Doppler spectroscopy of hydrogen and deuterium Balmer alpha line in an abnormal glow discharge

The results of hydrogen and deuterium Balmer alpha line shapes and line intensities study in an abnormal glow discharge are reported and analyzed. The Doppler shifts along line wings are used to determine energies of excited hydrogen and deuterium atoms. For 12 different cathodes, intensity and shape of line wings are examined and dependence upon cathode material is determined. Tentative explanation of line wings intensity dependence is related to the sputtering of cathode material and back-scattering coefficients of incident hydrogen or deuterium ions and atoms from cathode surface. The influence of the light reflected on a cathode surface to the line shape measurements along discharge axis is considered. In hydrogen, deuterium, and Ar+3%H/sub 2/ discharges, basic mechanisms of fast hydrogen generation and excitation are studied. The shape and intensities of the H/sub /spl alpha// line profiles in pure hydrogen and in argon-hydrogen mixture may be correlated with hydrogen atom-carrier gas collision excitation cross sections. In order to assess the importance of reflected fast hydrogen atoms back scattered from the cathode surface, for the Balmer line shape formation, a simulation program is used. The results are in a qualitative agreement with Balmer line shapes observations.     

Effects of collisions on linear and non-linear spectroscopic line shapes

A fundamental physical problem is the determination of atom-atom, atom-molecule and molecule-molecule differential and total scattering cross sections. In this work, a technique for studying atomic and molecular collisions using spectroscopic line shape analysis is discussed. Collisions occuring within an atomic or molecular sample influence the sample's absorptive or emissive properties. Consequently the line shapes associated with the linear or non-linear absorption of external fields by an atomic system reflect the collisional processes occuring in the gas. Explicit line shape expressions are derived characterizing linear or saturated absorption by two- or three-level “active” atoms which are undergoing collisions with perturber atoms. The line shapes may be broadened, shifted, narrowed, or distorted as a result of collisions which may be “phase-interrupting” or “velocity-changing” in nature. Systematic line shape studies can be used to obtain information on both the differential and total active atom-perturber scattering cross sections.     

谱线非均匀展宽条件下原子激发的计算方法

原子吸收谱线展宽对原子光激发有较大的影响.在速率方程理论中人们通常将原子谱线的均匀展宽和非均匀展宽都归于原子谱线的线型函数.本文中提出了一种将均匀展宽和非均匀展宽区别处理的原子激发计算方法,并指出当激发光与原子作用时间短于原子热运动平均碰撞时间时,这种区别对待均匀展宽和非均匀展宽的做法是十分必要的.文中还将本文提出的新方法应用于一个二能级原子跃迁过程的计算,计算结果显示两种方法具有很大差别.     

On a density matrix equation for atomic systems subject to an external radiation field interaction and collision processes

Approximations are examined which are necessary for a density matrix equation of Rautian's form describing the interaction of a gas medium with an external radiation field by considering collision processes between active atoms as well as active and perturber atoms. The collision integrals of the kinetic density matrix equation calculated by the Bogolyubov method involve exitation processes, inelastic and elastic collisions. The features of phase- and velocity changing collisions are discussed by specializing the collision terms. Some phenomenological collision models for velocity changing collisions are examined and conditions for their validity given.     

Collision frequency of Lennard-Jones fluids at high densities by equilibrium molecular dynamics simulation

Detailed classical molecular dynamics simulation of transport coefficients and collision frequencies at high densities in rare gases are presented in this paper with a view to investigate the likely cause of discrepancy between theory and experiments. The results, when compared with experiments, showed an underestimation of the viscosity calculated through the Green-Kubo formalism, but the results are in agreement with some other calculations performed by other groups. The origin of the underestimation was considered in the present work. Analyses of the transport coefficients showed a very high collision frequency which suggested that an atom might spend much less time in the neighbourhood of the fields of force of another atom. The distribution of atoms in the systems adjusts itself to a nearly Maxwellian type that resulted in a locally and temporarily slowly varying temperature. We showed that during collision, the time spent by an atom in the fields of force of other atoms is so small compared with its relaxation time, leading to a possible reduction in local velocity autocorrelation between atoms.     

