Semiclassical calculation of ionization rate for Rydberg hydrogen atoms near a metal surface

The ionization of Rydberg hydrogen atoms near a metal surface at different scaled energies above the classical saddle point energy has been discussed by using the semiclassical method. The results show that the atoms ionize by emitting a train of electron pulses. In order to reveal the chaotic and escape dynamical properties of this system in detail, the sensitive dependence of the ionization rate upon the scaled energy is discussed. As the scaled energy is close to the saddle point energy, the ionization process of the hydrogen atom is nearly the same as the case of hydrogen atom in an electric field. There is only a single pulse of electrons, with an exponentially decaying tail. With the increase of the scaled energy, the ionization rates are similar to the case of the hydrogen atom in parallel electric and magnetic field, a series of electron pulses appear in the ionization process. This is caused by classical chaos, which occurs for the metal surface. Our studies also suggest that the metal surface can play the role of both the electric and the magnetic fields. Our theoretical analysis will be useful for guiding experimental studies of the ionization of atoms near the metal surface.     

Terahertz emission from a metallic surface induced by a femtosecond optic pulse

Results of experimental and theoretical investigations on generation of terahertz radiation at the interaction of femtosecond laser pulses with a metal surface are presented. Investigations are performed with the laser pulse intensities higher compared with that used in papers [Opt. Lett.29, 2674 (2004); Opt. Lett.30, 1402 (2005)]. The most effective generation is observed for p-polarized optical pulses with incidence angles in the range 5°-10° (from the surface), depending on the kind of metal. For the copper, the exponential growth of terahertz pulse energy with the increase of optical pulse energy was registered. Theoretical interpretation for some of the experimental results is proposed based on the model of free electrons in metal.     

Dynamics of a radiating surface discharge

We have elaborated a code for numerical treatment of two-dimensional problems of the dynamics of a surface discharge with inclusion of radiative energy transfer. The gas dynamic, electric, and radiative properties of a surface discharge in atmospheric air have been studied experimentally for a total energy input of 100 J/cm in the input power range 3–6 MW/cm. A new technique involving a discharge sliding along a conducting surface is applied to initiate an extended surface discharge. The calculations agree satisfactorily with the experiments. The method elaborated here makes it possible to calculate a broad class of linear discharges with an eye to produce a radiation source with problem-oriented spectral power characteristics.     

Photoionization microscopy of hydrogen atom near a metal surface

We have studied the ionization of Rydberg hydrogen atom near a metal surface with a semiclassical analysis of photoionization microscopy. Interference patterns of the electron radial distribution are calculated at different scaled energies above the classical saddle point and at various atom-surface distances. We find that different types of trajectories contribute predominantly to different manifolds in a certain interference pattern. As the scaled energy increases, the structure of the interference pattern evolves smoothly and more types of trajectories emerge. As the atom approaches the metal surface closer, there are more types of trajectories contributing to the interference pattern as well. When the Rydberg atom comes very close to the metal surface or the scaled energy approaches the zero field ionization energy, the potential induced by the metal surface will make atomic system chaotic. The results also show that atoms near a metal surface exhibit similar properties like the atoms in the parallel electric and magnetic fields.     

Synthesis of carbon films in a plasma-chemical reactor based on beam-plasma discharge

A modification of the plasma-chemical reactor based on beam-plasma discharge is developed for the deposition of carbon (including diamond-like carbon (DLC)) films. The reactor differs from the existing devices by the simplicity of the procedure that makes it possible to control the energy characteristics of the ion beam incident on the film in the course of deposition. A method for the computer simulation of the parameters of the ion flux on an electrically insulated surface upon the modulation of the plasma potential is proposed. The method allows the prediction of the ion energy and ion flux that acts upon the deposited film. DLC films on metal substrates are produced. The charge deep-level transient spectroscopy is used to reveal the effect of the adsorbed water and alcohol vapors on the electrophysical properties of the films, which indicates that the films can be used as active adsorbing materials in chemical sensors.     

