Calculation of photoemission intensities for Cu(111): Discontinuities at zone boundaries

Application of a bulk band structure interpolation scheme has been made to the calculation of photoemission intensities. Comparison with recent experimental data obtained on the (111)face of copper in the photon energy range 11 ? hv ? 30 eV yields satisfactory agreement. Deviations from free-electron final states are shown to be very important.     

Ohmic Contacts to ZnSe-Based Materials

The formation of ohmic contacts to n- and p-type ZnSe is reviewed. The mechanisms for forming reasonable low-resistance ohmic contacts to n-ZnSe are well understood. This results from the fact that the Fermi energy level of ZnSe is unpinned and metals with sufficiently large work functions can make contact to n-type material. However, the situation is reversed for p-ZnSe where a large band gap and large electron affinity make it impossible to find metals with sufficiently large work functions to create an ohmic contact. Instead, the use of HgSe to form low barrier height Schottky contacts and of ZnSe/ZnTe multiple quantum wells (MQWs) to form ohmic contacts is reviewed. Although the MQWs can be used to form ohmic contacts to p-ZnSe, they degrade at high temperatures and high current densities. This is reviewed and shown to be a serious problem for applications to laser diodes.     

Electrical conductivities and degrees of ionic dissociation of 1:1 and 1:2 adducts of xenon difluoride with antimony,tantalum and niobium pentafluorides in the molten state

Electrical conductivities of [XeF₂.MF₅] and [XeF₂.2MF₅] (M = Sb, Ta, Nb) have been measured. These, together with viscosity measurements on [XeF₂.SbF₅], which has the highest specific conductivity, leads to a value of the order of 11% for the degree of dissociation of the compounds near their melting points.     

Bulk diffusion in the undamped Frenkel-Kontorova model

We present new simulation results for the undamped Frenkel-Kontorova model. These support the existence of the bulk diffusion coefficient D, which has been called into question by earlier simulation work of Schneider and Stoll. We point out that D can be studied by three independent routes, and we show that these all provide evidence for the existence of D and yield the same numerical value for it in the particular thermodynamic state examined.     

Simultaneous two-wavelength narrow-band output from a pulsed dye-laser

The Hänsch stage of a tunable dye laser is modified to allow two independently tunable beams to operate at any pair of wavelengths in the dye gain profile.     

Crystal structures of high-valent transition-metal chalcogenide fluorides

The structures of high-valent transition-metal chalcogenide fluorides are of interest since it is not clear whether they will be related to those of their oxide-fluoride analogues or to those of the transition-metal chalcogenide chlorides and bromides. This will depend on the extent to which substitution of fluorine for chlorine or bromine or sulphur, or selenium for oxygen has the more significant effect.The first single-crystal structure on a compound of this class, WSF₄, shows that it consists of octahedrally coordinated tungsten atoms linked by cis-bridged fluorine atoms into polymeric chains. This, together with work on the structure of ReSF₄, suggests that the tendency is to follow oxide-fluoride and fluoride structures.     

The mechanism of reaction of tetraalkyltins with mercuric carboxylates in methanol

In the reaction of mercuric carboxylates with tetraalkyltins in methanol, rate constrants for attack of the species (RCO₂)₂ Hg increase along the series R = t-Bu < Et < Me < Ph < ClCH₂CH₂ < MeOCH₂ < ClCH₂ which suggests an open transition state for these S_E2 substitutions.     

A method for the simultaneous detection of hydrogen fluoride and fluorine

A relatively simple and rapid method for detecting hydrogen fluoride and elemental fluorine is described. A solution containing sodium bicarbonate and potassium bromide is treated with the gas. Hydrogen fluoride immediately liberates carbon dioxide from the bicarbonate; elemental fluorine immediately colours the solution and then causes the evolution of oxygen, hydrogen peroxide and hydrogen fluoride. A mixture of fluorine and hydrogen fluoride simultaneously colours the solution and evolves carbon dioxide.     

Hydrogen adsorption on metal surfaces

Extensive calculations of the ground state properties of hydrogen chemisorbed on transition metal surfaces are presented. The calculations are performed using the effective medium theory. The results for the chemisorption energies on all the 3d, 4d and 5d metals presented are in good agreement with experiment. The trends along a particular row are shown to be dominated by the degree of filling of the d band. The full adiabatic potential energy surface is presented for a number of experimentally interesting systems, including H/Ni(111), H/Ni(110), H/W(100) and H/W(110). Equilibrium sites, bond lengths, vibrational frequencies and surface diffusion energies are deduced and compared with experiment. Again, agreement is good. The surface and adsorbate parameters determining those observables are discussed. It is shown that a simple canonical relationship exists between the perpendicular vibrational frequency and the metal-hydrogen bond length. This formulation, which is not based on pair potentials, should be useful as a first estimate of bond lengths from measured vibrational data.