limiting values of the temperature and particle sizes at which the ignition conditions take place are determined. The influence of the air humidity on the inertia of the process being investigated is analyzed. A comparison of numerical values of typical parameters of the process under investigation for 2D and 3D models is performed.     

Long-range repulsive interaction between molecules on a metal surface induced by charge transfer

The adsorption of a molecular electron donor on Au(111) is characterized by the spontaneous formation of a superlattice of monomers spaced several nanometers apart. The coverage-dependent molecular pair distributions obtained from scanning tunneling microscopy data reveal an intermolecular long-range repulsive potential, which decreases as the inverse of the molecular separation. Density functional theory calculations show a charge accumulation in the molecules due to electron donation into the metal. Our results suggest that electrostatic repulsion between molecules persists on the surface of a metal.     

Charge transfer of Rydberg H atoms at a metal surface

The charge transfer of Rydberg hydrogen atoms at a metal surface is investigated for the first time. The surface ionization of Stark states with various electron density distributions with respect to the surface is examined. Unlike the nonhydrogenic species studied previously, genuine control over the orientation of the electronic wave function in the surface-ionization process is demonstrated. A comparison of the results for a range of collisional velocities for the most redshifted Stark state with principal quantum numbers n=20-36 with the classical over-the-barrier approach shows a good agreement for the onset of the ion signal, but the shallow rise in signal is not accounted for. An excellent fit of the experimental results can be achieved using a simple semiempirical model.     

Self-energy of a charge near a metal surface

A quantum mechanical expression for the self-energy of a charged particle near a metal surface has been derived. The expression incorporates the interaction of the charge with the surface and bulk plasmons including their dispersions in the hydrodynamic model. It is found that the inclusion of the plasmons dispersions gives result for the saturated value of the image potential for copper closer to experiment.     

Direct observation of charge transfer at a MgO(111) surface

Transmission electron diffraction (TED) combined with direct methods have been used to study the sqrt[3]xsqrt[3]R30 degrees reconstruction on the polar (111) surface of MgO and refine the valence charge distribution. The surface is nonstoichiometric and is terminated by a single magnesium atom. A charge-compensating electron hole is localized in the next oxygen layer and there is a nominal charge transfer from the oxygen atoms to the top magnesium atom. The partial charges that we obtain for the surface atoms are in reasonable agreement with empirical bond-valence estimations.     

Reversible charge transfer in solids; binary approximation

A common theory of reversible charge transfer (RCT) kinetics for low and high excitation power with the use of the conditional concentration of acceptors is presented. A comparison with recently published RCT theory is given. The results are similar or identical only in restricted or fractal spaces of low dimension, and in the case of low concentration of acceptors, i.e. in all the cases when the conditions of the binary approximation are fulfilled. The deviation from the binary approximation results in a physically incorrect picture, the cause being due to the different space averaging procedure. The probability of the donor's cation state is an increasing function of the concentration of the acceptor and of the characteristic parameters of the rate constants. Received 26 May 1999     

Atoms interacting with a metal surface with adsorbates: local and non-local effects on the charge transfer

A model system representing a collisional Li atom interacting in the “on top” geometry with an Al surface partly covered with Li adsorbates is studied. The cases of a unique adsorbate and of a uniform adsorbate layer are considered and compared. The energies and widths of the atom levels are much modified in the vicinity of the adsorbate. This is interpreted in terms of molecularisation of the atomic and adsorbate levels. These results also determine the relative importance of the local and non-local effects of the adsorbates on the resonant charge transfer process in atom-surface collisions.     

Dissipation energy for a charge moving near a metal surface

The energy dissipated by a moving charge approaching a metal has been obtained for a wide range of the incoming velocities. Regime limits for very small or very great velocities are analysed as particular cases, and some experimental evidence for the system He⁺/Au is discussed in the light of our results.     

Theoretical study of the charge transfer in supported transition metal microclusters

The validity of the local charge neutrality approximation within the tight-binding method has been benchmarked against the global charge neutrality approximation. Calculations have been performed for Fe microclusters supported on the Ni(001) surface using a self-consistent spd tight-binding method parametrised to ab initio tight-binding linear muffin-tin orbital results. In order to enhance the effect of the hybridisation between both elements, we have considered an artificial strong interfacial relaxation of the geometries which were predicted by means of molecular dynamics calculations. Our results confirm the validity of the local charge neutrality approximation when a careful parametrisation is used and appropriate values for the charge in each site are elected.     

Static response to a charge impurity near a metal surface finite-barrier treatment

Summary The static response to a charge impurity near a metal surface is computed in the RPA approximation for the case of a finite barrier. Particular attention is given to the problem of the correct infinite-volume limit and to the contribution of the one-particle states with energy higher than the potential barrier, which has been found to be not negligible. An integral equation for the self-consistent potential is derived and solved numerically. Results for Na and Zn are exhibited, showing remarkable difference with respect to the infinite-barrier case.

