J.W. Rabalais

Time‐of‐flight scattering and recoiling spectrometer (TOF‐SARS) for surface analysis

Low energy (< 10 keV) ion scattering spectrometry [10.1] is becoming increasingly important as a surface analysis technique in three specific areas, i.e., surface elemental analysis [10.2–4], probing surface structure [10.5–16], and studying electronic transition probabilities [10.7,7–19] between ions or atoms and surfaces. This is largely due to the following recent advances: (i) impact collision ion scattering spectrometry [10.6] (ICISS) in which the scattering angle is close to 180°, thus simplifying the scattering geometry and allowing experimental determination of the shadow cone radii, (ii) the use of alkali primary ions [10.9, 10] which have low neutralization probabilities, leading to higher scattered ion fluxes, (iii) time-of-flight (TOF) techniques [10.20–23] with detection of both neutrals and ions in a multichannel mode in order to enhance sensitivity, (iv) scattered ion fractions [10.7,17] to probe the spatial distributions of electrons, and (v) the use of recoiling [10.24, 25] to determine the structure of light adsorbates on surfaces.

Performance of mass analyzed, low‐energy, dual ion beam system for materials research

Mass analyzed low‐energy ion beams delivered into a UHV growth chamber have enormous potential for novel materials studies. However, there are significant practical problems in the production of useful ion fluxes at energies down to a few electron volts. Many of these problems have been investigated during the testing of a unique new instrument. This instrument consists of a dual source, mass analyzed, low‐energy, ion beam system attached to an ultrahigh‐vacuum (UHV) deposition chamber which houses equipment for in situ Auger electron spectroscopy and reflection high‐energy electron diffraction analysis of the deposited material. A second UHV chamber, connected to the deposition chamber by means of a vacuum lock and sample transfer device, houses equipment for in situ low‐energy electron diffraction and time‐of‐flight scattering and recoiling spectrometry. The instrument is briefly described herein and data are presented to illustrate the effects of various parameters on the performance of the ion beam. T...

Scattering and recoiling imaging code (SARIC)

Abstract A new classical ion trajectory simulation program based on the binary collision approximation has been developed in order to support the results of time-of-flight scattering and recoiling spectrometry (TOF-SARS) and scattering and recoiling imaging spectrometry (SARIS). The code was designed to provide information directly related to the TOF-SARS and SARIS measurements and to operate efficiently on small personal computers. The calculation uses the Ziegler-Biersack-Littmark (ZBL) universal screening function or the Moliere screening function to simulate the three-dimensional motion of atomic particles and includes simultaneous collisions involving several atoms. For TOF-SARS, the program calculates the energy and time-of-flight distributions of scattered and recoiled particles, polar (incident) angle α-scans, and azimuthal angle δ-scans. For SARIS, the program provides images of the scattering and recoiling intensities in polar exit angle and azimuthal angle (β, δ)-space. A two-dimensional reliability factor (R) has been developed in order to obtain a quantitative comparison of experimental and simulated images. Examples of simulations are presented for Ni{100}, {110} and {111} surfaces and a Pt{111} surface. The R-factor is used to quantitatively compare the simulated Pt{111} image to an experimentally emulated image.

Element-, time- and spatially-resolved images of scattered and recoiled atoms

Abstract Element-specific, time- and spatially-resolved images of keV scattered and recoiled atomic ions plus fast atoms are exploited as a new technique of scattering and recoiling imaging spectrometry (SARIS). SARIS, an outgrowth of time-of-flight scattering and recoiling spectrometry (TOF-SARS), uses a large position-sensitive microchannel plate and TOF methods to capture images of energetic atoms which are scattered and recoiled from surfaces using a pulsed keV ion beam. The atoms are dispersed according to their velocities as a function of projectile/target atom masses and deflection angles. The spatial distributions of these atoms are captured by the MCP in time-resolved frames as short as 10 ns. Classical ion trajectory simulations provide a good description of the interactions, allowing direct simulation and interpretation of the experimental images. The images combine atomic scale microscopy and spatial averaging since they are created from a macroscopic surface area but they are directly related to the atomic arrangement of the surface at the sub-nanoscale level; the accuracy for measurement of interatomic spacings is expected to be better than 0.01 A. Example data is presented for Pt{111}and Au{110}.

Structure of benzene and phenol chemisorbed on Ni{110}

The chemisorption of benzene and phenol on a clean Ni{110}-(1 x 1) surface and an oxygen predosed Ni{110}-(3 X 1)-O surface near room temperature has been investigated by time-of-flight scattering and recoiling spectrometry accompanied by shadow cone calculations. The Ne scattering and H, C, and O recoiling fluxes exhibited strong angular anisotropies as a function of beam incident (alpha) and crystal azimuthal (delta) angles. These anisotropies are due to C and O atoms shadowing their neighboring atoms within the benzene molecules and resulting phenoxide species, demonstrating that scattering and recoiling spectrometry is capable of providing structural information on polyatomic molecular systems. The results show that both benzene and phenoxide are chemisorbed as molecules which have very good short-range order despite the absence of long-range order observable by low energy electron diffraction. Both benzene and phenoxide are oriented nearly parallel to the surface, with a maximum inclination angle of 15-degrees. The C atoms in the para positions of benzene and the C-O bond in phenoxide are oriented along the [001] azimuth. The C-H bond is bent out of the plane of the hexagonal ring so that the H atoms are above the C atom plane. Chemisorption on the oxygen predosed surface results in a reaction in which a H atom is abstracted from both benzene and phenol with the formation of surface hydroxide groups; the molecules remain well ordered on this surface also.

