Surface Loss Processes Research Articles

Comparing the luminescence processes of YVO4:Eu and core-shell YVO4@YF3 nanocrystals with bulk-YVO4:Eu

Comparative analysis of bulk, non-coated and core-shelled nanocrystalline YVO4:Eu was performed by means of time-resolved luminescence and VUV excitation luminescence spectroscopy techniques. Nanocrystalline YVO4:Eu samples – both as-prepared and YF3 core-shelled – have been synthesized by means of a microwave-assisted synthesis in ionic liquids, which allows to obtain 10–12nm nanoparticles with high crystallinity. The results show noticeable differences between bulk and nanocrystalline YVO4:Eu in photoluminescence experimental data, which explains by influence of the nanocrystal surface. A YF3 core-shell layer around YVO4:Eu nanoparticles partially recovers the intensity of the Eu3+ emission. It is demonstrated that the Eu3+ luminescence recovery is achieved at the expense of the suppression of the intrinsic emission but not due to the passivation of surface loss centers in core-shelled nanocrystals. It is also shown that surface loss processes are the reason of the degradation of energy transfer efficiency from the host lattice to Eu3+ under high-energy excitations in vacuum ultraviolet spectral range.

Quantifying stability influences on air pollution in Lanzhou, China, using a radon-based “stability monitor”: Seasonality and extreme events

A recently-developed radon-based technique is modified to quantify the seasonal influences of atmospheric stability on urban emissions in Lanzhou, China, based on 11 months of observations at three sites with contrasting pollution characteristics. Near-surface concentrations of primary (CO, SO2, NOx) and secondary (O3) gas phase pollutants responded to changing atmospheric stability in markedly different ways in winter and summer, primarily because monsoonal fetch changes strongly influenced the distance between measurement sites and their nearest upwind pollutant sources, but also due to mean diurnal changes in mixing depth. Typically, morning peak primary pollution concentrations increased by a factor of 2–5 from the most well-mixed to stable conditions, whereas nocturnal ozone concentrations reduced with increasing stability due to surface loss processes and the progressively reduced coupling between the nocturnal boundary layer and overlying free atmosphere. The majority of pollution exceedance events (cf. China National Air Quality Standard guideline values) occurred in winter, when all measurement stations were downwind of the city's main pollution sources, and were directly attributed to morning periods and stable atmospheric conditions. In the sheltered valley region of Lanzhou, extremes of winter nocturnal stability states represented a change in mean nocturnal wind speed of only 0.25 m s−1 (from 0.6 to 0.85 m s−1). Daily-integrated PM10 concentrations increased by a factor of 2 in winter from the most well-mixed to stable conditions, and were usually above guideline values at the industrial and residential sites for all atmospheric stability conditions. In summer, however, daily mean PM10 exceedances usually only occurred at the industrial site, under stable conditions. Finally, a simple model – based on mean radon concentrations between 1900 and 0400 h – is proposed to predict haze conditions in the city prior to commencement of the peak morning commuting time.

Effects of orbital evolution on lunar ice stability

[1] Many regions near the lunar poles are currently cold enough that surface water ice would be stable against sublimation losses for billions of years. However, most of these environments are currently too cold to efficiently drive ice downward by thermal diffusion, leaving impact burial as the primary means of protection from surface loss processes. In this respect, most of the present near-surface thermal environments on the Moon may actually be quite poor traps for water ice. This was not always the case. Long-term orbital changes have dramatically altered the lunar polar thermal environment. We develop a simple model of the evolution of the lunar orbit and spin axis to examine the thermal environments available for volatile deposition and retention in the past. Our calculations show that some early lunar polar environments were in the right temperature regime to have collected subsurface ice if a supply were available. However, a high-obliquity period, which occurred when the Moon was at about half its present distance from the Earth, would either have driven this ice out into space or deep into the lunar subsurface. Since that time, as the lunar obliquity has slowly decreased to its present value, environments have undergone their own thermal evolution, and each of the current cold traps experienced a period when they were most efficient at thermally burying ice. We examine the thermal history of a lunar polar crater to provide a framework for examining other processes effecting volatiles in the Moon's near-surface cold traps.

