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.     

双甘氨肽与高定向热解石墨的散射动力学研究

本文通过反应力场来研究中性双甘氨肽与高取向热解石墨的碰撞动力学过程，分别模拟初始入射角度为0o、20o、45o和70o以及入射能量为481.5 kJ/mol和表面温度为677 K的情况，并且确定和分析了散射产物的角度分布、平动能分布、内能分布和表面滞留时间分布. 双甘氨肽是一种多原子分子，具有较多的低频振动模式和较强的表面吸附相互作用，这些使得碰撞过程的表面滞留时间较长和容易造成能量损失，尤其是表面法线方向. 由于碰撞分子的动量沿表面法线方向上明显地发生损失，而沿表面平行方向上的动量则会大部分保留，因此，其散射角通常会呈现出超镜面反射分布，并且末态的平动能要远小于所谓硬立方模型的预测值. 本文加深了多肽分子与高取向热解石墨碰撞及其能量转移过程的认识和理解，有助于设计在稀薄大气中收集这类大分子的中性气体浓缩器.     

Theory of molecule-surface scattering at thermal and hyperthermal energies

A theoretical model of molecule-surface scattering is developed which includes energy and momentum transfers between the surface and projectile for both translational and rotational motions and internal mode excitation for the projectile molecule. The translation and rotation motions are treated in the classical limit, while a quantum treatment for internal vibrational mode excitation is used. The results of calculations are compared with recent high-precision measurements of the scattering of a beam of C(2)H(2) molecules from a clean, ordered LiF(001) surface at energies of up to nearly 1 eV. The calculated results for angular distributions and rotational excitations are in good agreement with experiment.     

Circular dichroism in K-shell ionization from fixed-in-space CO and N2 molecules

We have measured the angular distributions of 1s photoelectrons excited by circularly and linearly polarized light from fixed-in-space CO and N2 molecules, in the vicinity of their shape resonances. A strong circular dichroism, i.e., a strong dependence on the sense of rotation of the polarization vector of the photons, is found for both molecules. State-of-the-art one-electron multiple scattering and partially correlated random phase approximation calculations are in good agreement with many, but not all, aspects of the experimental data.     

Studies on scattering of laser radiation from viewing dump in tokamak Thomson scattering system

Thomson scattering is among one of the primary diagnostics used for tokamak plasma temperature and density measurements. Spuriously scattered stray signal further deteriorates the already existing poor signal-to-noise ratio in this technique. The present paper reports experimental investigations on the angular distribution of the scattered/stray radiation from graphite and a stack of laser blades dumps for different angles of incidence. Experimental results show a dependence of the stray signal on the dump-edge orientation. It is found that horizontally oriented dump edge produces a minimum level of stray light (around two orders less than in case of plane graphite tiles) in the horizontal plane, in which detectors for most of the diagnostics are installed. Studies are also made for p-polarized and s-polarized incident laser beams of 632.8?nm and 1064?nm wavelengths.     

Esdiad from CO on W(100): Evidence for non-vertically adsorbed CO

Mass-discriminating measurements of the angular distributions of electron-stimulated ion desorption (ESDIAD) from CO adsorbed on W(100) and coadsorbed with C and O have been performed. The O⁺ beams indicate normal and off-normal (by 5 to 13°) desorption which is interpreted as due to CO molecules bound in symmetric and in two types of unsymmetric bridges. Preadsorption of C suppresses the vertical state, while oxidation of the surface suppresses the off-normal states.     

Effect of the molecular structure on the gas-surface scattering studied by supersonic molecular beam

The experimental apparatus for investigating the gas-surface interaction has been newly developed. The coherent length of the helium, the energy resolution and the angular spread of the beam in the apparatus were established as ω= 16 nm, DE/E = 2.4%\Delta E/E = 2.4{\%} and Δθ= 0.5^○, respectively, through the measurements of the time-of-flight of He beam and of the angular intensity distributions of He scattered from LiF(001). The angular intensity distributions of Ar, N₂ and CO scattered from the LiF(001) surface along the [100] azimuthal direction were then measured as a function of incident translational energy. The effects of the molecular structural anisotropy and center-of-mass position on the gas-surface inelastic collision at the corrugated surface are discussed with predictions based on a recently developed simple classical theory of the ellipsoid-washboard model.     

Energy transfer in hyperthermal Xe-graphite surface scattering

The scattering of a hyperthermal Xe from a graphite (0001) surface has been studied using a molecular beam-surface scattering technique and molecular dynamics (MD) simulations. The angular and velocity distributions of scattered Xe atoms were measured at incidence energies from 0.45 to 3.5 eV, three incidence angles of 15°, 35° and 60° and the surface temperatures of 300 K and 550 K. The observed time-of-flight spectra exhibit a sharp velocity distribution with only one velocity component, which is ascribed to the direct inelastic scattering process. The angle-resolved energy ratios of the mean final translational energy over the mean incidence energy E_f/E_i agree well with those predicted based on the assumption of the conservation of the momentum parallel to the surface. The Hard-Cube model, where the mass of the cube is approximately 310 u, has reproduced the angle-resolved flux distributions of scattered Xe atoms. In the Hard-Cube model almost 80% of the normal component of the incidence translational energy is found to be lost in collision. The MD simulations reproduce well the experimental results by using the Brenner potential for intralayer C atoms and a Lennard-Jones potential for interlayer C–C pair interactions.     

