Crystallographically-dependent ripple formation on Sn surface irradiated with focused ion beam

The metallographically polished polycrystalline Sn surface was sputtered by 30 kV focused Ga⁺ ions at room temperature. The experiment was carried out using various FIB incidence angles (0°, 15°, 30°, and 45°) over a wide range of doses (10¹⁶–10¹⁸ ions/cm²). The surface morphology was carefully characterized under the optical microscope, scanning electron microscope (SEM) and atomic force microscope (AFM). Ripples were observed on the irradiated areas even at the normal FIB incidence angle, which is not consistent with the Bradley–Harper (BH) rippling model. The orientation of ripples relies on crystallographic orientation rather than projected ion beam direction as predicted by BH model. The ripple wavelength is independent of ion dose, while ripple amplitude increases with ion dose. It is found that the ripples are formed by self-organization due to anisotropic surface diffusion in the low melting point metal.     

Structure determination of the NiSi2(111) surface with medium-energy ion backscattering from individual monolayers

The surface structure of the epitaxial NiSi₂/Si(111) system has been determined applying new ion scattering methods. By detection of backscattered ions with ultrahigh energy resolution the signals from successive atomic layers are separated. Angular distributions of the yield of ions mainly backscattered from a single Ni monolayer directly provide the (sub)surface atom coordinates. In addition, analysis of the energy losses in the first atomic layer, which depend on the specific ion trajectories, allows an independent structure determination. Using either of the two methods, the NiSi₂(111) surface is found to have a bulklike topology, i.e. it is terminated by a Si---Ni---Si triple layer. Other surface structure models, such as termination by a Si double layer, are ruled out. The outermost Ni---Ni and Ni---Si interlayer spacings are found to be contracted with respect to their bulk values in the strained silicide by 0.05 ± 0.02 and 0.12 ± 0.02 Å, respectively.     

LEED structure determination of the Ni(1 1 1)(√3×√3)R30°-Sn surface

Quantitative low energy electron diffraction has been used to determine the structure of the Ni(1 1 1)(√3×√3)R30°-Sn surface phase. The results confirm that the surface layer comprises a substitutional alloy of composition Ni₂Sn as previously found by low energy ion scattering (LEIS), and also shows that there is no stacking fault at the substrate/alloy interface as has been found in (√3×√3)R30°-Sb surface alloys on Ag and Cu(1 1 1). The surface alloy layer is rumpled with the Sn atoms 0.45 ± 0.03 Å higher above the substrate than the surrounding Ni atoms. This rumpling amplitude is almost identical to that previously reported on the basis of the LEIS study. Comparison with similar results for Sn-induced surface alloy phases on Ni(1 0 0) and Ni(1 1 0) shows a clear trend to reduced rumpling with reduced surface atomic layer density, an effect which can be rationalised in terms of the different effects of valence electron charge smoothing at the surface.     

Transient behavior of the thermal ion emission from KCl(0 0 1) upon polarity reversal of the electric extraction field

The technique of polarity reversal of the external electric extraction field (strength: 10² V/cm) was applied to study the relaxation of the thermal ion emission from the KCl(0 0 1) single crystal surface. Transient currents of the K⁺ and K₂Cl⁺ ions upon switching from the emission suppression to the ion extraction mode were recorded as a function of the evaporation time, the temperature, and the time of field reversal. The temperature dependence of the time constants of the K⁺ ions obtained from the exponential decreases of the emission currents to their steady-state emission resulted as logτ_h(s)=−(13.39±0.56)+(12.42±0.49)10³/T in a high temperature interval of 826–930 K after a prolonged heating period and as logτ_l(s)=−(20.65±1.04)+(16.77±0.81)10³/T in a low temperature interval of 750–801 K at the initial stage of evaporation, with corresponding activation energies of E_h(K⁺)=2.47±0.14 eV and E_l(K⁺)=3.32±0.16 eV, respectively. The transient currents can be interpreted by a partial adsorption of the suppressed ion currents at the kinks of the surface steps. The differences in the high- and low-temperature runs may be attributed to a strong coarsening of the surface at higher temperatures, which occurs as a bunching of monosteps to macrosteps and/or to an enrichment and segregation of divalent impurities at the surface. The transient behavior of the molecular K₂Cl⁺ ions seems to be strongly correlated with that of the K⁺ ions. This correlation is possibly caused by changes of the strength or the sign of the local electrical field connected with the excess charge at the kinks.     