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Riassunto La risposta statica ad un'impurezza di carica in prossimità della superficie di un metallo è calcolata nell'approssimazione delle fasi a caso (RPA) nel caso di una barriera finita. Sono trattati con particolare cura i problemi del corretto limite di volume infinito e del contributo degli stati ad una particella con energia superiore alla barriera di potenziale, contributo che risulta non trascurabile. Si deriva e risolve numericamente un'equazione integrale per il potenziale autoconsistente. Sono presentati i risultati per Na e Zn, da cui si può rilevare una notevole differenza rispetto al caso della barriera infinita.

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Резюме В случае конечного барьера в приближении случайных фаз вычисляется статический отклик на заряженную примесь вблизи металлической поверхности. Особое внимание уделяется проблеме корректного предела бесконечного объема и вкладу одно-частичных состояний с энергией выше потенциального барьера, который, как получено, не является пренебрежимо малым. Выводится интегральное уравнение для самосогласованного потенциала и решается численно. Полученные результаы для Na и Zn обнаруживают значительное различие с результатами для случая бесконечного барьера.

     

Non-adiabatic charge transfer process of oxygen on metal surfaces

The dynamics of charge transfer processes of oxygen on metal surfaces are investigated. The analysis is based on a set of diabatic potential energy surfaces, each representing a different charged oxygen species. Empirical universal potential energy functions have been constructed that mimic the oxygen–silver, oxygen–aluminum and oxygen–cesium systems. The differences between the work functions of these metals are reflected in the potential parameters. The dynamics are followed by solving the multichannel time-dependent Schrödinger equation starting from oxygen in the gas phase. Only the direct short time part of the dynamics is followed leading to the creation O⁻₂ in the gas phase as well as dissociative chemisorption. A large portion of the wave function is trapped in molecular chemisorption charged states. It is found that the position of the crossing seam between potentials has a profound influence on the outcome.     

Partial charge transfer during the specific adsorption of halide anions on a metal electrode

The partial charge transfer from (to) an anion specifically adsorbed on an uncharged interphase boundary between a metal electrode and an electrolyte solution was studied. The energy of the system was presented as a quadratic functional in the orders of perturbation of the electron densities of the anion and metal. The linear terms of the expansion represent the traditional contribution to the energy term of the chemical potentials of separated starting subsystems. The quadratic terms describe the influence of the chemical hardnesses on the equilibrium bonding energy. Based on the condition of minimum of the energy functional expressions for the partial charge transfer, energy of stabilization, and adsorption bond length were obtained. The results were used to estimate the parameters of the chemisorption bonding of halide anions to a mercury electrode and to interpret experimental data on the properties of the interphase boundary between a mercury electrode and an alkali halide solution in water and acetonitrile. It was demonstrated that the formulas derived satisfactorily describe the key parameters of the bond even in the zeroth-order approximation.     

Resonant enhancement of charge transfer through surface states

A model is proposed that treats electrons at surfaces as a combination of two-dimensional and three-dimensional degrees of freedom. This yields a simple formula for the surface state induced resonant enhancement of the transfer of electrons through a surface. The model also yields analytic approximations for the transition between two-dimensional and three-dimensional distance laws in the correlations between electrons in surface states. Received 6 August 2002 Published online 31 October 2002 RID="a" ID="a"e-mail: rainer@sask.usask.ca     

Formation of a space charge during the interaction of an ion with a metal surface

When an ion approaches a metal surface, an induced charge forms in it, which affects the formation of electronic states in an ion-metal system. The spatial distribution of the electron density near the metal surface has been calculated in the case of the classical consideration and in the framework of the energy density functional approximation. It is shown that the space cloud exists in the form of a sphere segment; in this case, this cloud is shifted toward the approaching ion in the quantum-mechanical consideration.     

Semi-classical approximation and the problem of boundary conditions in the theory of relativistic particle radiation

Abstract

The process of relativistic particle radiation in an external field has been studied in the semi-classical approximation rather extensively. The main problem arising in the studies is to express the formula of the quantum theory of radiation in terms of classical quantities, for example of the classical trajectories. However, it still remains unclear how the particle trajectory is assigned, that is which particular initial or boundary conditions determine the trajectory in semi-classical approximation quantum theory of radiation.

We shall try to solve this problem. Its importance comes from the fact that in some cases one and the same boundary conditions may give rise to two or more trajectories. We demonstrate that this fact must necessarily be taken into account on deriving the classical limit for the formulae of the quantum theory of radiation, since it leads to a specific interference effect in radiation.

The method we used to deal with the problem is similar to the method employed by Fock to analyze the problem of a canonical transformation in classical and quantum mechanics.