On the sputtering of binary compounds

A simple physical model is presented to describe some aspects of the sputtering of compound targets. In particular, expressions are developed for the partial sputtering yields for binary systems in terms of the elemental sputtering rates, the stoichiometric concentrations and surface binding energy. The partial yields depend non-linearly on the bulk target concentrations. Comparison of the theoretical predictions with the data on sputtering of PtSi, NiSi and Cu3Au indicates that the general features are well described

Mass fractionation of the lunar surface by solar wind sputtering

The sputtering of the lunar surface by the solar wind is examined as a possible mechanism of mass fractionation. Simple arguments based on current theories of sputtering and the ballistics of the sputtered atoms suggest that most ejected atoms will have sufficiently high energy to escape lunar gravity. However, the fraction of atoms which falls back to the surface is enriched in the heavier atomic components relative to the lighter ones. This material is incorporated into the heavily radiation-damaged outer surfaces of grains where it is subject to resputtering. Over the course of several hundred years an equilibrium surface layer, enriched in heavier atoms, is found to form. The dependence of the calculated results upon the sputtering rate and on the details of the energy spectrum of sputtered particles is investigated. It is concluded that mass fractionation by solar wind sputtering is likely to be an important phenomenon on the lunar surface

Two-photon-induced birefringence in azo-dye bearing polyimide; the birefringence changes versus the writing power

Ultra-short high-intensity light pulses were utilized to induce the optical birefringence in a polyimide material possessing the azo-dye covalently bonded to the main chain. The obtained results showed that a two-photon absorption process was involved in a creation of the sample birefringence which, to the best of our knowledge, was not previously reported for polyimide materials. The growths and decays of birefringence were examined as functions of the pulse intensities. No damage to the material during the illumination process was detected in a wide range of optical powers applied. High birefringence level of the order of 0.005 was measured

Mass fractionation of the lunar surface by solar wind sputtering

The sputtering of the lunar surface by the solar wind is examined as a possible mechanism of mass fractionation. Simple arguments based on current theories of sputtering and the ballistics of the sputtered atoms suggest that most ejected atoms will have sufficiently high energy to escape lunar gravity. However, the fraction of atoms which falls back to the surface is enriched in the heavier atomic components in relation to the lighter ones. This material is incorporated into the heavily radiation-damaged outer surfaces of grains, where it is subject to resputtering. Calculations predict that an equilibrium surface layer, enriched in heavier atoms, will form with δ(^(18)O) ≈ +20‰ ≈ δ(^(30)Si) and that oxygen will be depleted on the surface layers of grains relative to the bulk composition by about 12.5%. These results are in fair agreement with experiment. The dependence of the calculated results upon the energy spectrum of sputtered particles is investigated. We conclude that mass fractionation by solar wind sputtering is likely to be an important phenomenon on the lunar surface but that the complex isotopic variations observed in lunar soils cannot be completely explained by this mechanism

Updated Nucleosynthesis Constraints on Unstable Relic Particles

We revisit the upper limits on the abundance of unstable massive relic particles provided by the success of Big-Bang Nucleosynthesis calculations. We use the cosmic microwave background data to constrain the baryon-to-photon ratio, and incorporate an extensively updated compilation of cross sections into a new calculation of the network of reactions induced by electromagnetic showers that create and destroy the light elements deuterium, he3, he4, li6 and li7. We derive analytic approximations that complement and check the full numerical calculations. Considerations of the abundances of he4 and li6 exclude exceptional regions of parameter space that would otherwise have been permitted by deuterium alone. We illustrate our results by applying them to massive gravitinos. If they weigh ~100 GeV, their primordial abundance should have been below about 10^{-13} of the total entropy. This would imply an upper limit on the reheating temperature of a few times 10^7 GeV, which could be a potential difficulty for some models of inflation. We discuss possible ways of evading this problem.Comment: 40 pages LaTeX, 18 eps figure

Theoretical calculation of the p− 6 Li radiative capture reaction

We present a new calculation of the Li6(p,γ) 7 Be radiative capture astrophysical S-factor in a cluster model framework. We consider several intercluster potentials, adjusted to reproduce the Be7 bound state properties and the p− 6 Li elastic scattering phase shifts. Using these potentials, we calculate the astrophysical S-factor, obtaining a good agreement with available data, and the photon angular distribution. Finally, we discuss the consequences of a hypothetical resonance-like structure on the S-factor