Quantum-resolved angular distributions of neutral products in electron-stimulated processes: desorption of CO from Pt(111)

The first quantum-resolved angular distribution measurements of electronically-excited neutral molecules which have undergone electron stimulated desorption (ESD) are presented. Two-dimensional imaging of laser resonance-enhanced multiphoton ionization (REMPI) is used to obtain angular distributions of CO^* in the v=0 vibrational level of the metastable a³Π_r state desorbed from CO/Pt(111) by 350 eV electrons. For saturation CO coverages at 90 K, sharp Gaussian distributions peaked about the surface normal (6° ± 0.5° half-width at half maximum) are observed, consistent with previously reported data acquired by ESDIAD (ESD ion angular distributions). The (1 + 1) photon REMPI scheme for state-specific CO^* detection involves the b³Σ(v = 0) ← a³Π(v = 0) transition at 283 nm, followed by photoionization at the same wavelength. In this paper, the overall experimental technique for REMPI imaging of products from electron stimulated processes is discussed. Thus specific CO^* data as a function of coverage and temperature is presented for comparison with the ESDIAD results.     

Quantum-resolved angular distributions of neutral products in electron-stimulated processes: NO desorption from and NO2 dissociation on Pt(111)

We present the first quantum-resolved angular distributions of ground-state neutral molecules which are products of electron stimulated desorption (ESD) and electron stimulated dissociation. Laser resonance-enhanced multiphoton ionization (REMPI) and two-dimensional imaging have been used to obtain angular distributions of NO desorbed by 350 eV electrons from O-precovered Pt(111). In a similar fashion, we have measured angular distributions for the NO product of NO₂ dissociation on clean and O-precovered Pt(111). In all cases, we observe narrow widths which are roughly the same as ion distributions determined by ESDIAD (ESD ion angular distributions). The angular distribution for NO ESD is sharply peaked (7° half-width at half maximum) along the surface normal for an O coverage (θ_o) of 0.25 monolayer (ML). The angular distribution of the NO product from dissociation of side-bonded NO₂ on clean Pt(111) is unexpectedly peaked about the surface normal, and thus does not reflect dissociative forces parallel to the surface or the 25° off-normal ground-state bond direction. On O-precovered Pt(111), where NO₂ is N-bonded, 6° off-normal beams are observed. When the substrate is precovered with θ_o > 0.5 ML, local disorder creates asymmetric site geometries which result in multiple peaked angular distributions with both normal and off-normal (9–10°) components; similar effects for NO ESD are observed. In all these studies, the NO angular distributions are invariant to rotational or vibrational state. This implies that the lateral translational degrees of freedom are essentially de-coupled from the internal modes of the molecule. The results are discussed in terms of desorption mechanisms, dissociative forces, site geometries, and disordered coadsorbate layers.     

Strain and defect densities in Si/Si1-xGex heterostructures investigated by ion scattering and X-ray diffraction

MBE-grown Si/Si_1-xGe_x heterostructures on (100) Si have been characterized by Rutherford backscattering spectroscopy (RBS), ion channeling and X-ray diffraction to investigate defect densities and tetragonal lattice distortion. Critical layer thickness and relaxation of strain by formation of misfit dislocations are strongly dependent on the growth temperature. A Si_0.67Ge_0.33 layer with a thickness of 2000 Å is found to be still fully strained at a growth temperature of 450°C, whereas the same layer grown at 550°C shows considerable strain relaxation by dislocations. To obtain better depth resolution than with conventional RBS, medium energy ion scattering (MEIS) experiments have been performed on Si/Ge superlattices with layer thicknesses of 10–40 Å. A position-sensitive toroidal electrostatic analyser was employed to detect the backscattered ions simultaneously over an angular range of 30° with an energy resolution of 1 keV FWHM for 250 keV He ions, corresponding to a depth resolution of about 10 Å.     

