Inelastic scattering of Ne atoms from a LiF(001) surface

Scattering experiments with a ²⁰Ne nozzle beam from a LiF(001) surface in the 〈100〉 azimuth are reported. The (11) and (1̄1̄) Bragg reflections show broad tails due to inelastic scattering. These tails can be attributed by time-of-flight measurements to single phonon scattering on acoustic modes. The inelastic contribution decreases rapidly with increasing energy of the phonons involved.     

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.     

A semiclassical treatment of atom-surface scattering: He + LiF(001)

The study of the diffraction of atoms by periodic surfaces is considered within a semiclassical framework. The semiclassical theory for atom-surface scattering is briefly reviewed, and the results are applied to HeLiF(001) scattering. Several model potentials for the atom-surface interaction are discussed, and some of the effects of these potentials on the classical trajectories are noted. The model potential which best describes the experimental results is found to approach a corrugated hard wall; the long range attractive part of the potential is found to be of secondary importance for incident angles θ_i < 50°.     

Selective adsorption of 4He on clean LiF (001) surfaces

The selective adsorption of ⁴He on in-situ cleaved LiF surfaces has been studied under improved resolution. The main results are as follows: (1) There are four bound states in the surface potential well, at energies of ?5.8, ?2.2, ?0.6 and ?0.1 meV. The lowest three levels were reported previously. (2) Most of the structure previously designated as “fine structure” is due either to transitions to these four levels via various small reciprocal lattice vectors or to the opening of diffraction channels. (3) The transitions involving the [01] and [01?] reciprocal lattice vectors (i.e., the ones nearly perpendicular to the incident wave vector) are strong; as much as 85% of the specular intensity may be removed. Transitions via the other small reciprocal lattice vectors are much weaker. (4) The widths of the lines are consistent with the velocity distribution, which has a half-width of about 2%. (5) The observed energies agree fairly well with those calculated by Tsuchida for a zeta-function potential, but are not consistent with a Morse potential.     

Electronic excitations during grazing scattering of hydrogen atoms on KI(001) and LiF(001) surfaces

The energy loss of hydrogen atoms with energies of 400 eV and 1 keV is studied in coincidence with the number of emitted electrons during grazing scattering from atomically clean and flat KI(001) and LiF(001) surfaces. The energy loss spectra for specific numbers of emitted electrons are analyzed in terms of a binary interaction model based on the formation of transient negative ions via local capture of valence band electrons from anion sites. Based on computer simulations we derive for this interaction scenario probabilities for the production of surface excitons, for electron loss to the conduction band of KI, for emission of electrons, and for formation of negative hydrogen ions. The pronounced differences of data obtained for the two surfaces are attributed to the different electronic structures of KI and LiF.     

External-field-induced formation of stabilized F 2 + centers in colored LiF crystals

The production of complexes, which are thermally stable at room temperature, with F ₂ ⁺ centers in radiation-colored LiF crystals by the combined action of different fields is investigated. The half-life of these laser centers produced by, specifically, hard UV radiation and a shock wave increased by almost two orders of magnitude. Fiz. Tverd. Tela (St. Petersburg) 40, 2044–2050 (November 1998)     

Heteroepitaxial growth of KCl and LiF on a cleaved (001) face of KBr

An initial stage of heteroepitaxial growth of KCl and LiF on a KBr substrate was studied using RHEED, EELS and AES. Both KCl and LiF films grow heteroepitaxially on a KBr substrate with their crystallographic axes parallel to that of substrate. In the case of KCl growth occurs in a well ordered layer-by-layer fashion and it is found that thickness of an intermediate region between KCl and KBr is less than 2 nm. In contrast three-dimensional island growth occurs in the case of LiF. Differences in lattice mismatch and solid solubility seem to determine the mechanism of heteroepitaxy of KCl and LiF on a KBr substrate.     

