P.F.A. Alkemade

Revisiting the 12C(d, p)13C reaction cross section using condensed gas targets

Abstract We have measured the differential cross section for the 12C(d, p)13C nuclear reaction at a laboratory scattering angle of 150° for incident deuteron energies near 972 keV. The measurements have been calibrated by comparison with the well known cross section for the 16O(d, P1)17O reaction at the same energy (972 keV) and angle, corresponding to the peak of a broad resonance in the latter reaction cross section. The technique utilized condensed targets of gases containing both carbon and oxygen atoms, viz. CO, CO2, and CH3OH. The three values thus obtained for the 12C(d, p)13C laboratory cross section at 972 keV are in excellent agreement (± 2%). A mean value of 27.8 ± 1.1 mb sr−1 is recommended. We conclude that, for CO adsorbed on the Pt(100) and Pt(111) surfaces at 185 K, the reported saturation coverages were stoichiometric.

The determination of absolute oxygen coverage by nuclear reaction analysis

Abstract The saturation of a Ni(100) surface with oxygen is demonstrated by NRA for both 16 O and 18 O. A value of 2.5 monolayers is found. The same saturation value is obtained for a surface first converted to the chemisorbed c(2 × 2) phase in 18 O, then taken to saturation in 16 O. This simple demonstration suggests that surface reaction mechanisms may be studied through isotopic labelling in conjuction with NRA methods.

Absolute surface carbon coverage determination via the 12C(3He, p)14N reaction

Abstract Absolute cross sections for the 12C(3He, p)14N nuclear reaction have been measured at E( 3 He ) ≈ 2.4 MeV with a precision of 4–5% for angles in the range 86° ⩽ θlab ⩽ 151°. The total cross section summed over po, p1 and p2 varies from 5 to 7 mb/sr in the angular range studied. Due to its high Q-value, this reaction provides a useful alternative to the familiar 12C(d, p)13C reaction for measuring submonolayer surface carbon coverage. This approach is of particular advantage for low-atomic-number substrates (e.g. Al, Si) which otherwise give rise to interfering reaction products.

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

Abstract The surface structure of the epitaxial NiSi 2 /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 NiSi 2 (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 A, respectively.

Deuterium depth distribution investigations in Zr and ZrO2

Abstract We have used the broad resonance at E( 3 He ) ≈ 0.63 MeV in the D + 3 He → p + 4 He nuclear reaction to probe the depth distribution of deuterium implanted at 50 keV energy into ZrO 2 and at 40 keV into ZrNb 2.5% specimens. Nuclear reaction data of two kinds, viz. the excitation functions and the proton energy spectra, have been used to extract the deuterium profiles. These data have been compared with secondary ion mass spectrometry (SIMS) measurements. The nuclear reaction analyses (NRA) reveal the basic features of the distributions - preservation of the primary implant features in the ZrO 2 specimen and redistribution including in-diffusion in the metal specimens. A summary is given of the procedures used to extract D profiles from the nuclear reaction measurements.

On the ambiguity in the analysis of Rutherford backscattering spectra

The ambiguity in the analysis of Rutherford backscattering spectra is investigated. A method is developed by which spectra of solids containing elements with almost equal atomic mass can be analysed. It is theoretically shown that, in cases where two elements in a solid have almost equal mass, a single RBS spectrum is sufficient to determine the composition of the solid unambiguously. In cases where N elements have almost equal mass, N — 1 spectra, measured under different experimental conditions, are, in principle, sufficient. The described method is applied to a spectrum of a sample consisting of a CuNi surface alloy on a copper substrate.

A new Monte Carlo model for the calculation of MEIS energy spectra

Abstract A Monte Carlo model for the simulation of the interaction of medium energy ions with crystalline solids is presented. In the simulations all energy loss events are taken into account to obtain simulated ion backscattering energy spectra. The inelastic energy loss in the ion-atom interactions is calculated using either one of three distinct models: (1) a continuous or impact parameter independent model; (2) the Oen and Robinson model; and (3) the dielectric response model of Lindhard. Calculated and measured spectra of medium energy He + -ions scattered from Cu(100) are compared. It is concluded that both the continuous model and the dielectric response model underestimate the impact parameter dependence of the inelastic energy loss. A good fit between simulated and experimental results is obtained for the Oen and Robinson model. The Monte Carlo simulations are used to analyze high energy resolution spectra of 100 keV H + backscattered from NiSi 2 /Si(111). It is shown that the relaxation of the outermost atomic layer can be determined by comparing measured with simulated energy losses.

Behaviour of the 12C(4He, 4He)12C elastic scattering cross section at Eα = 0.4–1.8 MeV

Abstract The energy dependence of the 12C(4He, 4He)12C elastic scattering cross section has been studied between 0.4 and 1.8 MeV incident energy at a laboratory angle of 150°. It is found that, within a precision of 2%, the measured energy dependence is of the screened Rutherford form. Assuming the 12C(4He, 4He)12C elastic scattering cross section is screened Rutherford at low energies, Eα ≈ 0.5 MeV, we conclude that it is screened Rutherford up to at least 1.8 MeV. This conclusion is inconsistent with an absolute cross section value derived from a comparison with the 12C(d, p)13C nuclear reaction yield.

Ion-induced Auger electron studies on Al(110)

Abstract The yield and energy spectra of KLL Auger electrons have been measured from a clean Al(110) crystal under impact of a 1.5 MeV He + beam in several random and channeling directions. Under channeling conditions we observe a reduction in the intensity of the electron continuum and of the KLL Auger signal. From an analysis of the spectral intensity yields we conclude that the transport elastic mean free path λ d for 1.4 keV electrons in Al is of the order of 70 A.

Ion-Induced Auger Electron Emission From Single Crystals: Experiment and Simulation

The energy spectra of the Si KLL and KLM Auger electrons produced by bombardment of a Si (100) single crystal with a 1 MeV He+ beam, are measured. The incident beam is aligned with a major crystallographic direction or with a random direction. The shapes of the measured Auger spectra are successfully explained using an efficient Monte Carlo simulation code for the electron transport. Different models for the energy loss distribution are compared with each other. In the comparison with the experiment, the local density approximation of the dielectric response model for the inelastic scattering cross section and the quantum mechanical phase-shift model for the elastic scattering cross section are used. The experiments and the Monte Carlo simulations show an enhancement of the spectral intensity within 150 eV of the initial Auger electron energy. The enhancement is caused by a reduction of the effective stopping power for the majority of the electrons created in the surface region.