Hardening of Metal Product Surface Layers by the Combined Gas Discharge Plasma

The properties of a low-pressure combined gas discharge initiated in a resonator chamber near the surface of a metal workpiece subjected to the superposition of high electromagnetic and electrostatic fields have been studied. It has been shown that, when a constant potential is applied to the workpiece, a layer forms on its surface in which positive ions or electrons (depending on the sign of the potential) accelerate to an energy as high as several hundred electronvolts. The interaction between accelerated particles and the workpiece causes the liberation of heat in the surface layer, which is sufficient for its complete melting. If the applied potential is positive, a hardening nanostructure arises on the workpiece surface. The physical parameters of the combined gas discharge have been estimated.     

An investigation into the effects of oxygen plasma discharge on tin-doped indium oxide thin films

The surfaces of tin-doped indium oxide (ITO) thin films for polymer light-emitting electrochemical cells (LECs) were modified by oxygen plasma discharge. The properties of the ITO surfaces were evaluated by means of the measurements of X-ray photoelectron spectroscopy (XPS), contact angle, surface free energy and polarity. The influence of surface properties of the ITO thin films on the performance of polymer LECs was investigated in terms of the turn-on voltage, injection current and luminance. When oxygen plasma discharge was employed to modify the ITO surfaces, the surface properties of ITO are optimized due to the improvement of surface stoichiometry and the enhancement of wettability. And the improved surface properties benefited from the oxygen plasma discharge is observed to decay with the time after the plasma discharge. The difference in chemical composition, surface free energy and polarity between the non-treated and treated ITO surfaces appears to become smaller with the increase of the time after plasma discharge. In addition, the electrical and optical performance of the devices is found to become worse with the increasing time after plasma discharge on ITO substrates. The results demonstrate that the device performance strongly depends on the ITO surface properties and the ITO/organic interface characteristics.     

Adsorption of V on a hematite (0 0 0 1) surface and its oxidation: Submonolayer coverage

The adsorption of submonolayer V on an idealized model hematite (0 0 0 1) surface and subsequent oxidation under atomic O adsorption are studied by density functional theory. The preferred adsorption sites, adsorption energy and configuration changes due to V and O adsorption are investigated. It is found that in most cases V forms threefold bonds with surface O atoms, inducing a large geometry change at the hematite surface and near surface region and a bond stretch between surface Fe and O. The adsorption energy is mainly decided by interplay between adsorbed metal-surface oxygen bonding and adsorbed metal - subsurface metal interaction. The relative energy of subsequent O adsorption and geometry depends on the reformed V/hematite structure. Electronic properties such as projected densities of states and chemical state change upon V adsorption are studied through both periodic slab and embedded cluster localized orbital calculations; both strong vanadium-oxygen and vanadium-iron interactions are found. While V generally donates electrons to a hematite surface, causing nearby Fe to be partially reduced, the Fe and V oxidization state depends very much on the coverage and detailed adsorption configuration. When the V/hematite system is exposed to atomic O, V is further oxidized and surface/near surface Fe is re-oxidized. Our theoretical results are compared with X-ray surface standing wave and X-ray photoelectron spectroscopic measurements. The influence of d-electron correlation on the predicted structures is briefly discussed, making use of the DFT + U scheme.     

大气压管板结构纳秒脉冲放电中时域X射线研究

纳秒脉冲放电能在大气压下产生高电子能量、高功率密度的低温等离子体,由于经典放电理论无法很好地解释纳秒脉冲放电中的现象,近年来以高能逃逸电子为基础的纳秒脉冲气体放电理论受到广泛关注.纳秒脉冲放电会产生高能逃逸电子,伴随产生X射线,研究X射线的特性可以间接反映高能逃逸电子的特性.本文利用纳秒脉冲电源在大气压下激励空气放电,通过金刚石光导探测器测量放电产生的X射线,研究不同电极间隙、阳极厚度下和空间不同位置测量的X射线特性.实验结果表明,在大气压下纳秒脉冲放电能产生上升沿约1 ns,脉宽约2 ns的X射线脉冲,其产生时间与纳秒脉冲电压峰值对应,经计算探测到的X射线能量约为2.3×10-3J.当增大电极间隙时,探测到的X射线能量减弱,因为增大电极间隙会减小电场强度和逃逸电子数,从而减少阳极的轫致辐射.电极间距大于50 mm后加速减弱,同时放电模式从弥散过渡到电晕.随着阳极厚度增加,阳极后方和放电腔侧面观察窗测得的X射线能量均有所减弱,在阳极后面探测的X射线能量减弱趋势更加明显,这说明X射线主要产生在阳极内表面,因此增加阳极厚度会使穿透阳极薄膜的X射线能量减少.     

