Scattering and recoiling mapping of the Kr–Pt(1 1 1) system by SARIS

The technique of angle resolved mapping of scattering and recoiling imaging spectra (SARIS) combined with computer simulations is demonstrated to be a valuable tool for characterization of atomic collision events on surfaces. The energy distributions of scattered Kr and fast recoiled Pt atoms from a Pt(1 1 1) surface were measured as a function of exit angle. The use of a large area microchannel plate detector and time-of-flight techniques decreases the collection time and increases the number of detected trajectories above that of other designs. Classical ion trajectory simulations using the three-dimensional scattering and recoiling imaging code are used to simulate the kinematics of the scattering and recoiling particles. It is shown that SARIS mapping allows one to probe the kinematics of both scattered and recoiled particles, the probability for their occurrence in specific trajectories, their detection probabilities, and their threshold detection velocity. The measured and simulated energy distributions agree quantitatively if the detection efficiency is taken into account. The observed value of the threshold detection velocity for Pt atoms, ν^th=3.78(5)×10⁴ m/s, is in good agreement with previous studies.     

Hyperthermal cluster-surface scattering

Using molecular-dynamics simulation, we study the processes occurring after impact of clusters on a rigid wall. Comparing the impact of model clusters consisting of 13 atoms, or of 13 diatomic molecules with varied bond strength, the systematics in the results of the collision process are investigated. Four regimes of impact-induced cluster fragmentation are identified: intact reflection, shattering into large fragments, complete fragmentation, and molecule dissociation. The effect of the number of degrees of freedom activated in the collision on the translational and internal energies of the reflected fragments is discussed in detail. As a rule, with increasing number of degrees of freedom which can be activated in the collision, the translational energy sinks. On the other hand, for weak intramolecular bonding, intramolecular vibrations are easily excited at small impact energies, reducing the resulting translational energy. The presence of even a very weak attractive well epsilon_w at the surface has a major influence on the sticking behavior of the clusters — and hence also on the absolute reflected energies — even at impact energies E₀ ≫ epsilon_w.     

Diffraction of fast metastable atoms by micrometric reflection gratings

Diffraction of thermal velocity metastable atoms by non-magnetic and magnetic reflection gratings of micrometric period has been observed. This observation is made possible by the use of an ultra narrow beam generated by metastability exchange. Grazing incidence angles are exploited to minimise the quenching of metastable atoms on the grating surface. Potential applications are beam splitting, atom holography and probing of micro-sized solid surfaces.     

Multiple scattering investigation of the 1T-TaS2 surface termination

Multiple scattering theory based on a cluster model is used to simulate full hemispherical X-ray photoelectron diffraction measurements on a 1T-TaS₂(0001) surface. Key points to determine the surface termination are discussed. As the commonly applied single scattering simulations do not give satisfying results, a multiple scattering approach has to be used to accurately simulate the full hemispherical photoelectron diffraction patterns. Differences and similarities between calculations of Ta and S terminated surfaces are presented along with experimental results at room temperature using both, the single and the multiple scattering approaches. We find that the surface is S terminated and that the quantitative difference between the calculations for both terminations permits to show the limits of the single scattering approach for solving surface termination problems. Moreover, by generalizing the results obtained using the multiple scattering approach, we discuss the application of this method to other similar systems.     

Scattering of low energy Na+ ions from a clean polycrystalline Ag surface

The energy distributions of low energy (E₀ = 0.4–3.2 keV) Na⁺ ions scattered from a clean polycrystalline Ag surface were measured. The angle between the incident beam and the surface was fixed at ψ = 45° while the scattering angle (θ) ranged from 50 to 130°. The cleanliness of the surface during the measurement was maintained by simultaneous deposition of Ag atoms from an effusion source. The obtained distributions considerably differ from the corresponding distributions of noble ions. Firstly, for all measured values of E₀ and θ, an intensive hump is observed in the high energy part of the distribution. In certain cases this hump is transformed into a peak. Secondly, the low energy part of the distribution is very pronounced, especially for higher values of E₀ and θ.     

Dissociation of a product of a surface reaction in the gas phase: XeF2 reaction with Si

Xenon difluoride interacts with Si(100)2 x 1 by atom abstraction, whereby a dangling bond abstracts a F atom from XeF2, scattering the complementary XeF. Partitioning of the reaction exothermicity produces sufficient XeF rovibrational excitation for dissociation to occur. The resulting F and Xe atoms are shown to arise from dissociation of XeF in the gas phase by demonstrating that the angle-resolved velocity distributions of F, Xe, and XeF conserve momentum, energy, and mass. This experiment documents the first observation of dissociation of a surface reaction product in the gas phase.     

