A re-evaluation of isotope shift constants

The field isotope shift constantC _unif for a uniform nuclear charge distribution has been re-evaluated fors-electrons for elements with 10≦Z≦ 95. A table is given which permitsC _unif to be found for any isotope pair.     

Angular-dependent XPS valence-band spectra from single-crystal copper

High-resolution angular-dependent X-ray photoemission (XPS) valence-band spectra from a copper single crystal with (001) orientation are presented and analyzed in terms of a direct-transition model of photoemission. This model includes the effects of photon momentum, spectrometer geometry, and, for very low angles of emission, refraction of electrons in leaving the surface, but neglects matrix-element variation. Theoretical calculations are found to be in good agreement with experiment for six distinct emission directions. For low (grazing) angles of emission, a sharpening of the d-band peak is observed that may be associated with surface-specific band-structure changes.     

Backward-wave interaction using step periodic structures

Periodicities introduced into hollow waveguides with step and turn symmetry are shown to have characteristics useful for backward-wave interaction in microwave electron tubes. Using Floquet's theorem, it is shown that the modes of a hollow waveguide can be shifted in phase over one period of the structure. Interaction with the shifted backward-wave mode is then possible over a range of frequencies. A gyrotron backward-wave oscillator is particularly suited to this structure; the interaction could be magnetically tuned over a range of frequencies, while a low phase constant is maintained. A lower phase constant allows the electron beam to interact coherently with the backward wave over longer distances, since any velocity spread in the electron beam forces some of the electrons to be out of step with the backward wave, thus reducing efficiency. A solid waveguide has the added advantages of being easily built for high frequencies and rugged for high temperatures.     

The angular dependence of plasmon loss features in XPS spectra from polycrystalline aluminum: Clean surfaces and effects of oxygen adsorption

The angular dependence of relative intensities of bulk- and surface-plasmon loss peaks associated with X-ray photoemission from the Al 2p core level has been measured for aluminum samples under ultrahigh vacuum conditions. At near-grazing electron exit angles, a sixfold increase in the single-surface-plasmon relative intensity is observed, while the single-bulk-plasmon relative intensity is decreased by ～20%. These experimental results are compared with theoretical calculations based on a dispersionless-plasmon jellium model and including both extrinsic and intrinsic plasmon creation. The presence of intrinsic creation is suggested and theoretical relative intensities are in good agreement with, although generally somewhat higher than, corresponding experimental values. For specimens with low coverages of oxygen (～0.15 monolayer), only surface-plasmon loss features are found to be associated with the O 1s core level, indicating near-surface adsorption.     

Surface analysis and angular distributions in x-ray photoelectron spectroscopy

A theoretical and experimental study of the application of x-ray-photoelectron angular distribution measurements to quantitative surface characterizations is presented. The basic theoretical model that has been used previously to analyze such angular distributions from flat surfaces in the absence of electron-diffraction (channeling) effects is discussed, including certain new generalizations and special cases pertinent to surface analysis. Previous experimental work is reviewed. The predictions of this model are also found to be consistent with new experimental data obtained from gold specimens with carbon-containing surface layers and from aluminum specimens with successive oxide- and carbon-containing layers. An order of magnitude increase in surface-layer relative intensities is observed at low electron escape angles relative to the surface. Also, effects due to x-ray refraction and reflection are found for very low angles of incidence, and these lead to approximately a four-fold increase in surface-layer relative intensities. Extensions of the theory to include the effects of non-uniform x-ray flux, a more realistic spectrometer acceptance function, non-uniformity of surface layers, and surface roughness are also considered, and numerical calculations for the specific case of a sinusoidally rough surface are presented. It is shown that rough-surface intensities will equal flat-surface intensities provided that both surfaces are clean and that no x-ray shading occurs. If surface layers are present, however, rough-surface angular distributions are predicted to deviate markedly from flat-surface distributions. By means of angular distribution measurements, it thus appears possible to selectively enhance near-surface contributions to photoelectron spectra, as well as to obtain information concerning electron mean free paths, surface layer thicknesses and uniformity, and perhaps surface roughnesses.     

Relative stabilities of acyl- and formyl-rhodium complexes

The various complexes RhCl₂(COR)(PPh₃)₂ and/or RhRCl₂CO(PPh₃)₂ (R  H, Me, Et, n-Pr, CH₂Cl, CH₂Ph, CH₂CH₂Ph, Cl) are generated in solution, and factors affecting the positions of equilibrium between the five-coordinate and six-coordinate isomers are discussed. The complex RhCl₂(COMe)[(Ph₂P)₂(CH₂)₃] is far more stable with respect to isomerization to a six-coordinate methyl isomer than are any of the triphenylphosphine complexes studied.     

Kaonic Hydrogen atom X-rays

The X-ray spectrum obtained with kaons stopping in liquid hydrogen has been measured. Possible candidates for X-ray lines from kaonic hydrogen atoms have been identified and the results compared with previous experiments and with theoretical predictions. X-ray lines from σ^?p atoms may also have been observed.     

Topology of generalized complex quotients

Consider the Hamiltonian action of a torus on a compact twisted generalized complex manifold M$M$. We first observe that Kirwan injectivity and surjectivity hold for ordinary equivariant cohomology in this setting. Then we prove that these two results hold for the twisted equivariant cohomology as well.