Inelastic scattering of helium from bulk longitudinal acoustic phonons in LiF(001)

The analysis of some recent time-of-flight spectra reported by Doak for He inelastic scattering from LiF(001) along the 〈110〉 azimuth reveals important contributions from bulk longitudinal acoustic (LA) phonons. The large intensity due to LA phonons compared to that due to Rayleigh waves is shown to be the result of resonance enhancement. The independent determination of the coupling to LA phonons and to Rayleigh waves permits an estimate of the atom-surface, potential softness.     

Evidence for F(-) formation by simultaneous double-electron capture during scattering of F(+) from a LiF(001) surface

Slow F(+) ions (v<0.1 a.u.) scattered from a clean and flat LiF(001) surface under a grazing angle of incidence exhibit a high probability for forming F(-) ions in the reflected beam, whereas no negative ions are found for neutral F(0) projectiles. From detailed studies of projectile energy loss and charge transfer, we find evidence for a correlated double-electron capture process in the formation of the F(-) ions.     

Subshell Resolved Measurements of Single Electron Capture in Slow Ne7+–Helium Collisions

Hyperfine Interactions - We have performed drying-humectation cyclical processes (CEBELCOR) on eight A36 low carbon steel coupons in NaCl solutions containing 1×10?2 M and...     

Surface Corrugation in Rotational and Diffractive Scattering of O2 from LiF (001)

A quantum dynamic calculation on a five-dimensional O₂/LiF (001) model system is performed using the multi-configuration time-dependent Hartree method. The obtained results show that the mechanism of rotational and diffractive excitation in details: Comparison with the rotational excited state, the initially non-rotational state is seen to favor the inelastic scattering in the rotational excitation process. The surface corrugation can damp the quantum interferences and produce a greater amount of rotational inelastic scattering at the expense of the elastic process in the rotational excitation process. The diffraction process and the average energy transferred into the rotational and diffractive mode are also discussed.     

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.     

Ne原子(e,2e)反应中反冲离子动量分析

利用冷靶反冲离子动量谱仪,对电子轰击Ne原子的单电离反应(e,2e)进行了研究,实验测量了70—3300eV入射能量情况下,反应过程中产生的一价反冲离子的动量分布,并对反冲离子的总动量进行了还原。介绍了一个简单的碰撞机制,据此着重分析了反冲离子纵向动量和横向动量二维谱形成的原因,该碰撞机制能够较好地解释较高能量入射时的实验结果。最后根据反冲离子的动量,估算了出射电子的能量范围,为下一步进行电子、离子的符合测量奠定了基础。     

Role of long range interaction in oxygen superstructure formation on Cu(001) and Ni(001)

On the basis of electronic structure calculations, we show that the long range Coulomb interaction provides the driving mechanism for oxygen overlayer formation on Cu(001). We illustrate that this interaction in the precursor c(2 x 2) phase induces a missing row reconstruction of Cu(001), and leads to the (2sqrt[2] x sqrt[2])R45 degrees O structure, which has strong covalent pO-dCu coupling. For the c(2 x 2)O overlayer on Ni(001) and Cu(001), we show that pO-dNi bonding is larger than pO-dCu and serves to neutralize the perturbation of the Coulomb interaction induced by the O overlayer. Consequently, c(2 x 2)O/Ni(001)) is stable while c(2 x 2)O/Cu(001) exists only in limited environments.     

Interface formation of iron on In-terminated InAs(001)

In situ scanning tunneling microscopy (STM) is used to investigate the nucleation and initial growth of iron on InAs(001) (4×2)/c(8×2) at room temperature and at 200 °C. Sequences of STM frames show the development at a specific location in the different stages of interface formation. At room temperature, initially 0.5 monolayer of iron grows in the form of small anisotropic clusters, which form long chains along [110] induced by the InAs substrate reconstruction. Further growth proceeds with the formation of isotropic 3d islands, resulting in a granular overlayer at higher coverages. Coalescence of the individual grains is strongly suppressed. At 200 °C the anisotropic chain structure is absent, but rectangular islands grow from the beginning. A decomposition of the substrate indicates intermixing. Room-temperature deposition followed by annealing at 200 °C leads to closed iron layers that are interspersed with pinholes, allowing diffusion of substrate material to the surface . PACS 68.37.Ef; 68.35.Ct; 68.55.Ac     

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.