Roger A. Pollak

Surface core-level shifts and surface valence change in mixed-valent YbAl2

Abstract High-resolution photoemission studies show that about two surface layers of mixed-valent YbAl2 are divalent. These layers exhibit large surface core-level shifts of Δs1=0.92 eV for the topmost and Δs2=0.35 eV for an underlying surface layer, allowing a separation of the divalent part of the spectrum into bulk and surface contributions. In this way, a mean valence of 2.4±0.1 can be derived for bulk YbAl2. The observed Δs1/Δs2 ratio is consistent with a recent theoretical prediction.

Oxidation behavior of PdSi compounds

Abstract The room temperature oxidation of PdSi, Pd 2 Si and Pd 4 Si has been studied using X-ray photoelectron spectroscopy (X-ray photoemission spectroscopy or electron spectroscopy for chemical analysis). We find that only silicon atoms in these silicides are oxidized and the oxidation of Pd 4 Si surfaces is enhanced compared with that of Pd 2 Si and PdSi, as is evidenced by both a higher silicon oxidation state and thicker oxide films. This behavior is discussed in terms of silicide stability and a spill-over effect where palladium atoms catalyze molecular oxygen dissociation.

Room‐temperature oxidation of Ni, Pd, and Pt silicides

The room‐temperature oxidation of Ni, Pd, and Pt silicides has been studied using electron spectroscopy for chemical analysis. It has been found that (1) Si atoms oxidize predominantly, (2) the oxidation of Si is enhanced in the metal‐rich silicides, and (3) the growth of the surface oxide layer leaves behind a phase enriched with metal.

On demand web-client technologies

This paper describes a comprehensive set of technologies that enables rich interaction paradigms for Web applications. These technologies improve the richness of user interfaces and the responsiveness of user interactions. Furthermore, they support disconnected or weakly connected modes of operation. The technologies can be used in conjunction with many Web browsers and client platforms, interacting with a J2EE™ server. The approach is based on projecting the server-side model-view-controller paradigm onto the client. This approach is firmly rooted in the Web paradigm and proposes a series of incremental extensions. Most of the described technologies have been adopted by IBM server (WebSphere®) and client products.

Laser‐pulsed plasma chemistry: Laser‐initiated plasma oxidation of niobium

We report the first observation of the chemical modification of a solid surface exposed to an ambient gas plasma initiated by the interaction of laser radiation with the same surface. A new technique, which we designate laser‐pulsed plasma chemistry (LPPC), is proposed for activating heterogeneous chemical reactions at solid surfaces in a gaseous ambient by means of a plasma initiated by laser radiation. Results for niobium metal in one atmosphere oxygen demonstrate single‐pulse, self‐limiting oxide growth induced by a pulsed CO2 laser. X‐ray photoelectron spectroscopy (XPS or ESCA) was used to monitor surface chemical composition changes and thickness control of thin (1 to 5 nm) reaction product layers. The dependence of single‐pulse oxide growth upon laser fluence is observed to be monotonic for oxide thicknesses up to 5 nm. Composition of the oxide Nb2O5−δ, formed by such an optically driven plasma, is similar to that formed by low‐temperature oxidation processes such as rf plasma oxidation; however, t...

Hybridization and bonding in transition metal compound superconductors: X-ray photoemission from crystalline and amorphous Nb3Ge.

Abstract An XPS (or ESCA) study of the high T c crystalline and the low T c amorphous phases of Nb 3 Ge indicates a change of the Nb-Ge bonding from covalent to metallic upon the amorphous to crystalline transition. The high T c superconductivity of Nb 3 Ge does not appear to stem only from an unusually high electronic density of states at the Fermi level but rather from another property such as a resonant enhancement of the electron-phonon coupling resulting from cation-anion hybridization at E F .

The electronic structure of SiO2, GeO2 and intermediate SixGe1−xO2 compositions: experiment and theory☆

Abstract Theoretical calculations are reported for SiO2, GeO2 and the intermediate composition Si.5Ge.5O2 which reproduce the main observed features and trends in experimental photoemission spectra. The agreement between the two establishes the importance of band theory in understanding the electronic structure of these materials, and demonstrates that detailed quantitative predictions are feasible for such complex materials in terms of the empirical tight-binding method. The calculations further establish that the structure in the valence bands is determined mainly by nearest- neighbor oxygen-oxygen interactions.

Interaction between chromium oxide and chromium silicide

We have used x‐ray induced photoemission spectroscopy and Rutherford backscattering spectroscopy to study the chemical reaction between Cr2O3 and CrSi2. We observed that upon annealing a Cr film on a Si substrate at 550 °C to form CrSi2, the native chromium surface oxide will decompose while a film of SiO2 will form when the CrSi2 growth front reaches the Cr2O3.

Metal Atom Reactions with Polymer Films

The chemical nature of the interface formed when an electropositive metal (chromium) is evaporated on the surface of oxygen-containing polymers has been investigated by synchrotron-radiation excited photoemission spectroscopy and by near-edge X-ray absorption fine stnrcture spectroscopy (NEXAFS). PMDA-ODA polyimide was studied along with two polymers intended to more simply model the complex chemical functionality of polyimide. Core-level spectra were resolved into distinct peaks, which were assigned to specific atoms in the polymer units, and changes in the spectra were monitored as chromium metal atoms were evaporated onto the surface. Similarly, NEXAFS spectra were taken as a function of chromium deposition. Results of these experiments show that chromium initially reacts very rapidly with surface carbonyl groups, but less strongly with ether oxygens or arene rings. In all polymers studied, formation of an electron-rich, carbide-like species was observed.

Surface and bulk valence of Y0.9Yb0.1Al2 and La0.9Yb0.1Al2

Photoemission and susceptibility measurements have been made on alloys of 10% YbAl2 with 90% YAl2 or LaAl2. Similar to YbAl2, Yb in the YAl2 host is intermediate valent in the bulk consistent with a single ion picture. Yb in the LaAl2 host has a valence of 2.0–2.1 in the bulk. In both cases the surface is completely divalent and the surface 4f emission is shifted to higher binding energies relative to the bulk 4f emission.