Model calculations of atom-surface scattering

The elastic scattering of light mass, thermal-energy atoms from simple surfaces is investigated. The surface is represented by the model of a single planar square array of hard spheres. The effect of the surface potential well is treated semiclassically by simply shifting the energy of the incident atom ; furthermore a constant imaginary term is added to the energy to account for inelastic scattering and adsorption. As in the multiple scattering formalism of LEED the total scattering matrix of the lattice is expanded in terms of the individual gas atom-surface atom t-matrices. Propagation of the incident atom on the surface is described in terms of a one particle Green's function propagator with complex energy. The terms in the multiple scattering series are summed to all orders, by using standard matrix inversion techniques. The size of the matrix to be inverted limits to ten the total number of phase shifts that are included in the calculation. Thermal effects are included through angle dependent Debye-Waller factors.Model calculations have been performed to study the intensity of the specular and the diffracted beams as a function of the angles of incidence. The importance of surface temperature (introduced by the Debye-Waller factors), the incident energy and the depth of the potential well of the gas-surface interaction are discussed. The main feature of the results is the decrease of the intensity of the specular beam in going from glancing incidence to normal incidence and the presence of structure due to the appearance and disappearance of diffracted beams across the surface. The azimuthal behavior of the specular beam is in agreement with experimental observations.     

Scattering of fast neutrons on238U,average energy and angular distributions of fission neutrons

Angular distributions of neutrons elastically and inelastically scattered from²³⁸U have been measured with a time-of-flight spectrometer at seven incident neutron energies between 1.5 and 5.5 MeV. Inelastic angular distributions for groups of unresolved levels are given for incoming neutron energies of 1.5, 1.9 and 2.3 MeV. The corresponding neutron cross-sections were obtained relative to then-p scattering cross-sections. The average energies and angular distributions of the fission neutrons were extracted from the measured fission neutron spectra at 1.5,1.9 and 2.3 MeV. Cross-section calculations based on a spherical optical model have shown to be inadequate to describe the neutron-nucleus interaction in case of strong nuclear deformation. The experimental reality may be better approached, instead, if the calculations are made using a potential which takes into account the deformation of the target nucleus. Some of the present measurements are interpreted in this theoretical perspective.     

Scattering of Li+ from LEED characterized W(110) and Si (111) surfaces at energies between 2 and 20 eV

Energy and angular distributions of Li⁺ ions scattered from W(110) and Si(111) surfaces have been measured for a wide range of scattering angles and for beam energies between 2 and 20 eV. The collision process can be explained in terms of binary collisions with single surface atoms, if the influence of the attractive part of the interaction potential is taken into account. A square well approximation for the potential makes it possible to predict the form of the energy spectrum as well as the behaviour of the angular distribution of the scattered particles for both systems. The influence of adsorbed atoms and molecules on the scattering behaviour has been investigated. Exposure of the W surface to H₂, O₂ and CO shows that surface coverages exceeding 10% of a monolayer very drastically influence the scattering. The results from the clean surfaces indicate that sufficiently precise measurements of the energy spectra of the scattered particles can yield very detailed information about the form of the interaction potential. The form of the energy spectra also contains information as to the extent of vibrational excitation of the surface atoms.     

Scattering of N2 from Ni(111)

A cold (T_rot<10 K) beam of N₂ with an initial translational energy of 0.40 eV strikes an Ni(111) surface at surface temperatures from 300 to 873 K at several incident angles from 15 to 60°. The rotational energy and angular distributions of the scattered molecules are probed using (2+1) resonance-enhanced multiphoton ionization. Molecules scattered in the specular direction have mean rotational energies that are independent of surface temperature, whereas those scattered at angles far from the specular show a dependence on surface temperature, caused likely by multiple collisions with the surface before escape. A rotational rainbow, seen in systems such as CO–Ni(111) and N₂–Ag(111), is not seen in this system. For molecules that scatter close to the specular direction, approximately 10% of the initial translational energy is converted into rotational energy of the scattered N₂. For surface temperatures above room temperature, the angular distributions indicate that molecules that scatter into low-J states also tend to exit in a broad peak (10–20° FWHM) near the specular, and this peak is broadened with increasing incident angle. The molecules that scatter into high-J states have a much broader distribution, indicating that they are trapped rotationally during the initial collision and suffer multiple collisions before leaving the surface.     

