A high transmission hemispherical energy analyzer for ion spectrometry

A hemispherical energy analyzer was constructed by using a novel approach to control the fringing electrostatic field. It provides several properties useful in ion spectrometers: namely, rather simple fabrication and compact size, high transmission efficiency at moderate resolution, and the capability to adjust resolution by changing the intersphere potentials. A computer program was developed to evaluate ion trajectories through the hemispherical analyzer. Data obtained from the trajectories were used to predict the characteristics of the analyzer. Experiments performed to determine the kinetic energy dependence of the absolute transmission and the resolution functions are in accord with theoretical calculations.     

Nutrient cycling responses to fire frequency in the Kruger National Park (South Africa) as indicated by Stable Isotope analysis

Fires, which are an intrinsic feature of southern African ecosystems, produce biogenic and pyrogenic losses of nitrogen (N) from plants and soils. Because of the long history of fires in these savannas, it was hypothesized that N 2 fixation by legumes balances the N losses caused by fires. In this study, the N 2 fixation activity of woody legumes was estimated by analyzing foliar i 15 N and proportional basal area of N 2 fixing species along experimental fire gradients in the Kruger National Park (South Africa). In addition, soil carbon (C) and N pools, foliar phosphorus (P) and gross N mineralization and nitrification rates were measured, to indicate the effects of fires on nutrient stocks and the possible N cycling processes modified by fires. Although observations of increased soil C/N and mineralization rates in frequently burned plots support previous reports of N losses caused by fires, soil %N did not decrease with increasing fire frequency (except in 1 plot), suggesting that N losses are replenished in burned areas. However, relative abundance and N 2 fixation of woody legumes decreased with fire frequency in two of the three fire gradients analyzed, suggesting that woody legume N 2 fixation is not the mechanism that balances N losses. The relatively constant %N along fire gradients suggests that these ecosystems have other mechanisms to balance the N lost by fires, which could include inputs by atmospheric deposition and N 2 fixation by forbs, grasses and soil cyanobacteria. Soil isotopic signatures have been previously used to infer patterns of fire history. However, the lack of a relationship between soil i 15 N and fire frequency found in this study indicates that the effects of fires on ecosystem i 15 N are unpredictable. Similar soil i 15 N along fire gradients may reflect the contrasting effects of increased N gaseous emissions (which increases i 15 N) and N 2 fixation other than that associated with woody legumes (which lowers i 15 N) on isotopic signatures.     

SONET-Framed Optical Packet Networking in the Context of Generalized Multiprotocol Label Switching

This article discusses the evolution of Synchronous Optical Network (SONET) towards support for shared packet media, particularly in light of new developments in the control plane, such as Generalized Multiprotocol Label Switching (GMPLS) and the (UNI) specification. Whereas the current SONET network is largely static and circuitcentered, and typically supports DS-1 and DS-3-level granularities, the emerging packet-centric, GMPLS-controlled optical network will be significantly more dynamic, and characterized by concatenated or virtually concatenated payloads such as STS-3c/12c/48c/192c, and, possibly, STS-1-24v (i.e., 'STS-24c'). In this environment, it will be shown that the differences between such packet-centric SONET networks and 'IP over Optical' network architectures begin to disappear.     

Polyethylene Sandwich Technique for Far Infrared Powder Samples

Recent research on the far infrared absorption of the free radical “tanane” (nitroxyde tetramethyl-2, 2, 6, 6,-piperidine oxyle, C₉H₁₈NO) in connection with its ferroelectric-ferroelastic transition at 14° C^{1, 2, 3} has shown that conventional sample preparation techniques are unsuitable. A new method which offers certain advantages has been developed and is described here since it has potential applicability to other powder samples and can be readily adapted for measurements at higher frequencies.     

Cartesian Dynamics of Simple Molecules VIII planar quadratomics (D3h symmetry)

The vibrations of planar A₃B type molecules with D_3h symmetry are analysed, by treating these molecules as a special case of A₂BC types with C_2v symmetry. Analytical expressions for the four normal mode frequencies are derived in terms of four force constants, and optimized values of the latter for the boron trihalides and sulphur trioxide are obtained and briefly discussed. Frequencies of various isotopic species are also calculated.     

Cartesian Dynamics of Simple Molecules III Non-Linear Triatomics (C2v Symmetry)

A simple spring model for the molecular vibrations of non-linear triatomic molecules with C_2v symmetry is described in terms of Cartesian co-ordinates. Analytical expressions for the stretching and bending mode frequencies are obtained and compared with previous derivations. Optimized values of force constants are calculated for models involving two or three adjustable parameters. The validity of the model is demonstrated by the satisfactory agreement between calculated and observed frequencies of isotopic species. The model provides a simple explanation of the near degeneracy of the symmetric and asymmetric stretching frequencies of H₂S and H₂Se.     

Cartesian Dynamics of Simple Molecules II Non-Centrosymmetric Linear Triatomics

A simple spring model for molecular vibrations, which uses Cartesian co-ordinates for both longitudinal and transverse displacements, is applied to non-centrosymmetric linear triatomic molecules. Analytical expressions for the stretching and bending mode frequencies are obtained, which are equivalent to those derived by conventional methods. For most molecules, the effects of the interaction between the outside atoms are shown to be negligible, but for N₂O, complex solutions are obtained unless this is included. The validity of the model is demonstrated by the satisfactory agreement between calculated and observed frequencies of isotopic species. For N₂O, frequency shifts resulting from isotopic substitution are explained by reference to the calculated eigenvectors.     

Anchoring transitions of liquid crystals on SiO x

Anchoring transitions were observed of nematic liquid crystal (LC) mixtures on obliquely evaporated SiO _x by varying the relative abundance of two components in the mixture. Of these two components, one has a longitudinal cyano group and another has lateral cyano groups. It was also found that both temperature and SiO _x thickness variations are able to shift the anchoring transitions. The anchoring on SiO _x has two origins: long‐range van der Waals potential and short‐range surface interactions. Since the two origins have opposite preference in LC director orientation the observed transition is caused by the change of their relative strength. Thermal absorption–desorption of molecules on SiO _x surface is important in determining the strength of short‐range interactions, whereas the layer thickness and optical properties of SiO _x are dependent on the van der Waals potential. Based on previous work by de Gennes we propose a model to describe the observed phenomenon.