Space weathering on near-Earth objects investigated by neutral-particle detection

The ion-sputtering (IS) process is active in many planetary environments in the solar system where plasma precipitates directly on the surface (for instance, Mercury, Moon and Europa). In particular, solar wind sputtering is one of the most important agents for the surface erosion of a near-Earth object (NEO), acting together with other surface release processes, such as photon stimulated desorption (PSD), thermal desorption (TD) and micrometeoroid impact vaporization (MIV). The energy distribution of the IS-released neutrals peaks at a few eVs and extends up to hundreds of eVs. Since all other release processes produce particles of lower energies, the presence of neutral atoms in the energy range above 10 eV and below a few keVs (sputtered high-energy atoms (SHEA)) identifies the IS process. SHEA easily escape from the NEO, due to NEO's extremely weak gravity. Detection and analysis of SHEA will give important information on surface-loss processes as well as on surface elemental composition. The investigation of the active release processes, as a function of the external conditions and the NEO surface properties, is crucial for obtaining a clear view of the body's present loss rate as well as for getting clues on its evolution, which depends significantly on space weather. In this work, an attempt to analyze processes that take place on the surface of these small airless bodies, as a result of their exposure to the space environment, has been realized. For this reason, a new space weathering model (space weathering on NEO-SPAWN) is presented. Moreover, an instrument concept of a neutral-particle analyzer specifically designed for the measurement of neutral density and the detection of SHEA from a NEO is proposed.

Vertical distribution of ozone in the planetary boundary layer at the Ming Tombs, Beijing

Surface ozone (O 3 ) and vertical O 3 distribution in the planetary boundary layer (PBL) at the Ming Tombs (40°17′15″N, 116°12′51″E), Beijing during September 7―12, 2001 were measured by ground based measurements and an in-situ tethersonde system. The results indicated that O 3 concentration measured at surface level agreed well with that measured by tethersonde system in daytime when active thermal mixing was dominated. Ozone showed the lowest concentration before the sunrise and then gradually increased in the morning and reached the maximum in the afternoon 14:00―17:00 (lst). After sunset, O 3 gradually decreased and resulted in low value below 200―300 m due to surface loss processes and chemical destruction in stable boundary layer characterized by temperature inversions. High O 3 was observed whenever there was pollutants transport from the metropolitan areas of Beijing. Our analysis suggested the complex terrain of the Ming Tombs region caused pollutants transported from Beijing to accumulate in the PBL, and resulted in severe O 3 pollution, with a maximum over 160×10 -9 , when the synoptic conditions was favorable for photochemical O 3 production.

Factors affecting degradation rates of five triazole fungicides in two soil types: 2. Field studies

