Understanding the phase-change mechanism of rewritable optical media.

Present-day multimedia strongly rely on rewritable phase-change optical memories. We demonstrate that, different from the current consensus, Ge(2)Sb(2)Te(5), the material of choice in DVD-RAM, does not possess the rocksalt structure but more likely consists of well-defined rigid building blocks that are randomly oriented in space consistent with cubic symmetry. Laser-induced amorphization results in drastic shortening of covalent bonds and a decrease in the mean-square relative displacement, demonstrating a substantial increase in the degree of short-range ordering, in sharp contrast to the amorphization of typical covalently bonded solids.

Development of XAFS theory.

A major goal of theoretical simulations of X-ray absorption fine structure (XAFS) is to provide calculations for the interpretation and analysis of experimental data in terms of geometrical and electronic information. The extended region or EXAFS (50-2000 eV above an absorption edge) contains geometric information about the pair distribution function, i.e.

New developments in the theory and interpretation of X-ray spectra based on fast parallel calculations.

There has been dramatic progress over the past decade both in theory and in ab initio calculations of X-ray absorption fine structure. Significant progress has also been made in understanding X-ray absorption near-edge structure (XANES). This contribution briefly reviews the developments in this field leading up to the current state.

Mass Absorption Coefficient of Tungsten and Tantalum, 1450 eV to 2350 eV: Experiment, Theory, and Application.

The mass absorption coefficients of tungsten and tantalum were measured with soft x-ray photons from 1450 eV to 2350 eV using an undulator source. This region includes the M3, M4, and M5 absorption edges. X-ray absorption fine structure was calculated within a real-space multiple scattering formalism; the predicted structure was observed for tungsten and to a lesser degree tantalum as well.

Hole counts from X-ray absorption spectra.

The interpretation of X-ray absorption spectra in terms of electronic structure has long been of interest. Hole counts derived from such spectra are often interpreted in terms of free-atom occupation numbers or Mülliken counts. It is shown here, however, that renormalized-atom (RA) and cellular counts are better choices to characterize the configuration of occupied electron states in molecules and condensed matter.

Progress and challenges in the theory and interpretation of X-ray spectra.

There has been dramatic progress over the past decade both in theory and in ab initio calculations of X-ray absorption fine structure (XAFS). Rapid progress is now being made in understanding X-ray absorption near-edge structure (XANES). This presentation reviews the developments in this field by many groups leading up to the current state of the art.

Pt L-edge XANES as a probe of Pt clusters.

The sensitivity of Pt L-edge XANES to local geometric and electronic structure in various Pt(n) clusters is investigated using the ab initio self-consistent FEFF8 code. Calculations based on FEFF8 are found to be in good agreement with experiment. For pure Pt clusters the XANES can distinguish between 2- and 3-dimensional clusters.

Effect of hydrogen adsorption on the x-ray absorption spectra of small Pt clusters.

Hydrogen adsorption on Pt(6)H(n) clusters leads to striking changes in the Pt L(2,3) x-ray absorption spectra. These effects are interpreted using a self-consistent real space Green's function approach. Calculations show that they are due largely to changes in the atomic background contribution to x-ray absorption (i.

Extended X-Ray absorption fine structure from hydrogen atoms in water.

We report the first quantitative measurement of extended x-ray absorption fine structure (EXAFS) from hydrogen atoms. A single oscillation is observed from gaseous water consistent with the location of the covalently bonded hydrogen in H 2O. The experimental phase and amplitude of the oscillation are in excellent agreement with curved wave multiple scattering calculations for isolated water molecules.

Shape resonances of oriented molecules: ab initio theory and experiment on hydrocarbon molecules.

We report ab initio calculations of the x-ray absorption cross section for the near edge x-ray absorption fine structure of C2H6, and C2H4, and C2H2 at the C K-edge, based on a full multiple scattering formalism. The angular dependence of the electric dipole transition in the calculations as well as the angular dependent experiments for the oriented molecules give a good opportunity to compare both. The resonance can be assigned to a sigma(*) shape resonance.