Determination of the Sn 4d line shape of the Sn/Ge(111) radical3 x radical3 and 3x3 surfaces

The Sn 4d line shapes of the Sn/Ge(111) sqrt[3]xsqrt[3] and 3x3 surfaces are currently under debate. By employing LEED, core-level, and valence band spectroscopy we have been able to determine the correct Sn 4d line shapes for these surfaces. Contrary to a recent study we conclude that the majority of the earlier reports present line shapes close to the correct ones. At 70 K we identify three 4d components in the 3x3 spectrum, two of which are identified with the two types of Sn atoms in the 3x3 cell. The third component is attributed to Sn atoms surrounding Ge substitutional defects.     

A collision-induced satellite in the Lyman profile due to H-H collisions

We present a theoretical profile of the Lyman line of atomic hydrogen perturbed by collisions with neutral hydrogen atoms and protons. We use a general unified theory in which the electric dipole moment varies during a collision. A collision-induced satellite appears on Lyman , correlated to the asymptotically forbidden transition of H₂. As a consequence, the appearance of the line wing between Lyman and Lyman is shown to be sensitive to the relative abundance of hydrogen ions and neutral atoms, and thereby to provide a temperature diagnostic for stellar atmospheres and laboratory plasmas. Received 15 January 2000 and Received in final form 17 May 2000     

On the variation of spectral line widths within stark-broadened multiplets

The Stark broadening of isolated ion lines is considered in the electron impact approximation. It is shown by an elementary formulation of the theory that, under certain plasma conditions, situations may arise in complex atoms where a relatively small number of collision-induced transitions predominate in the broadening, and consequently where appreciable variations of spectral line width are obtained within multiplets. Such lines are then particularly suitable for application of the Stark-broadening mechanism as a means of studying atomic structure and collision physics.     

Influence of line interference on the vibration-rotation band shapes

The shapes of the CO, v₃, CO₂, and v₃ N₂O fundamental vibration-rotation bands have been studied at various temperatures and in the presence of several perturbing gases. Also the half-widths of CO vibration-rotation lines have been measured at 78 K. In the region of line wings, the measured absorption coefficients deviate from those given by the superposition of Lorentzian profiles. These deviations are explained by the collision-induced line interference that causes redistribution of absorption inside the band. A theory of line mixing is formulated which is based on Markov approximation and on the strong collision model. Simple analytical expressions are obtained for the band shape. The computed shapes are in satisfactory agreement with the experimental results. The deviations from the Lorentz absorption observed in pure CO and in CO-N₂ at low temperature are partially ascribed to the formation of van der Waals dimers.     

Der Regenbogeneffekt und Interferenzerscheinungen bei molekularen Stößen

Crossed beam measurements of differential elastic scattering cross sections for collisions of alkali atoms with mercury and xenon atoms as well as with different molecules are presented. Special attention is given to the rainbow effect including supernumary rainbows and to interference effects connected with rainbow scattering. The observation of well resolved primary and supernumary rainbows together with interference patterns make the determination of a three parameter potential model possible. Partial wave numerical calculations of differential cross sections are used to evaluate potential parameters and to discuss the limits of the semiclassical rainbow theory. Elastic scattering between reactive collision partners is investigated too and interpreted in terms of a simple optical model.     

Electron capture in high-energy ion-atom collisions

We discuss the non-relativistic theory of electron capture from atoms (or ions) by ions when the relative velocity of the collision is greater than the orbital velocity of the captured electron. We emphasize the specific difficulties due to the two-body Coulomb potentials occurring in this process We show how the simplifications introduced by the small value of the electron to proton mass ratio can be used to provide a valuable tool to evaluate the adequacy of the existing theories. Extensive comparisons between theory and experiment are carried out and a number of new theoretical results are presented.     