Energy and charge distribution of helium atoms reflected from a Cu surface under grazing incidence

The energy and charge distributions of helium ions and atoms reflected from a Cu surface at grazing incidence have been measured at the energies 250 and 300 keV. The dependence of the relative number of helium atoms in the scattered beam on the scattering angle at incidence angles less than 2° and scattered particle energies below 150 keV has been discovered. It is shown that, for a theoretical estimation of the charge fractions of particles reflected from the target surface, charge-exchange cross sections used in calculations must be corrected when solids are considered instead of gases.     

Hyperthermal scattering of Cs from Pt(1 1 1): the effect of image charge on the ion-surface energy transfer

We have studied the energy exchange between hyperthermal (5-100 eV) Cs⁺ projectiles and a Pt(1 1 1) surface by measuring the kinetic energy of the scattered ions. The scattering geometry was chosen to be in-plane with specular scattering angles, and the energy of the scattered ions was analyzed as functions of incidence energy and angle. For low incidence energy (<40 eV), the energy transfer to the Pt surface is substantially enhanced due to the attractive image charge force between Cs⁺ and the surface. The image charge effects are highlighted by the different energy transfer on Pt(1 1 1) and Si(1 1 1) surfaces. Analysis of the experimental results using two- and three-dimensional theoretical models revealed a well depth of 1 eV for the image charge potential. Hyperthermal Cs⁺ ions scatter from Pt(1 1 1) predominantly via double collisions with Pt atoms, though the scattering phenomena are insensitive to the impact site at the surface.     

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.     

Simulated reaction dynamics of F atoms on partially fluorinated Si(100) surfaces

We have investigated the influence of translational excitation on the reactivity of atomic fluorine with the Si(100) surface via molecular dynamics simulations using a first-principles-derived interaction potential. Surface reactivity is contrasted for both clean and partially fluorinated surfaces with the results of previous simulations of F₂ molecules impinging on Si(100) surfaces, indicating many similarities between the dynamics of F atoms and F₂ molecules. Mechanisms for the reaction are proposed based on reactivity trends and scattered product energy and angular distributions, including evidence for the existence of a precursor-mediated adsorption pathway for low incident energy F atoms on partially fluorinated surfaces.     

Energy and charge distributions of fast hydrogen ions and atoms reflected from Cu in the case of grazing incidence

The energy and charge distributions of protons and hydrogen atoms reflected from the Cu surface in the case of grazing incidence angles are measured at energies of incident particles (H⁺ and H⁰) of 200 and 250 keV. The charged fractions of reflected particles are analyzed. A weak dependence of the neutral fraction of reflected particles on the scattering angle is discovered for incidence angles of 1°–2° and an energy of scattered particles of 60 keV or less. It is shown that the neutral fraction of reflected particles with an energy of 60–80 keV or more is independent of the scattering angle and is determined by the ratio of the cross sections for the electron capture and loss by ions in the material.     

Study on low-energy bombardment of Au (1 1 1) by noble metal atoms with molecular dynamics simulations

The low-energy bombardment of Au (1 1 1) surface by noble metal atoms is studied with molecular dynamics (MD) simulations. With the incident-energy dependence of adatom yields, sputtering yields, and vacancy yields for different projectiles, we find that the implantation of projectiles in shallow layers below surface can be distinguished by subplantation (in the first and second layers) and implantation (deeper than the third layer). The transition from subplantation to implantation occurs at the incident energy of about 45 eV for the low-energy bombardment of noble metal atoms on Au (1 1 1). The incident-energy dependence of defect yields is obviously different for the subplantation and implantation of projectiles. Based on our MD simulations, we discuss the influence of low-energy bombardment on film growth and the guide to the search for optimum deposition parameters.     

Simulations of electron transfer in grazing incidence ion–surface collisions

During the interaction of a low energy ion beam with a metallic surface in grazing incidence, electron transfer plays an important role in the final state of the scattered beam. In this work we compare two approaches—rate equations resolution and a Monte Carlo numerical code named ETISC1D—to describe the beam evolution and to compute the final populations of the various atomic and ionic species as well as the corresponding angular distributions. Results of both methods are compared to experimental data obtained by Hecht et al. for 2.3 keV He⁺(1s) and He*(1s2s, ³S₁) beams impinging on an Al(1 1 1) surface under 0.79° of incidence. The limitations of the rate equations method and the advantages of Monte Carlo simulations are pointed out. In particular, we show that although the rate equation approach can give a fast and rather accurate evaluation of the populations, it is unable to provide correct calculations of more sensitive quantities like angular distributions of scattered species as obtained with the ETISC1D code.     