Structure investigation of the nitrogen induced Cu(110)−(2 × 3) phase with 180° low energy impact collision ion scattering spectroscopy

The 180° low energy impact collision ion scattering spectroscopy with detection of noble gas neutrals (180°-NICISS) has been used to investigate the nitrogen saturated Cu(110) surface, which is known to exhibit a (2 × 3) diffraction pattern. The nitrogen induced (2 × 3) phase appears to be the result of a surface reconstruction of a new missing row type, in which every third 100 row of Cu atoms of the first layer is missing. The 180° NICISS patterns further indicate within an accuracy of 0.1–0.2 Å, that the double periodicity in the [1 0] direction is not due to the reconstruction of the Cu surface. Its origin has to be found in the arrangement of the N atoms.     

Relative sputtering yields from close-packed directions in nickel under low-energy Ar bombardment

A new approach to low-energy sputtering was developed wherein a monoenergetic Ar⁺ ion beam of the order of 10⁻⁹ A impinged onto a crystalline nickel target. Target preparation consisted of plating about 100 monolayers of high specific activity Ni-63 onto a coldrolled nickel substrate and then heating the target above its recrystallization temperature under ultra-high vacuum. The result was a highly ordered polycrystalline structure which, when sputtered, behaved like the (100) surface of a nickel single crystal. Approximately 25 percent of the surface atoms were Ni-63. Sputtered material was collected on a molybdenum foil which was subsequently analyzed by radiotracer techniques.

Experimental results concerning sputtering from [110] and [100] close-packed directions in nickel under bombardment by Ar⁺ ions of energy 25 eV to 600 eV are reported. The relative sputtering yields from [110] directions are presented as a function of incident ion energy for 75° and 15° ion incidence measured with respect to the [110] directions, and the extrapolated thresholds are compared with theoretical predictions.

In addition to the expected deposits on the foil due to sputtering from individual closepacked directions, secondary deposits occurred which are attributed to specular reflection of sputtered nickel atoms from the molybdenum foil.     

The elastic scattering of protons by deuterium

Proton-deuteron elastic scattering was investigated with a gas target. Excitation curves were taken at 45.28°, 90.26°, and 118.92° c.m. for lab proton energies from 4.5 to 11.5 MeV. Angular distribution were taken at 4.45, 5.45, 6.50, 7.46, 8.47, 9.47, 10.47 and 11.47 MeV from 30° to 140° c.m. Differential cross sections were measured with an absolute error of less than ±3%. The scattering data were compared with predictions derived from the scattering amplitudes of Aaron, Amado and Yam ¹⁰). No evidence for the existence of ³He excited states was found in this energy range.     

The S-wave pion–nucleon scattering lengths from pionic atoms using effective field theory

The pion–deuteron scattering length is computed to next-to-next-to-leading order in baryon chiral perturbation theory. A modified power-counting is then formulated which properly accounts for infrared enhancements engendered by the large size of the deuteron, as compared to the pion Compton wavelength. We use the precise experimental value of the real part of the pion–deuteron scattering length determined from the decay of pionic deuterium, together with constraints on pion–nucleon scattering lengths from the decay of pionic hydrogen, to extract the isovector and isoscalar S-wave pion–nucleon scattering lengths, a⁻ and a⁺, respectively. We find a⁻=[0.0936(±0.0011)]M_π⁻¹ and a⁺=[−0.0029(±0.0009)]M_π⁻¹.     