The criteria for registering pesticides persistent in soil are still a matter of debate. Amongst modern pesticides, several triazole fungicides are very persistent, though no deleterious effects on soil microbial processes have been reported. The behaviour of five such compounds (flutriafol, epoxiconazole, propiconazole, triadimefon and triadimenol) has been examined in two field trials utilising different agronomic treatments. These fungicides were applied in June 1996 at rates of 0.5 kg ha−1, and soil cores were taken to 20 cm depth at intervals over 2.5 years and analysed by extraction and high-pressure liquid chromatography. Triadimefon was quite rapidly reduced to triadimenol. Triadimenol, flutriafol and epoxiconazole were all very persistent with DT50 > 400 days, whilst propiconazole had DT50 c 200 days; behaviour was similar in the Rothamsted clay loam and Woburn sandy loam. Only flutriafol, the most polar and hence weakly sorbed of these fungicides, was appreciably leached, with traces reaching the 15–20 cm deep soil layer. Sprays applied to plots of fallow soil suffered loss of up to 50% of applied compound in the first four weeks, a loss eliminated by shallow incorporation, indicating an early role for surface loss processes such as photolysis and/or volatilisation. A young barley crop intercepted about one-third of the spray, though subsequent rain caused some wash-off. After one to two years, amounts of each compound remaining in the plots were similar for the three agronomic treatments, especially for flutriafol, though with a tendency for the incorporated plots to have the most chemical and the barley plots the least. Computer simulation of behaviour in the field using the model CALF, utilising sorption and degradation measurements made in laboratory incubations with these same soils together with daily climate measurements, overestimated persistence especially for flutriafol, epoxiconazole and triadimenol. This was due both to lack of inclusion of surface loss processes in the model, which caused initial deviations in the plots not receiving cultivation after spraying, and to a longer-term underestimation of breakdown in the field. This latter was especially noticeable for triadimenol, which was not detected after 2.5 years despite predictions of c 50% remaining. Thus field measurements of behaviour are desirable, as simulations based on laboratory measurements can overestimate persistence. © 1999 Society of Chemical Industry

Atoms, radicals and ions observed in plasmas - Their gas and surface chemistry

Abstract

Complex impedance analysis of heterogeneous catalysts containing mixed or fast ion conductors

Catalyst composites containing mixed oxide titanates and tungstates of high ac loss and insulating cements have been characterised by complex impedance spectroscopy coupled with other ac techniques. Near ambient temperature impedance curves show a whole or partial semi-circle inclined at 10 to 24° with the Z″ axis. As the temperature approaches 1000 K, a low frequency spur becomes dominant particularly for those composites containing fast ions. For materials containing mixed conductors an arc of a second semi-circle may appear at intermediate frequencies. This arc rapidly disappears on absorption of H 2 but can be re-established after reoxidation at 950 K. The principle use of impedance spectroscopy is in the determination of geometry independent parameters associated with different bulk and surface loss processes which can be correlated with catalytic factors such as preparation, chemisorption and behaviour in electromagnetic fields.

Low energy positron diffraction on Cu(111): Importance of surface loss processes at large angles of incidence

Intensities of positrons specularly diffracted from Cu(111) were measured at the Brandeis positron beam facility and analyzed in the energy range 8 eV<E<l34 eV for angles of incidence θ=25, 30, 35, 40, 45, 50, 52, 57, and 60°. These intensities were calculated for the known geometry of Cu(111) using a dynamical multiple scattering methodology. Above E=50 eV this methodology gives a useful account of the measured intensities using a constant imaginary optical potential of Vi=4 eV. At lower energies strong energy dependences occur, associated both with multiple elastic scattering phenomena within atomic layers of Cu parallel to the surface and with the thresholds of inelastic channels (e.g., plasmon creation). Use of the free electron calculation of Vi shows that energy dependence of inelastic processes is necessary to obtain a satisfactory description of the absolute magnitude of the diffracted intensities below E=50 eV. Detailed comparison of the calculated and observed diffraction intensities reveals the necessity of incorporating surface loss processes explicitly into the model in order to achieve a quantitative description of the measured intensities for E<40 eV and θ>40°.

Surface loss processes in the alkali fluorides studied by electron energy loss spectroscopy (EELS)

Electron energy loss spectroscopy has been used to investigate excitation processes in the band gap of the alkali fluorides. All the fluorides were found to exhibit surface exciton losses and this observation is shown to be consistent with the model of Cox and Williams [Surface Sci.175 (1986) L782] for the surface exciton loss. Other peaks in the loss spectra were shown to arise at beam induced point defects in the surface but no identification of the defect type could be made. Excitation of plasmon losses in metal liberated by the electron beam were observed in some cases but it was found that the generation of free alkali metal could be considerably reduced or even eliminated by using primary beam energies of 50 eV and below.