The peculiarities of the low-energy ion scattering by polycrystal targets

In the present work, experimental and computer simulation studies of low-energy (E₀ = 80-500 eV) Cs⁺ ions scattering on Ta, W, Re target surfaces and K⁺ ions scattering on Ti, V, Cr target surfaces have been performed for more accurate definition of mechanism of scattering, with a purpose of evaluation of an opportunity of use of slow ions scattering as a tool of surface layers analysis. The choice of the targets was based on the fact that the ratios of atomic masses of target atoms and ions μ = m₂/m₁ were almost the same for all cases considered and greater than 1 (direct mass ratio) however, the difference of binding energies of target atoms in the cases of Cs⁺ and K⁺ scattering was almost twice as much. It has been noticed that the dependencies of the relative energy retained by scattering ions at the maximum of energy distribution versus the initial energy E_m/E₀ (E₀) have a similar shape in all cases. The relative energy retained by scattering ions increases while the initial energy of incidence ions decreases. The curves are placed above each other relative to the binding energies of target atoms, to show what this says about the influence of binding energy on a process of scattering of low-energy ions. The correlation between value of energy change maintained by an ion for different values of E₀ in the case of scattering by targets with different masses of atoms and its binding energies is experimentally established. The contrary behavior of the E_m/E₀ (E₀) dependencies concerning the target atom binding energy quantity E_b for cases with direct (μ > 1) and inverse (μ < 1) mass ratio of colliding particles is established. The comparison of experimental energy distributions with calculated histograms shows that the binary collision approximation cannot elucidate the abnormally great shift in the maxima of relative energy distributions towards greater energy retained by scattering ions.     

Reactive scattering of a supersonic oxygen atom beam O + I2

Reactive scattering of O atoms with I₂ molecules has been studied at an initial translational energy E = 43 kJ mol^-1 using a supersonic beam of O atoms seeded in He and E = 18 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 were obtained which show predominantly rebound scattering at both initial translational energies. Scattering in the forward direction has a product translational energy distribution similar to that predicted for a long-lived collision complex but scattering in the backward direction has a much higher product translational energy. The greater predominance of rebound scattering observed for O + I₂ compared with O + Br₂ may be attributed to the greater exoergicity of the O + I₂ reaction. However, comparison with the O + IBr reaction which exhibits a long-lived collision complex mechanism indicates that the predominance of backward scattering for O + I₂ also arises from diminished forward scattering from larger impact parameter collisions.     

Interaction of low energy (5–10 keV) argon atoms and ions with a Cu(100) surface; Ion fraction,ionization and neutralization

The ion fractions η⁺ of low energy (5–10 keV) argon particles scattered from a Cu(100) surface, are measured with a time of flight spectrometer. Neutral as well as charged projectiles are used. The scattering angle θ is 30°. The results for different angles of incidence ψ and crystal directions are reported. For scattering in the 〈100〉 direction, with a ψ-value of 15° and a primary energy E₀ of 5 and 10 keV, the ion fractions for the quasi single scattering peak, η⁺_QS, are 1.5 and 6.1% respectively. When E₀ is between 5 and 10 keV a reionization process with a constant reionization probability occurs during the violent interaction. This process, but also neutralization along the outgoing trajectory, determines η⁺_QS. With ions as projectiles, an energy difference of about 16 eV is observed between the quasi single scattering peaks in the spectra of all scattered particles and of ions only. The ion fraction for the quasi double scattering peak, η⁺_QD. depends largely upon E₀, indicating that the efficiency of the reionization process increases with E₀. A qualitative discussion of the data is given, using the reionization process and the interatomic neutralization processes along the trajectory of the scattered particles.     