Dynamical properties of Ag-Cu binary alloy from molecular dynamics simulation

Molecular dynamics simulations (MD) of dynamical properties of molten binary Ag-Cu alloy is presented at various temperature above the eutetic temperature. Atoms in the system have been modelled through an interatomic Lennard-Jones potential interaction. The structure, through the effective pair distribution function allows to determine the Enksog collision frequency as well as the coordination of atoms in the first shell. The surface traction, which is the force per unit area between the species shows a long separation oscillation about the value zero, while the collision frequency of pairs of atoms increase with increasing temperature. The adhesion energy between components found to be 3.4178 J/m². In agreement with theory, we found a decrease in surface tension of Ag-Cu alloy as temperature increases. Separation of atoms pairs in the first shell might be responsible for a non linear relationship found between temperature and coordination number in present calculations.     

Differential cross sections for chemically reactive systems

A quantum mechanical theory is developed for the differential and total elastic and reactive scattering cross sections for an electronically adiabatic bimolecular exchange reaction, with the restriction that the three atoms are constrained to move on a straight line, but with the whole system free to rotate in three dimensions. The introduction of a set of natural collision coordinates, together with a vibrationally adiabatic approximation, is used to reduce the scattering problem to the solution of one-dimensional Schrödinger equations. Semi-classical expressions for the elastic and reactive phase shifts are derived. The partial wave summations that occur in the theory are evaluated by semi-classical techniques and elastic and reactive differential cross sections are calculated for three different kinds of potential surface. A feature of the calculations is the appearance of a new kind of rainbow, which is named a ‘cubic’ rainbow since it arises when the deflection function varies cubically with impact parameter. The classical and semi-classical theory of cubic rainbows is developed.     

Scattering of O? from a graphite surface

Recently an extensive series of measurements has been presented for the angular distributions of oxygen molecules scattered from a graphite surface. Incident translational energies ranged from 291 to 614 meV with surface temperatures from 150 to 500 K. The measurements were taken with a fixed angle of 90° between the source beam and the detector and the angular distributions consisted of a single broad peak with the most probable intensity located at an angle slightly larger than the 45° specular position. Analysis with the hard cubes model for atom-surface scattering indicated that the scattering is primarily a single collision event with a surface having a collective effective mass much larger than a single carbon atom. Limited analysis with a classical diatomic molecular scattering theory was also presented. In this paper a more complete analysis using the classical diatomic molecular scattering theory is presented. The energy and temperature dependence of the observed angular distributions are well described as single collision events with a surface having an effective mass of 1.8 carbon graphite rings. In agreement with the earlier analysis and with other experiments, this suggests a large cooperative response of the carbon atoms in the outermost graphene layer.     

A density matrix formulation of the theory of magnetic resonance spectra in slowly reorienting systems

When a spin system is coupled to a molecule or part of a molecule which is reorienting at a rate comparable to the width of the powder spectrum, the line shape can be calculated from a density matrix which is a function both of spin operators and of the Euler angles describing the molecular orientation. The problem of calculating such line shapes is eased by using the super-operator formalism and choosing symmetry adapted basis operators in the Liouville space of spins and orientations. Diffusion and strong collision models for molecular reorientation are discussed.     

微波器件微放电阈值计算的蒙特卡罗方法研究

为了精准快速地计算微波器件中微放电效应的阈值,在传统蒙特卡罗方法的基础上,提出了三种不同的蒙特卡罗方法,分别对二次电子的初始能量、出射角度和初始相位等参数进行随机,结合四阶龙格-库塔法和Furman模型计算电子的运动轨迹和单次碰撞产生的二次电子发射系数,然后应用不同的方法计算有效二次电子发射系数作为微放电效应的判据.以平板传输线TEM模式为研究对象,采用四种不同的蒙特卡罗方法计算微放电阈值,并与统计模型结果进行对比.结果表明单电子-多碰撞蒙特卡罗方法误差最小,而且稳定性最好.     