Scattering and recoiling imaging spectrometer (SARIS)

An ultrahigh vacuum spectrometer system has been designed and constructed for obtaining spatial- and time-resolved, element-specific images of atoms that are scattered and recoiled from surfaces. A pulsed noble gas ion beam in the 1–5 keV range is used to scatter and recoil atoms from a surface. A large, position-sensitive microchannel plate detector with resistive anode encoder, that is sensitive both to ions and fast neutrals records the spatial distribution patterns of the emitted atoms. The use of time-of-flight methods allows capture of these patterns in time windows as short as 10 ns. The sensitivity of these patterns to the details of surface structure provides the basis for a scattering and recoiling imaging spectrometry (SARIS). The primary ion beam current is ∼0.1 nA/cm2, supplied in 20 ns pulses at a rate of 30 kHz, resulting in ∼5×102 ions/pulse; images with adequate statistics can be obtained in several seconds with a total ion dose of <1010 ions/cm2. The SARIS technique can provide unique, e...

Dissociative scattering of 1.5–4.5 keV N+2 and N+ on gold and graphite surfaces

Scattering of molecular nitrogen ions in the 1.5–4.5 keV range from gold and graphite surfaces results in a small fraction of surviving molecules and molecular ions in addition to atoms and atomic ions resulting from dissociation. The kinetic energy (Ek ) distributions of scattered N+2 and N+ ions have been measured directly by means of an electrostatic sector analyzer (ESA) and the velocity distributions of the scattered N2 and N neutrals plus ions have been measured by time‐of‐flight (TOF) techniques. Scattered ion fractions were determined from the TOF measurements. The relative Ek distributions of the scattered atomic ions indicate that dissociation from excited repulsive electronic states which are populated during the collision dominate the mechanism, rather than purely vibrational or rotational excitation from the X 2Σ+g ground state of N+2 . The excited dissociative C 2Σ+u and D 2Πu states of N+2 are accessible by Franck–Condon transitions from the X2Σ+g state. The data are consistent with a mecha...

Oxygen induced added‐row reconstruction of the Ni{110} surface

The oxygen induced reconstructed phases of the Ni{110} surface have been studied by time‐of‐flight scattering and recoiling spectrometry (TOF–SARS). The substrate structures are determined from experimental measurements of azimuthal angle (δ) and polar incident angle (α) anisotropies in the scattered Ne intensities coupled with classical trajectory simulations for shadow cone analysis. By monitoring features in the TOF–SARS scans that are unique to specific phases, it is possible to follow the migration of the first‐layer Ni atoms as a function of O2 exposure. The results show that upon increasing exposures of the clean Ni{110}–(1×1) surface to O2, a series of LEED patterns [initial p(3×1), p(2×1), and final p(3×1)] is produced corresponding to three surface phases which differ only in the density of the first‐layer Ni 〈001〉 rows. These nascent ‘‘added rows’’ are stabilized by bonding to oxygen atoms which reside in the long‐bridge positions along the 〈001〉 rows. Structural models for the three phases are...

BN formation from bombardment of boron with N2

Abstract Boron nitride formation from bombardment of boron with 0.5–4 keV N 2 + ions is studied by in situ XPS and AES measurements. A dynamic process of nitrogen implantation and surface sputtering leads to the formation of a BN layer whose thickness is comparable to the range of the nitrogen ions in the boron. The maximal nitrogen entrapment is determined by the stoichiometry of BN; excess nitrogen is released by sputtering and possibly diffusion. No diffusion of nitrogen to the bulk nor expansion of the BN layer is detected at room temperature for prolonged bombardment. The results are in accord with a simple collisional altered layer model that is presented.

Detection of low energy neutrals by a channel electron multiplier

Abstract A method for determining relative channel electron multiplier (CEM) detection efficiencies for low energy neutrals using the technique of direct recoiling is described. The method uses time-of-flight analysis of low-energy atoms recoiling from an energetic ion/surface collision. The system used for demonstration is 1–10 keV Ne+ scattering from a polyimide resin surface, producing H and C recoils. The behavior of the CEM detection efficiency γ can be divided into two regions: A flat plateau region is observed at high velocity while below a specific velocity, νH = 4.4 × 107 and νC = 3.2 × 107 cm/s, γ decreases linearly with velocity with slope d γ d ν = 6.4 × 10 −8 s/cm . This behavior is discussed in terms of secondary electron emission by a kinetic ejection process.