Absorption and Recurrence Spectra of Nonhydrogenic Rydberg Atom Near a Metal Surface

Multielectron to theoretical treatments atoms near a metal surface are essentially more complicated than hydrogen atom with regard By using the semicalssical dosed orbit theory generalized to the multielecton atoms, we study the dynamical properties of the Rydberg lithium atom near a metal surface. The photoabsorption spectra and recurrence spectra of this system have also been calculated. Considering the effect of the ionic core potential of the Rydberg lithium atom, the number of the closed orbits increases, which leads to more peaks in the recurrence spectra than the case of hydrogen atom near a metal surface. This result shows that the core-scattered effects play an important role in nonhydrogenic atoms. This study is a new application of the closed-orbit theory and is of potential experimental interest.     

Comments on surface electronic properties of TiC and TiN

Some recent experimental and theoretical results on surface electronic properties of a special class of refractory transition metal compounds (TiC, TiN and ZrN) and particularly, on surface states, the surface charge self-consistency and the stability of the polar (111) surface, are discussed within a simple qualitative framework based on the use of energy weighted averages as the main tool of the analysis.     

介质材料二次电子发射特性对微波击穿的影响

以介质填充的平行板放电结构为例,本文主要研究了介质填充后微波低气压放电和微放电的物理过程.为了探究介质材料特性对微波低气压放电和微放电阈值的影响,本文采用自主研发的二次电子发射特性测量装置,测量了7种常见介质材料的二次电子发射系数和二次电子能谱.依据二次电子发射过程中介质表面正带电的稳定条件,计算了介质材料稳态表面电位与二次电子发射系数以及能谱参数的关系.在放电结构中引入与表面电位相应的等效直流电场后,依据电子扩散模型和微放电中电子谐振条件,分别探讨了介质表面稳态表面电位的大小对微波低气压放电和微放电阈值的影响.结果表明,介质材料的二次电子发射系数以及能谱参数越大,介质材料的稳态表面电位也越大,对应的微波低气压放电和微放电阈值也越大.所得结论对于填充介质的选择有一定的理论指导价值.     

Chemisorption of CO and H on Pd, Pt and Au nanoclusters: a DFT approach

We have performed a theoretical study of pure metal nanoparticles (Pd/Pt/Au) and their interaction with CO molecules and atomic hydrogen, in order to investigate chemisorption effects of relevance to catalysis by nanoparticles. First-principles density-functional local relaxations are used to investigate the effect of CO and H adsorption on six structural motifs. The results of the energetic crossover and structural deformations are analyzed in terms of the interplay between metal-metal interactions (including internal and surface stress) and CO-metal and H-metal interactions. It is found that H adsorption releases surface stress, thus favoring 5-fold symmetry motifs, whereas CO adsorption produces a flattening of the potential energy surface of the metal clusters.     

On energy transfer and generation of an electric current in the vicinity of an electrode dipped in an electrolyte and strongly heated by a current passing through it

Processes occurring when a metal electrode dipped in an electrolyte is heated by intense evaporation of the electrolyte are considered in terms of a physically rigorous model. Based on the Onsager principle of least energy dissipation rate in nonequilibrium processes, the fractions of thermal energy that are spent on heating and evaporating the electrolyte and on heating the vapor are found. The energy is released within the vapor-gas sheath when an electric current flows between the electrode and electrolyte surface. It is found that the electrolyte vapor temperature exceeds 1300 K. Analytical expressions are derived for the vapor-gas sheath thickness, the electrolyte vapor pressure, and the velocity of the vapor escaping the discharge zone. It is shown that field evaporation of thermally activated negative ions from the electrolyte surface cannot provide an electric current with densities found in experiments but is responsible for the generation of free electrons near the electrolyte surface. These electrons arise when the ions decay via collisions with excited molecules.     