Isothermal desorption of hydrogen from polycrystalline diamond films

The kinetics of isothermal H₂ desorption from polycrystalline diamond are studied in real time. The surface H coverage (θ_H) is measured by mass analyzing the recoiled H₊ ion signal during the desorption. We find that the H₂ desorption is 1st order in θ_H with an activation energy of 69 ± 6 kcal/mol and a prefactor of 10^{10.5 ± 0.9} s⁻¹. We suggest that formation of a C---C π-bond on the clean surface plays a key role in H₂ desorption from diamond, a view consistent with previous theoretical calculations of H₂ desorption from diamond.     

Surface structure and electron density dependence of scattered Ne ion fractions from the Si(1 0 0)-(2×1) surface

The magnitudes and azimuthal anisotropies of 4 keV Ne⁺ scattered ion fractions from the Si(1 0 0)-(2×1) two-domain surface have been measured by means of time-of-flight scattering and recoiling spectrometry. The absolute values of these ion fractions as well as their dependence on surface structure and electron density have been determined. By investigating the trajectories of the scattered Ne⁺, a clear correlation is demonstrated between these experimentally observed surviving ion fractions of Ne⁺ and the fraction of ions that scatters from the topmost layer of the surface. This is interpreted in terms of a model in which the neutralization probability of Ne⁺ is proportional to the local substrate electronic charge density.     

TL and PTTL in natural fluorite previously irradiated with gamma rays and heavy ions

The effect of annealing temperature on the phototransfer thermoluminescence (PTTL) signal was studied to determine the appropriate annealing temperature for treating the natural powder before irradiation. The temperatures used to anneal virgin natural fluorite samples (only natural dose without giving the samples any artificial doses) were 150, 250, 350, 450, 550, 650 and 750°C for a duration of 1 h in each case. The results show that the PTTL response did not change for anneal temperatures up to 450°C, but at higher temperatures the signal decreased rapidly. The height of the 90°C peak decreased by two orders of magnitude as the anneal temperature increased from 450 to 750°C, whilst the height of the 180°C peak decreased by three orders of magnitude between the same two annealing temperatures. In order to investigate the effect of previous gamma rays and heavy ion irradiation on thermoluminescence (TL) and PTTL signals, powdered samples of natural fluorite from Cornwall, England, were annealed at 500°C and then irradiated (at GSI, Darmstadt, Germany) with ¹⁶¹Dy ions of energy 13 Mev/n; the range of fluences used was from 10⁴ to 10¹² ion cm⁻². Identical samples were given gamma doses in the range 1 Gy to 2.6 × 10⁴ Gy in order to compare the effects of gamma rays and heavy ions. The sensitivities of TL and PTTL were studied by giving the samples a gamma test dose of 1 Gy after annealing the samples at 500°C for 30 min in order to eliminate the TL resulting from previous gamma or heavy ion irradiation.     

Single crystal RuO2/Ti and RuO2/TiO2 interface: LEED, Auger and XPS study

The interactions at the evolving RuO₂/titanium interface have been studied by LEED, AES and XPS. Titanium films of up to 5 monolayers were evaporated onto well ordered and ion sputtered ruthenium dioxide crystal surfaces of (110) and (100) orientation. Stabilization of the surface oxygen content under thermal treatment in UHV (up to 600°C) with increasing titanium coverage was established. After extended (up to 4 h) annealing in O₂ at 600°C an epitaxial ordering of TiO₂ on RuO₂(110) was observed. The (1 × 1) LEED patterns from the epitaxial layer exhibit a reduced background level when compared to the RuO₂ substrate itself. These findings are correlated with the XPS data and are interpreted in connection with the disappearance of the defect RuO₂ phase in the surface layer of the RuO₂. The appearance of the (1 × 2) surface reconstruction at the RuO₂(100)/Ti interface is discussed in the context of maximum cation coordination by oxygen atoms.     

Elastic scattering of B on B in the energy range from 4 to 15 MeV

Angular distributions (at E_c.m. = 4, 6.5 and 10.5 MeV) and excitation functions (at 30°, 60° and 90°) are given for the elastic scattering of ¹⁰B on ¹⁰B. The results of an optical model (OM) analysis are presented and discussed. In the energy range covered the Sommerfeld parameter η varies from 4.4 to 2.3.     

