Study of low energy noble gas ion reflection from monocrystalline surfaces; Influence of thermal vibrations of the surface atoms: II. Calculational evaluation of the sensitivity for model parameters and possibilities for estimation

The sensitivity of the characteristics of low energy noble gas ion reflection from monocrystalline surfaces for thermal properties of the target atoms has been investigated by computer simulation. In addition the uncertainties in comparing experimental results with calculations, introduced by a not well-known interaction potential, have been examined. The calculations have been carried out for 6 keV Ar⁺ ions reflected from a vibrating Cu〈100〉 chain. To achieve the above presented object we varied the mean square value $\text{u}{}^2$ and the correlation coefficients of the atomic thermal displacements. The used ion-atom interaction potential (a Thomas-Fermi potential in the Molière approximation) has been varied by changing the screening length aF. Under certain conditions the shape of the energy spectra of specularly reflected particles depends pronouncedly on both $\text{u}{}^2$ and aF. The effects are the most pronounced for scattering angles between about 20° and 30°. The angular distribution shows also a distinct and simultaneous sensitivity for the used potential and the target-temperature. A most interesting feature is the occurrence of a QT peak at higher temperatures, resulting from quasi triple collisions from surface “thermal pit” structures. At a given scattering angle the cut-off temperature of this QT peak can be related to the mean square displacements of the involved atoms. This cut-off temperature appears to be (almost) independent from the used potential, allowing an estimation of $\text{u}{}^2$. The intensity of the QS peak and the QD peak depend exclusively on the mean square differences of thermal displacements of neighbouring atoms. Correlated atomic displacements have some influence on the angular distributions and on the QT peak intensity. Possibilities to estimate model quantities are discussed briefly.     

Three-dimensional spatiokinetic distributions of sputtered andscattered products of Ar+ and Cu+ impacts onto theCu surface: molecular dynamics simulations

Energy and angular distributions of reflections and sputtered atoms are essential inputs for feature profile evolution simulations. Molecular dynamics simulations are used to compute the three-dimensional energy and angular distributions for reflected and sputtered products when both Ar⁺ and Cu⁺ ions bombard a copper surface. We term these “spatiokinetic” distribution functions (SKDF's). We show by example that SKDF's for reflected Ar⁺ ions focus as the incident angle &thetas;_i (normal=0°) is increased from 60-75° and broaden as the incident energy E_i is increased from 55-175 eV. We show that the SKDF's for glancing-angle reflected Cu⁺ ions focus when E_i is increased from 55-175 eV. We show that the SKDF's for copper atoms sputtered by 175 eV Ar⁺ are insensitive to &thetas;_i;. We report total sputter yields for Ar⁺ and Cu⁺ ions at 55 and 175 eV for incident angles between 0° and 85°, and sticking probabilities for Cu⁺ ions for these energies and angles. Comparison to representative experimental results (Doughty et al., 1997) is given     

Energy dependence of optical-model parameters for the interaction of 6Li and 7Li Ions with 28Si nuclei at low energies

The results of a joint analysis of experimental data (angular distributions for scattering and total reaction cross sections) are presented for the case of the interaction of ^6,7Li ions with ²⁸Si nuclei at energies between 7.5 and 32 MeV. The respective calculations were performed on the basis of a deformed potential of the optical model by using the SPI-GENOA code. The energy dependences of the parameters of the macroscopic optical model and the total cross section for the (^6,7Li+²⁸Si) reactions were obtained over the energy interval from 7.5 to 32 MeV. The calculated angular distributions and total reaction cross sections are in good agreement with experimental data.     

Supernumerary rainbows in the angular distribution of scattered projectiles for grazing collisions of fast atoms with a LiF(001) surface

Fast atoms with keV energies are scattered under a grazing angle of incidence from a clean and flat LiF(001) surface. For scattering along low index azimuthal directions within the surface plane ("axial surface channeling") we observe pronounced peak structures in the angular distributions for scattered projectiles that are attributed to "supernumerary rainbows." This phenomenon can be understood in the framework of quantum scattering only and is observed here up to projectile energies of 20 keV. We demonstrate that the interaction potential and, in particular, its corrugation for fast atomic projectiles at surfaces can be derived with a high accuracy.     

