Azimuthal Features Research Articles

X-ray photoelectron spectroscopic and ion scattering study of the SrTiO 3(0 0 1) surface

The chemical and structural properties of several as-received (unspecified and TiO 2 terminated) SrTiO 3(0 0 1) surfaces have been analyzed prior to and following UHV and O 2 annealing using X-ray photoelectron spectroscopy (XPS), time-of-flight scattering and recoiling spectrometry (TOF-SARS), and low energy electron diffraction (LEED). Well resolved 1×1 LEED patterns were noted for the TiO 2 terminated surface. Simulations of the TOF-SARS azimuthal scans indicate that the O atoms are situated 0.1 Å above the Ti layer (surface plane). This TiO 2 terminated surface reconstructed following O 2 annealing resulting in the loss of the azimuthal TOF-SARS features and the LEED pattern. This disordered surface also exhibited a stronger Sr TOF-SARS signal, and a high binding energy (HBE) component in the Sr 3d, 3p and 4p XPS spectra similar to that observed in the unspecified surfaces. Cleaning with organic solvents and exposure to air further enhanced the HBE components, suggesting Sr–C formation. No HBE components were observed in the Sr spectra from the TiO 2 terminated surface. Valence band XPS spectra reveal the presence of surface states and Ti 3d sub-band states.

Modulation of cosmic rays in the heliosphere over 11 and 22 year cycles: a modelling perspective

The long term modulation of galactic cosmic rays in the heliosphere deals with the causes of the 1 l-year and 22year solar cycles, while short term modulation is associated with effects of the order of a few solar rotations. During the past decade, with four spacecraft (Pioneer 10 & 11, Voyager 1 & 2) at various relatively large distances from the Sun, and with Ulysses inside of 5 AU and IMP 8 at Earth, the emphasis has changed to understand modulation as being applicable to three distinctive areas of the heliosphere. They are conveniently called the inner, middle and outer heliosphere. The outer heliosphere is about studying the effects of the configuration of the heliosphere, its interfaces with the interstellar medium and the termination shock - all aspects of great importance to cosmic ray modulation. The middle heliosphere is the region where shorter term effects like corotating regions and the effects of coronal mass ejections lose their characteristic features and merge into bigger entities of different scales which have profound effects on medium to long term modulation. The inner heliosphere that Ulysses explores is characterized by many short term events, superimposed on the long-term baseline, which have apart from their strong temporal and radial dependence easily recognizable latitudinal and azimuthal features. Compatibility studies between the results of modelling and observations at Earth and beyond have been done with reasonable success over the years. This review focuses on theory and modelling, and the understanding of the basic mechanisms of galactic cosmic ray modulation in the heliosphere, with emphasis on long-term modulation cycles.

Characterization of the surface structure of CH3 and CF3 terminated n-alkanethiol monolayers self assembled on Au{111}

Surface elemental and structural characterization of hexadecanethiol and heptadecanethiol (C16 and C17 for short) and 16,16,16-trifluorohexadecanethiol (FC16) self-assembled monolayers (SAMs) on a Au{111} surface have been obtained from time-of-flight scattering and recoiling spectrometry. The clean Au surface was also characterized in order to identify the azimuthal orientation of the SAMs with respect to the substrate. Classical ion trajectory simulations were used to relate the experimental scattering and recoiling data to the surface structure. The scattered and recoiled atoms originate from the outermost five–six atomic layers, and azimuthal anisotropy was observed in the measurements. The results provide a model for the SAMs in which the alkyl chains chemisorb with the S atoms situated above the face-centered-cubic (fcc) threefold sites of the Au{111} surface to form a continuous film with a (√3×√3)R30° structure that fully covers the Au surface. The orientation of the molecular axis azimuth of the SAMs relative to the Au azimuthal directions was determined. The data indicate that the molecular chains have specific tilt and twist angles relative to the Au surface and six coexisting domains resulting from the six equivalent tilt directions of the molecular axis. Dramatic changes in the anisotropic patterns of the ion scattering azimuthal scans from the surfaces of the SAMs with different terminations were observed. These phenomena result from the different tilt angles of the CH3 and CF3 groups. The data are consistent with free rotation of both the CH3 and CF3 groups. The C16 SAM exhibited the best azimuthal features and was more resistant to radiation damage from the incident Ar+ scattering beam than the other films. Due to the tilt angle of the SAMs, an “ion’s eye view” of the structure, i.e., the positions of the atomic cores as experienced by the incoming keV ions, reveals a regular array of sloping cavities within each unit cell.

Numerical Simulations of Moon—Ringlet Interaction

Nonaxisymmetric ring features excited by perturbations of shepherd satellites are studied in terms of direct particle simulations using Aarseth's N-body integrator combined with the calculation of particle-particle impacts. Interaction parameters typical to Saturn's F-ring are investigated. The generation of clumps by external satellites is verified, but the interparticle collisions tend to smooth sharp features. Using F-ring parameters the clumps are observed to cover the total azimuthal length, but it is not clear whether these azimuthally overlapping clumps would be detectable in the actual F-ring. Gravitational scattering by ring particles increases the velocity dispersion, smearing regular azimuthal features at least in the rings of low optical depths. Considerable accretion is observed to occur, particles sticking pairwise to each other, even if the tendency of the particles to accrete is artificially reduced in the simulations. A new explanation for the braided appearance of the F-ring is proposed, based on the interaction between the shepherding satellites and the ring containing embedded moonlets. In our model the braiding is a dynamic phenomenon: the braids are destroyed and recreated in a cyclical manner.

The influence of various moonlets on the optical depth profile in planetary rings

Our probability concept for characterizing the long-term gravitational influence of one moonlet on planetary rings has been generalized to: 1. (i) eccentric orbits of a single moonlet, 2. (ii) n co-orbital moonlets of different masses. We have applied a special scaling property of the three-body problem, which permits the generalization of the results of the scattering process, once they are obtained for a certain satellite, to moonlets of arbitrary mass. In this way, further test particle calculations can be avoided. Our results for each of the two cases are as follows: 1. (i) A narrow confined ringlet divides a wide gap which has evolved around the orbit of the moon. The radial density profile of that ringlet contains information about the size as well as the semimajor axis a m and the eccentricity e m of the orbit of the embedded moonlet. 2. (ii) Here the innermost ringlet profile is strongly modified by the small coorbital satellites. However, unambiguous hints of the existence of all moonlets can be gathered from the ringlet profile, but only if the moonlets are really isolated (in size) from the bulk of the ring particles and if they differ enough in size from each other. Otherwise, the typical density features of comparable moonlets interfere in a way that makes it impossible to distinguish between them. Then, additional information is needed to identify the number of embedded satellites, such as azimuthal ringlet features.