Study of the angular and energy distributions of Na+ and K+ ions which have passed through thin copper films

The angular and energy distributions of alkaline Na⁺ and K⁺ ions which have passed through thin Cu films in different crystal states are studied. The ion energy E₀ is varied from 10 to 40 keV, and the incidence angle. ranges from 0° to 60°. The angular aperture of the detector is ～0.5°, which allows the form of the angular distribution of ions which have passed through the solid thin films as a function of the energy, the angle of primary-ion beam incidence, and the layer thickness to be studied in detail. It is shown that, in the range E₀ = 10.40 keV, the energy loss ΔE of those ions that have passed increases linearly as the energy of incident ions increases. The energy loss increases with increasing ion mass in the case of singly charged ions. The surface amorphization of single- and polycrystalline films leads to an increase (by 150–200 eV) in the energy loss caused by the diffuse propagation of ions and to loss-peak broadening. It is probable that surface amorphization is accompanied by an increase in the number of atoms experiencing multiple collisions with atoms of the film, which leads to an increase in the average energy loss by ions that have passed through films.     

The dynamical origin of non-normal energy scaling and the effect of surface temperature on the trapping of low molecular weight alkanes on Pt(111)

Molecular classical dynamical simulations of alkanes trapping on platinum surfaces were performed to examine the origin of non-normal energy scaling for molecular adsorption. Conversion of normal to parallel translational energy at normal incidence and conversion of parallel translational energy into normal translational energy at glancing angles are the primary mechanisms which produce non-normal energy scaling of alkanes trapping on cold Pt(111). In addition, a tendency to convert rotational energy gained in the first gas-surface collision into normal translational energy for collisions at glancing incidence further increases the degree of non-normal energy scaling. Increasing surface temperature is shown to have little effect on energy transfer processes in the first bounce but increasing influence on subsequent bounces. Despite difficulties in defining trapping at high surface temperatures, simulations indicate that the initial trapping probability of ethane on Pt(111) does not fall by more than a factor of two over the surface temperature range of 100–700 K.     

Accommodation coefficients for low-energy ions on metal surfaces

The interaction of low-energy ions (E = 3 to 100 eV) with the surface of a solid cannot be treated in terms of gas dynamics. The scattering of particles at an energy of E _o > 20 eV may be explained in terms of binary collisions with the atoms of the target. The validity of the single collision model with free surface atoms for medium energies from 10 to 100 eV and even down to 1 eV was confirmed on the basis of both experimental and computational data. This paper describes experimental studies of the secondary ion emission from the (100) and (110) faces of a Mo single crystal and both experimental and theoretical studies of alkaline ion accommodation coefficient on polycrystalline Mo within the energy range E = 3 to 50 eV with a varying direction of the primary ion beam from normal to the glancing angle of incidence (ρ = 0 to 75°). On the basis of the retarding potential method using a spherical condenser whose central electrode was the target, we measured the energy distribution of secondary ions. Calculations have been performed for the energy of scattered ions and the high energy portion of accommodation coefficient on the basis of single and double binary collisions using the Born-Mayer potential and taking into consideration the influence of adsorption forces at the surface of the target.     

Anisotropy of the repulsive intermolecular potential from rotationally inelastic scattering

Structure is observed in recoil velocity distributions of potassium atoms scattered inelastically from N₂ and CO molecules at CMS angles?>π/2 and collision energies 0.34≦E≦1.24 eV. It is caused by rotational excitation mainly. Individual rotational transitions are not resolved. The quasi-continuous recoil velocity distributions extend between well defined bounds, the upper corresponding to elastic scattering, the lower marking a largest amount of energy transferred into molecular rotation at given? andE. This maximal transfer increases with angle and — forE?0.64 eV — in near proportion toE. At all? andE it is larger for CO than forN ₂, amounting to about 0.42E forN ₂ and 0.64E for CO at?=150°. At or very close to each of their bounds the distributions exhibit pronounced maxima. A third intermediate maximum is present for CO, but missing for N₂. The scattering in these experiments is dominated by the repulsive core of the interaction potential. On the basis of classical scattering from a rigid, ellipsoidal, initially nonrotating potential shell, the observations can be qualitatively understood as follows: The recoil velocity dependence of the orientation averaged cross section at fixed observation angle will exhibit classical, integrable singularities at extremal amounts of rotational energy transfer. If the center of symmetry of the shell coincides with the center of mass of the molecule, there will be two such extremal transfers,ΔE=0 and a largest transfer possible at the chosen angle. If the centers do not coincide, the singularity at the largest transfer will split into two, one corresponding to the short, the other to the long “lever arm” of the shell. K + N₂ is an example of the centrical, K+CO of the excentrical case. The finite energy losses, at which the singularities occur, are — at each angle — strongly dependent on the anisotropy and excentricity of the shell.