Multiple ion scattering

The energy spectra and the angular distributions of noble gas ions, reflected from a metal surface, yield information about a number of important properties of this surface. A large number of investigations have been carried out in the past decades, not only to get insight into the interaction mechanism but also to develop methods for applying of the knowledge gained. To obtain information about the outermost surface layer, it appears necessary to use noble gas ions as primary particles, and to detect the scattered particles in the charged state only; the fraction of noble gas ions reaching the detector, after reflection from target atoms in the second layer, is very small because of the high probability of neutralization. However, this is only valid if the initial energy of the incoming ions is relatively low, namely ? 10 KeV. Under certain experimental circumstances it appears justifiable, down to a lower limit of about 20 eV, to conceive the interaction of these ions with the target atoms as single collisions. The relation between the initial energy and the post-collision energy is then very simple if the collision is an elastic one; it depends upon the scattering angle and upon the ratio of the two given masses only and not upon the interaction potential. The shift of the peaks in an energy spectrum is caused by inelastic collisions and is relatively small in the considered energy region. The causes of this shift will be discussed. As opposed to single collisions, the post-collision energies after a multiple collision depend largely upon the interaction potential. Attention will be paid to the search for these potentials. Utilization of the multiple collision phenomenon in the study of surface geometry is hampered by the vibrational motion of the surface atoms. As a result, the energy spectra are blurred and a shift of the so-called quasi-single and quasi-double peaks can occur. Under certain conditions a third peak emerges which can give additional information about the surface vibrations. The intensity of the scattered ions depends upon the cross section for scattering. Recently it has been shown that the relation between this cross section and the initial energy has an oscillating character for certain combinations of incident ions and target atoms. This phenomenon has very important consequences, e.g. in the use of single ion scattering as an analytical tool. To investigate surface structures it appears that single and multiple ion scattering, combined with LEED and AES, can provide valuable information.     

Influence of the image interaction on ion desorption processes

A classical treatment of the interaction between an ion and a conducting surface during an ion desorption process is presented. Analytical expressions have been obtained for the trajectories of desorbing ions as well as for the trajectories of ions trapped by the image potential. The distortion by the image potential of the energy and angular distribution of desorbing ions is estimated. Our objective has been to provide a guide for experimentalists to the surprisingly large changes in such distributions caused by the image forces.     

Angular distributions of fast electrons, ions, and Bremsstrahlung x/gamma-rays in intense laser interaction with solid targets

We study the angular distributions of fast electrons, ions, and bremsstrahlung x/ gamma-rays generated during the interaction of an ultrashort intense laser pulse with solid targets. A relation is found on the angular directions for fast electrons and ions as a function of the particle's kinetic energy, experienced Coulomb potential changes, and the incident angle of the laser pulse. It is valid independent of the acceleration mechanisms and the polarization of the laser pulse, as confirmed by particle-in-cell simulations. The angular distribution of bremsstrahlung x/gamma-rays is presented to show explicitly its correlation with the corresponding angular distributions of electrons.     

Investigation of the reaction <Superscript>208</Superscript>Pb(<Superscript>18</Superscript>O, <Emphasis Type="Italic">f</Emphasis>): Folding angular distributions of fission fragments and gamma-ray multiplicity

Correlations between folding angular distributions of fission fragments and the gamma-ray multiplicity are studied for ¹⁸O + ²⁰⁸Pb interactions at energies of the beam of ¹⁸O ions in the range E _lab = 78–198.5 MeV. The probabilities are determined for complete-and incomplete-fusion processes inevitably followed by the fission of nuclei formed in these processes. It is found that the probability of incomplete fusion followed by fission increases with increasing energy of bombarding ions. It is shown that, for the incomplete-fusion process, folding angular distributions of fission fragments have a two-component structure. The width of folding angular distributions (FWHM) for complete fusion grows linearly with increasing energy of ¹⁸O ions. The multiplicity of gamma rays from fission fragments as a function of the linear-momentum transfer behaves differently for different energies of projectile ions. This circumstance is explained here by the distinction between the average angular momenta of participant nuclei in the fusion and fission channels, which is due to the difference in the probabilities of fission in the cases where different numbers of nucleons are captured by the target nucleus.     

Experimental verification of the technique for calculating light scattering in turbid media and determination of the single-scattering albedo based on the spectroscopy of elastically reflected electrons

The equations of light transfer in a turbid medium and transfer equations for electrons undergoing only elastic collisions in a solid body are shown to be equivalent. The angular distributions of elastically reflected electrons were calculated using the DISORT and MDOM optical codes. The calculation data are compared with the results of experimental measurements for the angular distributions of elastically reflected electrons and with calculations according to the Monte-Carlo method. Optical codes are shown to be highly efficient in calculating the angular distributions of electrons elastically reflected from multilayer media.     

Reconstruction of heterogeneous single-channel neutralization kinetics by the method of fast-ion grazing scattering from metal surfaces

A method is suggested for the unambiguous reconstruction of the heterogeneous slow-ion neutralization kinetics near the surface of a conductor. The method is based on the special features of fast ion grazing scattering with above-thermal energies of translational motion along the normal. It is shown that the angular distributions of fast particles reflected from the surface are related to the slow-ion neutralization rate by a simple algebraic expression. The method allows the reconstruction of the coordinate dependence for the neutralization rate and the interaction potentials. Its possibilities are demonstrated by the example of neutralizing fast He⁺ ions (ion energies E₁≈2 keV and glancing angles θ₀≈0.5°–0.8°) scattered from the A1(111) surface.     

