R.A. Outlaw

TPD and ESD studies of the interaction of hydrogen and oxygen on polycrystalline zirconium

Temperature programmed desorption (TPD) and electron stimulated desorption (ESD) have been used to study the interaction of hydrogen and oxygen on a polycrystalline zirconium surface. ESD and TPD features due to hydrogen bonding directly both to oxygen and to Zr are identified. Deuterium adsorption studies reveal that the presence of an oxide layer retards hydrogen adsorption but that the oxygen attracts bulk hydrogen forming a hydroxylated surface oxide layer.

Electron-stimulated desorption of atomic oxygen from polycrystalline Ag

Abstract Emission of neutral oxygen atoms from an oxygen-charged polycrystalline Ag wire has been examined by using electron-stimulated desorption (ESD) in conjunction with threshold ionization detection (appearance potential). A quadrupole mass spectrometer (QMS) was used to detect the neutral oxygen atoms, but the ionizer was operated in the appearance potential (AP) mode to make it possible to distinguish the oxygen atoms from products formed by collision of the oxygen atoms with the walls. Loss of the reactive oxygen atoms was also minimized by enclosing the ESD chamber in fused silica. With a primary beam energy of 100 eV, the ESD cross section for oxygen atom desorption was found to be 7 × 10 -19 cm 2 at 100 °C, and an ESD threshold was found to exist below 34 eV. Flash desorption of oxygen allowed estimation of the bulk diffusivity of oxygen through polycrystalline Ag. A value of 2.64 × 10 -6 cm 2 s -1 at 500 °C was obtained.

Electron stimulated desorption of oxygen atoms and ions from an oxidized AgZr alloy surface

Abstract Electron stimulated desorption (ESD) is a complex process in which the ion and neutral yields are functions of many variables. In order to examine some of these dependencies which have not been considered previously, a study of the composition and ESD behavior of a Ag 0.5 wt % Zr (Ag0.5Zr) alloy membrane has been conducted. Zr was added to the Ag primarily to increase the O concentration at the surface and thereby enhance the ESD signal. The surface was sputter cleaned, saturated with O 2 , sputter cleaned again and then annealed at 250°C until no further changes occurred in the surface composition according to ion scattering spectroscopy (ISS) and Auger electron spectroscopy (AES). ISS data suggest that the resulting surface consists of islands of Zr oxide partially covering the Ag surface. Neutrals emitted in ESD are quite difficult to detect so a technique was developed to determine the O neutral-to-ion ratio of 1.1 × 10 7 for a primary beam energy of 1 keV. By monitoring the surface O concentration as a function of time with AES, a total ESD cross section of 3.9 × 10 −19 cm 2 was determined. The ESD O + energy distribution spectrum was obtained and compared with that obtained from O-charged Ag. Also, the effect of primary beam incidence angle on the ESD yield and ion energy distribution was examined and found to be significant.

Proton damage in GaAs solar cells

A simplified model for the short-circuit current reduction caused by proton-induced radiation damage is described. The model accounts for the nonuniformity of defect production within heteroface GaAs shallow junction solar cells. The results from the model show agreement with the strong energy dependence observed in proton radiation damage experiments.

Sulfur segregation in titanium and selected titanium alloys

Abstract The segregation of S in electrotransport-purified polycrystalline α-Ti and in several polycrystalline Ti alloys has been studied by Auger electron spectroscopy and ion scattering spectroscopy in the temperature range extending from 20 to 1000°C. The chemisorbed oxygen and carbon on Ti were observed to disappear at T ≈ 400°C after which the S signal increased to levels approaching 0.5 monolayer. At lower temperatures the presence of the surface oxygen and carbon appeared to inhibit the segregation, presumably because there were no available surfaces sites for the S emerging from the bulk. The activation energy for the S segregation in pure polycrystalline Ti was determined to be 16.7 kcal/mol, which, when compared to S segregation from single-crystal Ti, is quite small and suggest grain boundary or defect diffusion segregation kinetics. In the Ti-aluminide alloys, the presence of Al appeared to enhance the retention of surface oxygen which, in turn, substantially reduced the S segregation. The γ alloy, with its high Al content, exhibited the greatest retention of surface oxygen and the smallest quantity of S segregation ( T ≈ 1000°C). On the other hand, the β-21S alloy exhibited a greater segregation rate for the S than did α-Ti.

Auger electron intensity variations in oxygen-charged silver

Auger intensity variations over an oxygen-charged polycrystalline silver surface have been observed by studies of Auger images and line scans of selected adjacent grains which were determined to be the (421) and (221) orientations. The observed contrast (M4,5VV transition) between the grains is produced by the variation in the detected Auger electrons caused by the different directions (interatomic direction) of forward focusing in each grain. The contrast produced by the Ag Auger electrons was found to be strongly dependent on the surface order of the grains, but that of the O Auger electrons was not, presumably because the atoms were randomly distributed throughout the Ag surface or subsurface. The contrast observed between the grains at the lower Auger energies (N1VV and N1N2,3V transitions) appeared to be produced by constructive interference from multiple scattering. The N1N2,3V electron at 29 eV, for example, gave higher contrast than that of the N1VV transition at 78 eV.

Finite size corrections to Madelung’s number

A correction to the usual Madelung form for the electrostatic energy of cubic ionic crystals due to the finite size of real atomic ions is derived.

Continuous electron stimulated desorption using a ZrO2/Ag permeation membrane

During the development of an atomic oxygen beam generator for laboratory simulation of the atmospheric conditions in low Earth orbit, a new technique for performing electron stimulated desorption (ESD) in a continuous manner has been developed. In this technique, oxygen permeates through a Ag membrane at elevated temperature thereby providing a continuous supply of oxygen atoms to the 1000‐A ZrO2 coating at the vacuum interface. ESD then results in a large peak of neutral O2 molecules which ultimately decays into steady‐state desorption. The ESD signal is linear with respect to primary beam flux (3.5×10−2 O2 molecules per electron at a primary beam energy of 1 keV) but nonlinear with respect to primary beam energy.