P. H. Citrin

Unimportance of siloxene in the luminescence of porous silicon

Near‐edge‐ and extended‐x‐ray absorption fine structure measurements, as well as luminescence excitation and emission spectra, were obtained from samples of porous Si and siloxene. Combined, these data establish that, contrary to a recently proposed explanation, siloxene is not generally responsible for the observed room‐temperature luminescence in porous Si.

A soft/hard x‐ray beamline for surface EXAFS studies in the energy range 0.8–15 keV

A new ultrahigh‐vacuum x‐ray beamline for surface EXAFS experiments has been constructed at the 2.5‐GeV storage ring at Brookhaven National Laboratory, USA. Its design minimizes or eliminates a variety of problems inherent in other beamlines, providing state‐of‐the‐art intensity, brightness, resolution, and beam stability over an extremely wide range of energy. Performance is discussed with initial results obtained after the recent x‐ray ring shutdown.

X-ray absorption spectroscopy applied to surface structure : SEXAFS and NEXAFS

Abstract The application of both surface extended X-ray absorption fine structure (SEXAFS) for determining adatom-substrate distances and adsorption sites, and near-edge X-ray absorption fine structure (NEXAFS) for determining adsorbed molecular orientations and bonding changes, is simple, direct, and now routine. The events leading up to developing these methods, however, were neither simple nor direct, and by no means were they routine. Communities of X-ray spectroscopists, synchrotron radiation researchers, and surface scientists overlapped to create a particularly special and fertile environment. This article presents a brief overview of these scientific and sociological events from a personal perspective, with emphasis on some of the earlier history not familiar to many surface science practitioners. Comments about the capabilities, limitations, and accomplishments of SEXAFS and NEXAFS are also mentioned.

Can electrical deactivation of highly Si‐doped GaAs be explained by autocompensation?

Using near‐edge x‐ray absorption fine structure, the first experimental determination of Si atom concentrations occupying As sites in Si‐doped GaAs (100) is reported. The measurements reveal that at high doping levels (≳1019 cm−3) in molecular‐beam‐epitaxy‐grown samples, the number of such p‐type Si atoms is insufficient to account for the observed large reduction of free‐carriers.

Core-hole lifetime and screening are different in the surface of W(110)

High resolution 4f photoemission spectra from clean W(110) show that the natural lifetime width and the singularity index, which characterizes the conduction electron screening, are both larger in the first atomic layer than in the bulk. The phonon broadening of the surface and bulk components are smaller than earlier theoretical estimates. The excess broadening at the surface is compatible with a simple Debye model. These findings are very different from the interpretation previously given surface-atom core-level line shapes and have implications extending to other systems.

Sexafs: New horizons in surface structure determinations

Abstract As early as the 1930s, it had been observed that the absorption cross section in the X-ray regime had a complex structure as a function of energy extending as far as 1000 eV above an absorption threshold.1 These measurements were made by using a low‐power conventional X-ray tube, a dispersive Bragg spectrometer, and film as a detector. After some correct and incorrect speculation concerning the origin of this extended X-ray absorption fine structure (EXAFS), the field remained essentially dormant through the 1950s. In the late 1960s the measurements were revived,2 this time using modern counting and automation techniques. The previous interpretation3–5 of the structure as arising from the backscattering of the ejected photoelectron from the atom near the absorbing site was strongly supported. The idea that this phenomenon might be used as a local structural probe was born. However, even with the experimental advances the measurements were difficult and time-consuming because of the low power of t...

Surface-ion core-level shifts in alkali halides

Abstract Surface-ion core-level shifts from evaporated films of K, Rb, and Cs chlorides, bromides and iodides have been identified in high resolution photoemission spectra. The measured shifts of the alkali-metal surface ions in the six NaCl-structure compounds average 0.45 ± 0.03 eV to higher binding energy. Subtracting from these shifts the change in Madelung potential at the surface gives the corresponding change in the surface polarization energies, averaging 0.17 ± 0.02 eV. Shifts of the halide surface ions are found to be significantly smaller. All of these results are in qualitative agreement with theoretical expectations of surface Madelung potential and polarization energy changes, both of which are positive for the cation but of opposite sign for the anion. The data from the Cs compounds have a much larger scatter due to the polymorphism upon thin film growth.