Ground‐based and MErcury Surface, Space ENvironment, GEochemistry, and Ranging observations detected Ca0 and Ca+ in the exosphere of Mercury as well as unexpectedly high levels of sulfur on Mercury's surface. The mineral oldhamite ((Mg,Ca)S) could be a predominant component of the Mercury surface, particularly within the hollows identified within craters, and could therefore serve as a source of the observed exospheric calcium. Laboratory measurements on the photon‐stimulated desorption (PSD) of CaS powder (an analog for oldhamite) at a wavelength of λ = 355 nm have been conducted, utilizing resonance‐enhanced multiphoton ionization time‐of‐flight mass spectrometry to determine the yields and velocity distributions of Ca0. The desorbing Ca0 could be fit using two Maxwell‐Boltzmann components: a 600 (±30) K thermal component and a 1389 (±121) K nonthermal component, the latter accounting for ~25% of the observed signal. Cross sections for PSD using 3.4 eV photons were found to be 1.1 (±0.7) × 10−20 cm2 for Ca0 and 3.2 (±0.9) × 10−24 cm2 for Ca+. Adopting these cross sections, a Monte Carlo model of the release of Ca0 by PSD from the Tyagaraja crater finds the neutral microexosphere created from this process to be substantial even if only 1% CaS is assumed in the hollows. Diffuse reflectance UV‐visible measurements were made on the CaS powder to determine a bandgap, Eg, of 2.81 (±0.14) eV via the Tauc method.
The adsorption of K and I on the surface of the high-T c cuprate BSCCO-2212 is investigated with lowenergy ͑0.8 to 2 keV͒ Na + ion scattering and density functional theory ͑DFT͒. Samples were cleaved in ultrahigh vacuum and charge-resolved spectra of the scattered ions were collected with time-of-flight. The spectra contain a single peak representing Na scattered from Bi, as the clean surfaces are terminated by BiO. The neutralization of scattered Na depends on the local potential above the target site, and the angular dependence indicates that the clean surface has an inhomogeneous potential. Neutralization is dependent on the coverage of I, but independent of K adsorption. DFT suggests high-symmetry sites for the adsorption of both I and K, and that the potential above the Bi sites is altered by I by an amount consistent with the experimental findings, while the potential is not affected by K adsorption. DFT also enables an experimental determination of the "freezing distance," which is the effective point beyond which charge exchange does not occur, to be 1.6Ϯ 0.1 Å from the outermost Bi layer.
The surface charge generated on an Al0.24Ga0.76As∕GaAs quantum well sample by electron bombardment was monitored by measuring the change in the conductivity of the channel. Upon turning off the electron bombardment the surface charge on adsorbed layers of xenon and water at 8K decays in room temperature darkness with a lifetime τ=0.30±0.02s. The average charging efficiency, μ0, defined as the ratio of the charge collected by the surface to the beam current times the charging time, is μ0≃0.001. Surface charging proves to be an effective method for contactless gating of field effect devices.
The high-T c cuprate Bi-2212 is sputtered with 500 eV Ar + ions and changes to the surface composition are investigated with low energy (2 keV) Na + ion scattering. It is shown that ion bombardment leads to the development of a Bi-O overlayer, differing in structure from the underlying material and making the surface highly resistant to further sputtering-induced changes. In contrast, sputtering and ion scattering simulations would suggest that atoms from lower layers should be present at the surface as a consequence of the kinematics of the sputtering process. It is thus concluded that the Bi-O layer forms because the surfactant effects of Bi reduce the surface energy.
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