A theoretical explanation of the surface diffusion mechanism in metal electrodes in contact with electrolytes

A theoretical explanation for the surface diffusion mechanism observed in columnar structured metal electrodes in contact with electrolytes is given. The potential energy of a surface metal atom on which ions forming part of the supporting electrolyte are adsorbed is described by means of an anharmonic oscillator curve whereas the energy of a surface metal atom liberated from any adsorption interaction is approximated by a harmonic oscillator energy fuction. Geometric arguments allow to define a symmetry factor δ for which experimental values were previously obtained. A qualitative interpretation of the value of δ has been made.     

Characteristics of thin films of hexagonal boron nitride mixed with copper controlled by a magnetron co-sputtering deposition technique

Thin film properties, such as surface morphology, internal stress and surface energy, of hexagonal boron nitride/copper mixtures, produced by a magnetron co-sputtering deposition technique, were investigated. Techniques included scanning electron microscopy, film deflection and contact angle measurements. It was possible to fabricate nano-scale islands on the surface of the film and to control the size of the islands by adjusting the sputter discharge conditions. The internal stress and surface energy can be significantly modified depending on these conditions indicating that they are dependent variables. This technique of co-sputtering deposition is a promising candidate for the control of nano-structures on a surface and for controlling internal stress and surface energy.     

The effect of air plasma on barrier dielectric surface in dielectric barrier discharge

Barrier dielectric is an important part of atmospheric pressure dielectric barrier discharge (AP-DBD), which partly affects discharge characteristics. Conversely, discharge plasma also has influence on dielectric surface properties. To investigate this influence, some experiments were carried out on a home-built AP-DBD system with glass plate as barrier dielectric. Surface wettability was evaluated by water contact angles on a drop shape analysis system. The morphologies and chemical compositions of the glass sample surfaces were observed by field-emission scanning electron microscope (FESEM) and energy dispersive X-ray spectroscopy (EDS) attached to FESEM. The results show that water contact angles decrease as discharge energy increases, micro-discharge etching zones are formed into glass surface and different from the control glass in surface micro-structure and chemical compositions.     

Interaction of a surface glow discharge with a gas flow

A surface glow discharge in a gas flow is of particular interest as a possible tool for controlling the flow past hypersonic aircrafts. Using a hydrodynamic model of glow discharge, two-dimensional calculations for a kilovolt surface discharge in nitrogen at a pressure of 0.5 Torr are carried out in a stationary gas, as well as in a flow with a velocity of 1000 m/s. The discharge structure and plasma parameters are investigated near a charged electrode. It is shown that the electron energy in a cathode layer reaches 250–300 eV. Discharge is sustained by secondary electron emission. The influence of a high-speed gas flow on the discharge is considered. It is shown that the cathode layer configuration is flow-resistant. The distributions of the electric field and electron energy, as well as the ionization rate profile in the cathode layer, do not change qualitatively under the action of the flow. The basic effect of the flow’s influence is a sharp decrease in the region of the quasineutral plasma surrounding the cathode layer due to fast convective transport of ions.     

Regular set of gas discharges on the surface of a dielectric in a quasi-optical microwave beam

Various ways of producing a regular set of plasma objects in the field of a quasi-optical microwave beam using passive vibrators are considered. These objects provide the basis for a periodic vortex structure on the surface of an immersed dielectric body. The properties of a deeply subcritical surface discharge in the flow, as well as its structure and spatial and energy characteristics, are described.     

Correspondence between classical dynamics and recurrence spectra of Rydberg hydrogen atom near a metal surface

The chaotic behaviours of the Rydberg hydrogen atom near a metal surface are presented. A numerical comparison of Poincare surfaces of section with recurrence spectra for a few selected scaled energies indicates the correspondence between classical motion and quantum properties of an excited electron. Both results demonstrate that the scaled energy dominates sensitively the dynamical properties of system. There exists a critical scaled energy εc, for ε 〈 εc, the system is near-integrable, and as the decrease of ε the spectrum is gradually rendered regular and finally turns into a pure Coulomb field situation. On the contrary, if ε 〉 εc, with the increase of ε, the system tends to be non-integrable, the ergodic motion in phase space presages that chaotic motion appears, and more and more electrons are adsorbed on the metal surface, thus the spectrum becomes gradually simple.