Study Of Surface Interactions Research Articles

Observation of 7Be on the surface of LDEF spacecraft

THELong Duration Exposure Facility (LDEF), an orbiting unmanned satellite, was recently returned to Earth after almost six years in space. From radioactivity measurements, we have found significant quantities of the isotope7Be on the leading edge (but only on the leading edge) of LDEF. Although the absolute atmospheric concentration of 7Be needed to explain this detection is extremely small (10–7atoms cm–3), its concentration at LDEF's altitude (310 km) must be several orders of magnitude higher than in the stratosphere below, where it is produced by cosmic-ray reactions with atmospheric nitrogen and oxygen nuclei. To explain the presence of 7Be on the surface of LDEF, it must first be rapidly and efficiently transported to high altitudes, and then adsorbed onto the surface of the spacecraft. Neither process had been expected. Our detection may therefore lead to the use of 7Be as an exo-atmospheric tracer, as well as to studies of surface interactions in space.

A scanning tunneling microscope for ultrahigh vacuum atom–surface interaction studies

A compact, mechanically stable and thermally compensated tunneling microscope has been designed for use in an ultrahigh vacuum (UHV) surface characterization and analysis system. Very small concentric tubular scanning elements provide mechanical rigidity, thermal stability, and minimal cross coupling of the X, Y, and Z motions. The compact load-locking design provides full compatibility with the rest of the UHV analysis system without requiring that a disproportionately large fraction of the vacuum system be devoted to the microscope. Reported here is the design of the microscope and load-lock system, and the results of tests using the microscope in air to image the surfaces of graphite, gold, and platinum.

High-resolution He-scattering apparatus for gas–surface interaction studies

A high-resolution apparatus designed for the study of elastic and inelastic scattering of thermal helium atoms from crystal surfaces is presented. The highly expanded He nozzle beam has an energy spread ΔE/E of about 1.4% and is collimated to 0.2°. The angle subtended by the detector opening as seen from the sample is also 0.2°. Beam intensities as low as 10−6 of the specular beam intensity from a low-temperature clean Pt(111) surface are detectable. Pseudorandom chopping with a resolution of 2.5 μs (flight path 790 mm) is used for time-of-flight (TOF) analysis of the scattered helium. The base pressure in the sample chamber is in the low 10−11 mbar. The capabilities of the apparatus are demonstrated for physisorbed Xe adlayers on Pt(111). The results presented are obtained by using He scattering in various modes: coherent inelastic, coherent elastic, and incoherent (diffuse) elastic. This technique allows for a nondestructive nearly exhaustive characterization of the thermodynamics, structure, and dynamics of physisorbed adlayers on arbitrary substrates.

Surface interaction study of TiC single crystal with energetic hydrogen ions by AES and FDS

Changes in atomic composition in the near surface region of single–crystal titanium carbide (TiC) due to hydrogen or deuterium ion irradiation in the keV range were studied with the use of AES (Auger electron spectroscopy) and FDS (flash desorption spectroscopy). Depth profiles obtained with sputter-AES showed that carbon atoms were preferentially depleted due to the hydrogen ion irradiation. The amount of depleted carbon approached the equilibrium value above the hydrogen ion fluence of 2×1017 ions/cm2. Depth profiles of this depletion region were found to be similar to the damage distributions caused by the incident particles. The depleted region recovered completely by annealing at 1000 °C for 30 s. The result of FDS measurements suggests that the hydrogen atoms which were implanted into the near surface region replaced the carbon atoms of the TiC lattice to form titanium hydride.

