Atom diffraction from magnetic gratings in the thin phase-grating approximation

The problem of atom diffraction from a reflecting magnetic diffraction grating is solved in the thin phase-grating approximation. The general problem for scalar diffraction is modelled using a semi-classical method in which the grating potential is separated into a reflecting term and a diffracting term. The trajectory of the atom in the reflecting potential is solved classically and the atom wave function in the diffracting potential found by integrating the phase change along the classical trajectory. The diffraction orders are obtained after Fourier transforming the result. This can be done independently of the grating potential resulting in a general formula for the diffraction efficiencies. The general result is applied to the problem of atom diffraction from a magnetic grating. Several approximations are required to reduce the problem to a form amenable to analytic solution. The results are compared with an accurate numerical method. Received 1st February 2001 and Received in final form 8 March 2001     

Selective adsorption in elastic and inelastic scattering of He and Ne from LiF (001) surface

New experimental evidence is presented for selective adsorption in the inelastic, as well the elastic scattering of He and Ne from LiF. For the inelastic scattering a condition for the effect is derived, by which it is possible to obtain, merely from the position of minima in the inelastic spectrum, an averaged excitation spectrum. The results thus found are entirely consistent with the present knowledge of the surface dynamics of LiF.     

Theoretical studies of atom-solid elastic scattering by iterative integration using the Green function

The elastic scattering of low-energy light atoms from a perfect crystalline surface is studied by an iterative integration scheme using the Green function. The atom-solid interaction is represented by the often used Morse type surface potential. A varying number of closed and open channels is included in the calculation, according to necessicity. For a beam incident along the cyrstal symmetry directions, a scheme to utilize the symmetry condition for efficient computation is proposed. The diffraction intensities at a bound state resonance (selective adsorption) are calculated by properly selecting a reference potential for the calculation of the Green function. The calculations yield the resonant diffraction intensity patterns in agreement with previous calculations using a different numerical technique and with the experimental observations for the HeLiF and HeNaF systems. A calculation including 69 allowed diffracted beams (open channels) for the HeLiF system at normal incidence is also presented and comparison with experimental results is made to estimate the periodic potential parameter.     

Rydberg transition frequencies from the local density approximation

A method is given that extracts accurate Rydberg excitations from density functional calculations in the local density approximation, despite the short-ranged potential. For the case of He and Ne, the asymptotic quantum defects predicted by the calculations are in less than 5% error, yielding transition frequency errors of less than 0.1 eV.     

菲涅耳衍射解析表达式的物理意义

文中讨论了圆孔菲涅耳衍射解析表达式的物理意义,指出了精确解(在菲湿耳近似下)与近似解的区别,并与用数值积分法计算的结果进行比较,证明了圆孔菲涅耳衍射解析表达式的正确性。     

Helium scattering from metal surfaces: potential determination and crystallography

We describe a method for determining the interaction potential between a helium atom and a metal surface from diffraction data. Results are presented for He/Ni(110) and He/Cu(110). For He/Cu(110) the potential 3s in good agreement with a first principles calculation but this is not true for He/Ni(110). We also show that a surface relaxation of 10% produces measurable changes in intensities.     

Bound state resonances in atom-solid scattering

Theoretical studies and numerical calculations of bound state resonances (selective adsorption) in the elastic scattering of low-energy light atoms from a perfect crystalline surface are made. The surface-atom potential used is the sum of Yukawa-6 pair potentials with the pair potential parameters fixed to yield the correct bound state energies for the HeLiF system. The scattering equation is numerically integrated using basis sets of open and closed channels of varying number. It is found that numerically accurate solutions are obtained only if the basis set includes the bound state in resonance, all diffracted channels coupled strongly to it by the periodic surface potential, and the specular and other low-order diffracted channels. The calculations yield specular minimum and (11) and (21) diffraction maximum under (01) selective adsorption conditions in agreement with experimental observations for the HeLiF and HeNaF systems. These results appear independent of the potential model. The atomic band structure due to the surface periodic potential is found to yield splitting of the specular minima and/or broadening of the minima. It is suggested that studying such splittings will yield information on the periodic surface potential.     

