Alberto Herrera-Gomez

Modern theory and applications of photocathodes

A theory of photoemission is presented which has proven over the last thirty years to provide a very useful description of this process, both for fundamental and practical applications. This is the Three Step Model of Spicer. By treating photoemission in terms of three successive steps-- optical absorption, electrons transport, and escape across the surface, it allows photoemission to be related to parameters of the emitter, such as optical absorption coefficient, electron scattering mechanisms, and the height of the potential barrier at the surface. Using simple equations and established parameters, this theory is very useful in predicting the performance of cathodes and in understanding in detail the unexpected phenomena appearing when photocathodes are pushed into new practical domains.

Chemical depth profile of ultrathin nitrided SiO2 films

Ultrathin nitrided SiO2/Si(001) films were studied using angle-resolved x-ray photoemission spectroscopy. The structure of the oxynitride depended on the nitridation process. Under one type of nitridation the film kept the structure of the SiO2, with N assuming O sites. By taking advantage of the nonuniformity on the chemical depth profile, the Si 2p chemical shift was determined for those Si atoms bonded to three O and one N atom, and for those bonded to two O and two N atoms. The stoichiometry depth profile was recognized through a simple method that allowed the input of physical constrains.

Identification of bound water through infrared spectroscopy in methylcellulose

This paper describes the interaction of water with solid polymers. A portion of the water adsorbed in hydrophilic materials is strongly bound to individual sites (bound water), while an additional amount is bound less firmly to the polymeric chain (free water). Both, free and bound water, have been the subject of many studies that have uncovered, in some cases, qualitative evidence of the presence of bound water. In this paper we report a method to quantify the fraction of water adsorbed in the first layer in contact with an hydrophilic material. The measurement was carried out on infrared spectroscopy data of methylcellulose films exposed to several humid environments. The quantification was based on the determination of the change in the natural vibration frequency brought about by the interaction of water molecules with the solid.

Photoemission from the Sr/Si(001) interface

The growth of Sr on n-type Si(001) was studied in detail for coverages between 0 and 1 monolayer (ML) using core level photoemission spectroscopy. In a similar manner, the Sr saturation coverage was studied in the 600–925 °C temperature range. Data analysis was carried out by a method that allows accurate determination of the band-bending shifts. Using this method it is possible to pinpoint the formation and destruction of chemical species from bungled core level photoemission data without needing to know details of the chemical composition of the spectra. Through this analysis it was established that the interaction between Sr and Si breaks down the binding energy difference between upward and downward Si dimer atoms. In addition, it was found that the saturation coverage exhibits a clear plateau at 1 ML around 650 °C, and a slope change at 1/3 ML around 850 °C. The surface band bending suffers a discontinuous increase as the Sr coverage surpasses 12 ML and as the low energy electron diffraction symmetry...

Analysis of the water bound to a polymer matrix by infrared spectroscopy

This article focuses on the interaction of water with solid polymers. A portion of the water adsorbed in hydrophilic materials is strongly bound to individual sites (bound water), while an additional amount is bound less firmly to the polymeric chain (free water). Both free and bound water have been the subject of many studies that have uncovered, in some cases, qualitative evidence of the presence of water directly bound. In this article, we report a method to determine the different fractions of water adsorbed in hydrophilic materials. One fraction corresponds to the first layer in contact with the adsorbing solid and the other corresponds to the remaining water absorbed layers. We were able to determine said quantification by means of measuring the change in the natural vibration frequency brought about by the interaction of water molecules with the solid. The measurement was done on films of methylcellulose exposed to humid environments using infrared spectroscopy and gravimetric static microclime sor...

Elastic anomaly for Sr Ti O 3 thin films grown on Si(001)

X-ray diffraction measurements have revealed a negative Poissons ratio for

Calculated electron energy distribution of negative electron affinity cathodes

\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}

Physics of high-intensity nanosecond electron source: Charge limit phenomenon in GaAs photocathodes

thin films grown on Si(001). X-ray absorption fine-structure measurements demonstrate that this elastic anomaly is driven by the interfacial polarization of the

Evolution of the crystallographic position of As impurities in heavily doped Si crystals as their electrical activity changes

\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}

Electron transverse energy distribution in GaAs negative electron affinity cathodes: Calculations compared to experiments

layers. First-principles density-functional calculations support these conclusions. It is suggested that this phenomenon may be common for heteroepitaxial growth of materials that possess an ionic polarizability.