一种考虑几何屏蔽效应的计算“电子-分子”散射总截面的可加性规则修正方法

在考虑分子内原子间的几何屏蔽效应随电子入射能量变化的基础上, 提出了一种能够在中、高能区准确计算“电子-分子”散射总截面的可加性规则修正方法. 利用这一修正后的可加性规则并使用“电子-C, H, O, N原子”散射总截面的实验数据, 在50—5000 eV内计算了电子被NO, N₂O, NO₂和C₂H₆分子散射的总截面, 且将计算结果与实验结果及其他理论结果进行了比较. 结果表明, 利用这一方法修正过的可加性规则进行计 关键词： 电子散射 可加性规则 总截面 几何屏蔽效应     

Molecular orientation effects in gas-surface dynamical processes

We report studies of chemical reactions on single crystalline Si surfaces induced by oriented molecular beams. We discuss the obtained steric effects on Si from the viewpoint of the strength of the gas-surface interaction, in comparison with the steric effects appearing on the graphite surface. Oriented molecular beams demonstrate the possibility for controlling surface chemical reactions by varying the orientation of the incident molecules. The steric effects found on Si surfaces hint at new ways of material fabrication on Si surfaces.     

Effects of relativistic kinematics in heavy ion elastic scattering

Relativistic corrections to the reaction kinematic parameters were made for elastic scattering of ⁶Li, ¹²C and ⁴⁰Ar from ⁴⁰Ca, ⁹⁰Zr and ²⁰⁸Pb targets at incident energies between 20 and 100 MeV/nucleon. The results of optical model calculations show that the effects of such corrections are important when describing the angular distributions of elastic scattering cross sections for heavy ion scattering at incident energies as low as around 40 MeV/nucleon. The effects on the total reaction cross sections on the other hand, were found to be small within the energy range studied when the optical model potential is fixed.     

Exact finite-range DWBA description of the60Ni(160,12C)64Zn reaction at 60 MeV incident energy

Cross section angular distributions of⁶⁰Ni(¹⁶O,¹²C)⁶⁴Zn reactions leading to three strongly excited states at 60 MeV incident energy and the¹⁶O+⁶⁰Ni and¹²C+⁶⁴Zn elastic scattering at 60 MeV respectively 45 MeV and 54 MeV have been measured using aQ3D magnetic spectrograph. EFR-DWBA calculations assuming the transfer of anα-cluster in its 0s ground state are able to describe the general features of the strongly oscillating experimental angular distributions using a surface transparent optical model potential. The optical model parameters used in the DWBA calculations are obtained from fits of the elastic scattering data of incident and exit channels. The importance of “correct” optical model parameters in the exit channels for relative spectroscopic factors will be discussed and the extracted relative spectroscopic factors will be compared to previous (⁶Li,d) results.     

Reactive scattering of a supersonic oxygen atom beam O+ICl

Reactive scattering of O atoms with ICl molecules has been studied at an initial translational energy E = 40 kJ mol^-1 using a supersonic beam of O atoms seeded in He and at E = 15 kJ mol^-1 using O atoms seeded in Ne. Velocity distributions of OI product were measured by cross-correlation time-of-flight analysis. Full contour maps of the differential reaction cross-section have been obtained which show peaking almost equally in the forward and backward directions at both initial translational energies. The product translational energy distributions are consistent with a long-lived O-I-Cl collision complex dissociating via a loose transition state. The stability of the O-I-Cl complex is attributed to the low electronegativity of the central I atom compared with the peripheral atoms. This electronegativity ordering rule also determines the stability of the intermediates in the other reactions of oxygen atoms with halogen molecules. The mild peaking of the product angular distributions for O + ICl and IBr indicates that collision complexes have quite modest collision angular momenta L ～ 40 ? corresponding to impact parameters b ～ 1·4 Å and that the angular momentum of the OI molecule in the loose transition state may be approximately half the product orbital angular momentum.     

中子与裂变核238U反应双微分截面的理论分析

根据中子与原子核U及其同位素反应的总截面,去弹性散射截面和弹性散射角分布的实验数据,获得了入射中子能量0.1—20MeV的一组普适中子与U及其同位素反应的光学模型势参数.应用光学模型,核裂变理论,耦合道理论,扭曲波玻恩近似理论,宽度涨落修正的Hauser-Feshbach理论和预平衡反应的激子模型,计算和分析了中子与²³⁸U反应的所有截面、角分布、能谱和双微分截面.理论计算与实验数据进行了分析比较     

Associative desorption of adsorbed oxygen promoted by translational energy of incident O2 molecules

Effects of the translational energy of incident oxygen molecules on surface processes from dissociative adsorption to subsequent associative desorption on Pt(1 1 1) have been investigated by means of supersonic molecular beam scattering. The reaction rate of associative desorption increases with the incident translational energy. The enhanced associative desorption can be attributed to nonthermal diffusion of dissociated oxygen, promoted by the translational energy of incoming molecules.