A comparison of self broadening and shift of helium,neon and argon emission lines

Self broadening (van der Waals and resonance) and shift of Ne emission lines and van der Waals broadening and shift of He emission lines have been measured using a high pressure (0.5–3.0 atom), low current discharge. These results are compared with previous measurements in Ar and He to obtain a complete comparison of self broadening and shift of He, Ne and Ar emission lines. Oscillator strengths for the resonance transitions are obtained from the resonance broadening coefficients. The trend of the van der Waals broadening coefficients for the three noble gases is correctly predicted by a theory due to HINDMARSHet al.⁽⁴⁾ in which a Lennard-Jones potential is used in the impact theory formalism. The measured line shifts cannot be accounted for by this theory and reflect the need for more accurate quantum mechanical calculations.     

The scattering of atoms from a corrugated wall model with the GR method in the system He/LiF(001)

In this communication we present a new method, called GR, to calculate the diffraction probabilities of atomic beams from a hard corrugated surface model. The method is applied to the system He/LiF(001). We show that the method gives a convergent solution, within 10^?4 in unitarity, when β₀ ? 0.12, where β₀ is the dimensionless total corrugation amplitude. This limit suggests that the method converges for “any” crystallographic surface of interest. The agreement between the experimental data and the calculated curves is good. From this agreement we conclude that the surface corrugation for He/LiF(001) is cosine-like and the “best” corrugated surface is given.     

The electronic factor in catalysis by metals

By assuming a Lennard-Jones 12-6 potential between He and Li⁺, and He and F^? in a LiF crystal, the specular and diffracted intensities of He from a LiF(001) crystal surface are obtained by direct integration of the coupled differential equations. Numerical results are obtained by using 3–29 partial wave models for glancing incidence at θⁱ = 60–90°. The 11 partial wave model gives fairly good results in comparison with other models having more waves. Experiments of Williams in which the incident conditions are varied by rotating the crystal are discussed.     

Helium, neon and argon diffraction from Ru(0001)

We present an experimental and theoretical study of He, Ne and Ar diffraction from the Ru(0001) surface. Close-coupling calculations were performed to estimate the corrugation function and the potential well depth in the atom-surface interaction in all three cases. DFT (density functional theory) calculations, including van der Waals dispersion forces, were used to validate the close-coupling results and to further analyze the experimental results. Our DFT calculations indicate that, in the incident energy range 20-150?meV, anticorrugating effects are present in the case of He and Ar diffraction, whereas normal corrugation is observed with Ne beams.     

Behavior of the photoelectron boundary layer with sinusoidal timedependence

At high-incident photon intensities (X-rays, UV, or visible), a photoelectron boundary layer can form. This layer's properties, and especially its electric dipole moment, change in a nonlinear way as the light intensity varies in time. We show here that the dipole moment obeys an exact dynamical equation, which can be used as the basis for analytic approximation schemes for a driver of arbitrary time dependence. An approximate analytic solution for a sinusoidally oscillating light source is presented, and compared with a particle-pusher numerical simulation. Scaling laws are extracted from the analytic solution     

A uniform quasi-classical description of radiative transitions for asymmetric rare-gas atom-atom collisions

The uniform quasi-classical approximation [14] is used to describe the optical spectra formed during asymmetric collisions between atoms of rare gases in which one of the atoms is in a metastable state. We consider the reactions He(2¹S) + Ne → He(1¹S) + Ne + ?ω and Ar(³P₂) + He → Ar(¹S) + He + ?ω, in which the optical transition mechanisms are typical of most rare gases. Quasi-molecular terms of excited states and radiative widths calculated in a unified semiempirical approach are used. Spectral characteristics are calculated for thermal collision energies in the entire frequency range, including the center and both wings of the forbidden line. For the blue wing, our results are consistent with the widely used Condon approximation at collision energies E ≥200 cm^?1. At lower collision energies and in the region of the red wing and center of the forbidden line, the spectral distributions that cannot be described in the Condon approximation are reproduced in the uniform quasi-classical approximation. Comparison with quantum-mechanical calculations by the strong-coupling method confirms the high accuracy of the uniform quasi-classical approximation in the entire range of radiation frequencies.     

Diffraction of fast atomic projectiles during grazing scattering from a LiF(001) surface

Light atoms and molecules with energies from 300 eV to 25 keV are scattered under a grazing angle of incidence from a LiF(001) surface. For impact of neutral projectiles along low index directions for strings of atoms in the surface plane we observe a defined pattern of intensity spots in the angular distribution of reflected particles which is consistently described using concepts of diffraction theory and specific features of grazing scattering of atoms from insulator surfaces. Experimental results for scattering of H, D, 3He, and 4He atoms as well as H2 and D2 molecules can be unequivocally referred to atom diffraction with de Broglie wavelengths as low as about 0.001 Angstroms.     