强激光场中氢负离子双光子电离能量谱的研究

利用强场近似(Strong field approximation,SFA)方法研究氢负离子(H-)在强激光场中双光子电离的能量谱,所得到的电离谱随角度的变化规律与实验结果符合得很好.进一步的研究表明,H-离子在强激光场中双光子电离的能量谱与有质动力能有关.激光场强度越大,光电子的有质动力能也越大,能量谱向左移动越明显...     

强激光场中氢负离子双光子电离能量谱的研究

利用强场近似（Strong field approximation,SFA）方法研究氢负离子（H ）在强激光场中双光子电离的能量谱,所得到的电离谱随角度的变化规律与实验结果符合得很好.进一步的研究表明, H 离子在强激光场中双光子电离的能量谱与有质动力能有关.激光场强度越大,光电子的有质动力能也越大,能量谱向左移动越明显.我们的结果表明,使用强场近似是一种研究负离子在强激光场中电离过程的有效方法.     

Low energy ion reflection from metal single crystals

Energy and angular distributions of ions reflected from metal single crystals have been computed. The neutralisation process is involved in the computing model. The result indicates a strong dependence on the surface trajectories of the reflected particles.     

Spatial and angular distribution of the heavy ion charge under channeling in crystals

A kinetic theory of passage of multiply charged heavy ions through crystals is developed that allows for diffusion in the transverse momentum space and ion-crystal charge exchange. The theory provides an adequate explanation for the observed angular distributions of heavy ions passing through oriented crystals, makes it possible to calculate the partial angular distributions of different charge states, and treats the discovered effects of “cooling” and “heating” of channeled ion beams in physical terms. The angular and spatial distribution of channeled ions with different energies is calculated. Whether a channeled beam of multiply charged heavy ions will be cooled or heated is related to the dependence of the electron capture and loss probabilities on the impact parameter when the ions interact with atomic chains. This interaction governs the run of the angular and spatial distribution of the channeled ion charge.     

Dissociation dynamics of H+2 in intense laser fields: investigation of photofragments from single vibrational levels

We present a photodissociation experiment on H+2 with ultrashort laser pulses ( >/=130 fs) at peak intensities of 相似文献   

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.     

Temperature dependence of angular characteristics of boron nitride sputtering

The angular dependence of the sputtering yield and the spatial distribution of particles ejecting from a boron nitride polycrystal with a wurtzite structure in the temperature range from 0 to 2800°C under bombardment with 300 eV xenon ions are calculated by the molecular dynamics method. A reduction of steepness of the curves of angular dependence of boron nitride sputtering with increasing temperature is revealed. Features of the obtained distributions are analyzed on the basis of mechanisms of interaction of slow heavy ions with surface target atoms.     

Low-energy ion scattering by monatomic films

The angular and energy distributions of Cs⁺ and Xe⁺ ions scattered by monatomic crystalline films have been calculated via the molecular dynamics method in the scope of the multiparticle interaction mechanism. The calculated dependences of scattered heavy ions with a low initial energy E ₀ = 40 eV on the atomic mass, crystal lattice type, interatomic distance, and binding energy of the film atoms are discussed and compared with the experiment. The presented data can be used to predict the physical properties of a surface covered with a monatomic layer of foreign atoms during experimental surface studies based on the backscattering of ions with low initial energies.     

Angular distributions of fragments originating from the spontaneous fission of oriented nuclei and problem of the conservation of the spin projection onto the symmetry axis of a fissile nucleus

The concept of transition fission states, which was successfully used to describe the angular distributions of fragments for the spontaneous and low-energy induced fission of axisymmetric nuclei, proves to be correct if the spin projection onto the symmetry axis of a fissile nucleus is an integral of the motion for the external region from the descent of the fissile nucleus from the external fission barrier to the scission point. Upon heating a fissile nucleus in this region to temperatures of T ≈ 1 MeV (this is predicted by many theoretical models of the fission process), the Coriolis interaction uniformly mixes the possible projections of the fissile-nucleus spin for the case of low spin values, this leading to the loss of memory about transition fission states in the asymptotic region where the angular distributions of fragments are formed. Within quantum-mechanical fission theory, which takes into account deviations from A. Bohr’s formula, the angular distributions of fragments are calculated for spontaneously fissile nuclei aligned by an external magnetic field at ultralow temperatures, and it is shown that an analysis of experimental angular distributions of fragments would make it possible to solve the problem of spin-projection conservation for fissile nuclei in the external region.