Induced molecular transport due to surface acoustic waves

An investigation into the possiblity of inducing a net transport of molecules via their interaction with a propagating surface acoustic wave is reported here. A theory is presented predicting such a transport and giving a formula for the pumping rate of a device consisting of two evacuated chambers connected by a channel of rectangular cross section containing a surface on which a surface acoustic wave is propagating. This formula gives the parametric dependence of the pumping rate on the wavelength, amplitude and velocity of the surface acoustic wave, and the mean molecular velocity of the gas. The results of an experimental investigation are also given. The experiment, conducted with air at approximately 3 μ and a surface acoustic wave on a lithium niobate substrate, confirms the existence of transport and gives a pumping rate which compares favorably with the theoretical prediction. Possible applications of this effect include its use in vacuum technology and studies of surfaces and surface interactions.

Proton backscattering as a technique for light ion surface interaction studies in CTR materials investigations

Analysis of near-surface helium and hydrogen isotope depth profiles by a specialized proton backscattering technique has been demonstrated as a means to characterize the concentration, initial position, and subsequent migration behavior of all Z atom species (except 1H) in metals. The projected range (R p ) of helium implanted at 50 keV in Nb, V Ti and Cu has been measured and, for the latter two metals, has been found to agree with theoretical calculations within 100 Å (3%). Deuterium has also been detected and profiled (in Ti) using this technique. Detection sensitivity has been demonstrated at the 7 at % level for D in Ti and at the 0.5 at % level for 4He in Cu. In addition, surface and bulk distributions of carbon, oxygen and 3He have been profiled and, in principle, the depth distributions of tritium, 6Li, 7Li, Be, B, N, and F are also simultaneously resolvable if contained within the foil samples. The technique has been used to investigate the effect of radiation damage and in situ annealing on implanted helium profiles in copper. From these observations the mechanism for helium release from the end-of-range location has been delineated.

A study of surface interactions on γ-alumina, silica-alumina, and silica-magnesia using the EPR spectra of adsorbed nitric oxide

The EPR spectrum of the adsorbed NO molecule was used to determine crystal and magnetic field interactions on the surface of γ-alumina, silica-alumina, and silica-magnesia. The crystal-field splitting, Δ, of the 2pπ ∗ levels of NO was found to be 0.75 eV on dehydrated γ-alumina and 0.60 eV on dehydrated silica-alumina. The spectrum on silica-magnesia indicated two adsorption sites with Δ equal to 0.60 and 0.33 eV. Spectra of NO on γ-alumina and silica-alumina show considerable hyperfine coupling with an 27Al atom at the adsorption site. The spectra were very similar to the spectrum of NO on a decationated Y zeolite. The maximum number of adsorption sites (as indicated by the spin concentration) was found to be 4(± 1) × 10 12 sites/cm 2 for 7-alumina and two types of silica-alumina. Silica-magnesia had 1.1(± 0.3) × 10 12 sites/cm 2. The number of sites and the shape of the spectra were a function of the degree of dehydration. Water added back to dehydrated γ-alumina and silica-alumina also affected the spectra. It appears that these adsorption sites are the α sites which Peri has studied using the IR spectra of adsorbed CO 2.

Radiotracer Studies of Carbon Black Surface Interactions with Organic Systems. I. Experimental Approach and Initial Results from Chemisorption and Vinyl Polymerization Studies

Abstract Radiotracer techniques have been applied to the study of interactions of carbon black surface functional groups with two chosen organic systems. The basic reaction mechanisms demonstrated in this study may have implications in elastomer reinforcement. Direct radiochemical evidence supports the conclusions of Hallum and Drushel (based on less direct polarographic data) that surface quinonic groups exhibit hydrogen abstraction activity toward tertiary hydrogens in paraffinic hydrocarbons. Studies on the system carbon black and styrene using tritium radiotracer have provided direct evidence that phenolic hydrogens participate in the polymerization acceleration and graft polymer formation reaction and are transferred to the growing polystyrene chains as postulated by Donnet. Several methods have been developed for specifically labelling certain oxygenated functional groups on the carbon surface with tritium and for tritium labelling carbon black in aromatic hydrogen positions. The techniques developed in this work and the basic reaction mechanisms derived will permit this investigation to be extended into a radiochemical study of carbon black surface interactions with elastomer related systems of interest to the rubber industry.