Fresnel diffraction mirror for an atomic wave

We have experimentally demonstrated a material-independent mirror for atomic waves that uses the Fresnel diffraction at an array of parallel ridges. He* (2 (3)S(1)) and Ne* (1s(3)) atomic waves were reflected coherently on a silicon plate with a microfabricated grating structure, consisting of narrow wall-like ridges. We measured the reflectivity at grazing incidence as a function of the incident velocity and angle. Our data show that the reflectivity on this type of mirror depends only on the distance between the ridges, the wavelength, and the incident angle, but is insensitive to the material of the grating structure. The reflectivity is observed to increase by 2 orders of magnitude, compared to that of a flat polished silicon surface, where the reflection is caused by the attractive surface potential. For He* atoms, the measured reflectivity exceeds 10% for normal incident velocities below about 25 cm/s.     

Coherence during scattering of fast H atoms from a LiF(001) surface

The coherence for diffraction effects during grazing scattering of fast hydrogen and helium atoms from a LiF(001) surface with energies up to some keV is investigated via the coincident detection of two-dimensional angular distributions for scattered projectiles with their energy loss. For keV H atoms, we identify electronic excitations of the target surface as the dominant mechanism for decoherence, whereas for He atoms this contribution is small. The suppression of electronic excitations owing to the band gap of insulators plays an essential role for preserving quantum coherence and thus for the application of fast atom diffraction as a surface analytical tool.     

有温度反馈阶跃引入负反应性瞬变的解

考虑有温度反馈反应堆阶跃引入负反应性过程的变化规律,对点堆中子动力学方程的解进行研究,给出不同的分析模型,分析发现,（1）用于瞬发超临界的分析模型及其结果不能用于引入大的负反应性过程的分析.（2）在相同的初始功率时,先驱核瞬跳近似模型的结果要优于目前比较普遍的中子瞬跳近似模型的结果.（3）无论初始功率与引入负阶跃反应性的大小,温度瞬跳近似模型的结果总是优于先驱核瞬跳近似与中子瞬跳近似模型的结果.故而温度瞬跳近似模型是目前最为准确的解析模型. 关键词： 中子动力学 温度反馈 反应性 点堆     

有温度反馈阶跃引入负反应性瞬变的解

考虑有温度反馈反应堆阶跃引入负反应性过程的变化规律,对点堆中子动力学方程的解进行研究,给出不同的分析模型,分析发现,（1）用于瞬发超临界的分析模型及其结果不能用于引入大的负反应性过程的分析.（2）在相同的初始功率时,先驱核瞬跳近似模型的结果要优于目前比较普遍的中子瞬跳近似模型的结果.（3）无论初始功率与引入负阶跃反应性的大小,温度瞬跳近似模型的结果总是优于先驱核瞬跳近似与中子瞬跳近似模型的结果.故而温度瞬跳近似模型是目前最为准确的解析模型.     

Li-和Na-单电子解离过程的研究

采用交叉束方法 ,利用负离子源产生的 3— 19keV的Li- 和Na- 轰击惰性气体靶He ,Ne和Ar ,通过静电偏转和位置灵敏探测器区分碰撞后中性粒子束和负离子束 ,测量了不同碰撞系统的中性粒子计数与相应入射负离子计数的比值R(E) ,并得到R(E)与入射负离子能量、负离子种类和靶原子种类的关系. The count ratios R of the neutralized atoms of final state to projectiles Li -and Na -in collision with He, Ne and Ar are measured in the energy range of 3-19 keV. It is found that the count ratios R increase slowly with the collision energy in whole experimental energy range for He, Ne and Ar. For Li -→He, Ne, Ar Collisions, R(He)≈R(Ar)>R(Ne), and for Na -→He, Ne, Ar Collisions, R(He)>R(Ar)> R(Ne).     

Logarithmically accurate total cross sections of fast electron scattering by atoms

A simple analytical formula for the total integrated cross section of fast electron scattering by atoms is obtained. This formula, being logarithmically accurate, is derived from the approximate rule of sum that does not take into account exchange effects in the atomic wave function (Hartree approximation). It is shown that exchange effects can be neglected in that scattered electron energy range where the first Born approximation is valid. The results of calculation for He, Ne, and Ar atoms agree well with experimental data.