Inelastic electron scattering from He

The spectra of electrons scattered inelastically from ⁴He have been measured at incident energies from 150 MeV to 400 MeV for scattering angles from 38° to 90°. Through the use of a liquid ⁴He target, a high momentum resolution (≈0.25%) was obtained, and the 20 MeV 0⁺ state of the -particle was observed for the first time in electron scattering. The excitation energy and the total width of this state were determined and are in good agreement with the results from other experimental methods. It was found that the total disintegration cross section appears be smaller than 2(σ_p+σ_n) by more than an order of magnitude at the lowest q²(≈ 0.33 fm⁻²). In earlier work the assumption of a total cross section of 2(σ_p+σ_n) was found to be quite accurate for higher q². As a new result, the partial radiative width for the 0⁺ state is determined to be Γ_rad = 1.1±0.3 meV.     

Interaction of 11 MeV neutrons with Y

Differential cross section for scattering of 11 MeV neutrons by ⁸⁹Y were measured using the Ohio University beam swinger time-of-flight facility. Measurements were taken in the angular range between 15° and 145°. Empirical optical model parameters have been obtained from the measured elastic scattering data. Deformation parameters were obtained for low-lying excited states using these optical-model parameters in a DWBA collective formalism. A comparison with deformation parameters in neighboring even-even nuclei ⁸⁸Sr and ⁹⁰Zr is presented. The weak-coupling model is used to describe neutron scattering to low-lying states in ⁸⁹Y.     

Compton scattering by the proton through θcms = 75° and 90° in the Δ-resonance region

Differential cross sections for Compton scattering by the proton have been measured in the energy interval between 200 and 500 MeV at scattering angles of θcms = 75° and θcms = 90° using the CATS, the CATS/TRAJAN, and the COPP setups with the Glasgow Tagger at MAMI (Mainz). The data are compared with predictions from dispersion theory using photo-meson amplitudes from the recent VPI solution SM95. The experiment and the theoretical procedure are described in detail. It is found that the experiment and predictions are in agreement as far as the energy dependence of the differential cross sections in the Δ-range is concerned. However, there is evidence that a scaling down of the resonance part of the M1+3/2 photo-meson amplitude by (2.8 ± 0.9)% is required in comparison with the VPI analysis. The deduced value of the M1+3/2-photoproduction amplitude at the resonance energy of 320 MeV is: |M1+3/2| = (39.6 ± 0.4) × 10−3 mπ+−1.     

An investigation of the Si(111)−(√3 × √3)R30°−Ag surface by Li impact collision ion scattering spectroscopy

The Si(111)−(√3 × √3)R30°−Ag surface has been investigated using the technique of Li⁺ impact collision ion scattering spectroscopy. Typical LEED √3 domain sizes were estimated to be on the order of 150 Å for a 1 ML coverage of Ag, with the √3 structure persisting for coverages of Ag up to 35 ML. Silver islanding was found to influence the appearance of the 5 keV Li⁺ ICISS angular scans even for 1 ML coverages of Ag deposited at 480°C. A detailed structural analysis of the Si(111)−√3−Ag surface (0.25 ML deposition) involved the comparison of 5 keV Li⁺ ICISS experimental data along the [11 ], [ 10] and [2 1] azimuths with computer simulations of the scattered ion intensities based on previously proposed models for the √3 surface. Nine structurally different models were tested, and only the missing-top-layer (MTL) and the honeycomb-chained-trimer (HCT) models were found to be consistent with all the experimental results. An estimate of 0.4 Å for the maximum downward vertical displacement of the Ag atoms with resect to the surface Si atoms in the MTL model is made. The effects of increased thermal vibrational amplitude in the simulation of Si---Ag shadowing effects is also discussed. The interpretations of previous noble gas ICISS results are shown to be inconsistent with the present alkali metal ICISS study of the √3 surface.