薄膜生长中的表面动力学(Ⅰ)

本文较全面地从理论上研究了薄膜生长过程中原子在表面上的和种动力学表现，涉及的内容包括亚单层生长时，原子在表面上的扩散，粘接，成核，以及已经形成的原子岛之间的相互作用，兼并，失稳，退化等一系列过程，作为研究的基础。在本文的第一部分（即0-6章）中，我们首先介绍了目前这方面理论研究中所主要使用的各种方法。例如，第一性原理计算，分子动力学模拟。蒙特卡罗模拟。速率方程和过渡态（TST）理论等。基于这些研究。我们介绍给读者为什么原子成岛时在低温下选择分形状，而在高温时则选择紧致状。这一过程可以用经典的扩散子限制集聚理论（Diffusion-Limited Aggregation,DLA)。然而当有表面活性剂存在时形核的规律安全相反，由此提出了一个反应限制集聚理论（Reaction-Limited Aggregation,RLA)，这两个理论目前可以很好的解释亚单层生长时的一般形核规律。接下来我们讨论了长程相互作用对生长初期原子形核的影响。并进一步得出了相应的标度理论。在第6章我们系统地研究了分了吸附对二维原子岛形状的控制性，从而提出了边-角原子扩散的对称破缺模型。     

薄膜生长中的表面动力学(Ⅰ)

本文较全面地从理论上研究了薄膜生长过程中原子在表面上的各种动力学表现 ,涉及的内容包括亚单层生长时 ,原子在表面上的扩散 ,粘接 ,成核 ,以及已经形成的原子岛之间的相互作用 ,兼并 ,失稳 ,退化等一系列过程。作为研究的基础 ,在本文的第一部分 (即0～ 6章 )中 ,我们首先介绍了目前这方面理论研究中所主要使用的各种方法。例如 ,第一性原理计算 ,分子动力学模拟 ,蒙特卡罗模拟 ,速率方程和过渡态 (TST)理论等。基于这些研究 ,我们介绍给读者为什么原子成岛时在低温下选择分形状 ,而在高温时则选择紧致状。这一过程可以用经典的扩散子限制集聚理论 (Diffusion_LimitedAggregation ,DLA)。然而当有表面活性剂存在时形核的规律完全相反 ,由此提出了一个反应限制集聚理论 (Reaction_LimitedAggregation,RLA)。这两个理论目前可以很好地解释亚单层生长时的一般形核规律。接下来我们讨论了长程相互作用对生长初期原子形核的影响 ,并进一步得出了相应的标度理论。在第 6章我们系统地研究了分了吸附对二维原子岛形状的控制性 ,从而提出了边 角原子扩散的对称破缺模型。     

Line-shape of dark line and maser emission profile in CPT

The paper is concerned with the line shapes of resonance phenomena observed in Coherent Population Trapping (CPT) applied to alkali atoms in a cell containing a buffer gas. Significant asymmetries and departures from a Lorentzian shape have been observed in connection with the measurement of dark lines and CPT maser emission profiles. Measurements are reported as a function of the power and frequency tuning of the laser used to create the CPT phenomenon. The paper reports on different experimental conditions and a comparison between theory and experiments is made for the cases of cesium and rubidium in a buffer gas. Received 3 March 2000 and Received in final form 10 April 2000     

Charge density determination in icosahedral AlPdMn quasicrystal using quantitative convergent beam electron diffraction

Charge density distribution in icosahedral AlPdMn quasicrystal has been studied on a single-crystal specimen by using quantitative convergent beam electron diffraction (QCBED) technique. The QCBED systematic row method was used in the refinement of structure factors. To refine the low-order structure factors, the wave-mechanical formulation of electron diffraction dynamical theory was used in the calculation of electron diffraction intensities for the quasicrystal in fitting the experimental intensity line scan profiles. The shapes of atomic surfaces (occupation domains) were described with symmetry-adapted series of surface harmonics. An iterative procedure was used in determination of structure factors of the quasicrystal. The structure factors of nine strongest symmetry inequivalent reflections according to X-ray diffraction experiment were refined with QCBED technique. The average of refinement results for a given reflection performed on several CBED patterns, which were slightly different in orientation and sample thickness, and on different line scans, was taken as the value of structure factor for the reflection. The obtained structure factors for electrons were transformed into X-ray structure factors with Mott formula. The bonding charge density map for the quasicrystal was constructed with the obtained nine structure factors. Assuming that the atoms are spheres, the gain or loss of electrons for different atoms were calculated. It shows that identical atoms can have different valences at different kinds of positions. The bonding charge is localized along certain directions.