Quantitative modeling of \textit{in situ} x-ray reflectivity during organic molecule thin film growth

Synchrotron-based x-ray reflectivity is increasingly employed as an \textit{in situ} probe of surface morphology during thin film growth, but complete interpretation of the results requires modeling the growth process. Many models have been developed and employed for this purpose, yet no detailed, comparative studies of their scope and accuracy exists in the literature. Using experimental data obtained from hyperthermal deposition of pentane and diindenoperylene (DIP) on SiO$_2$, we compare and contrast three such models, both with each other and with detailed characterization of the surface morphology using ex-situ atomic force microscopy (AFM). These two systems each exhibit particular phenomena of broader interest: pentacene/SiO$_2$ exhibits a rapid transition from rough to smooth growth. DIP/SiO$_2$, under the conditions employed here, exhibits growth rate acceleration due to a different sticking probability between the substrate and film. In general, \textit{independent of which model is used}, we find good agreement between the surface morphology obtained from fits to the \insitu x-ray data with the actual morphology at early times. This agreement deteriorates at later time, once the root-mean squared (rms) film roughness exceeds about 1 ML. A second observation is that, because layer coverages are under-determined by the evolution of a single point on the reflectivity curve, we find that the best fits to reflectivity data --- corresponding to the lowest values of $\chi_\nu^2$ --- do not necessarily yield the best agreement between simulated and measured surface morphologies. Instead, it appears critical that the model reproduce all local extrema in the data. In addition to showing that layer morphologies can be extracted from a minimal set of data, the methodology established here provides a basis for improving models of multilayer growth by comparison to real systems.Comment: 34 pages (double-spaced, including figures and references), 10 figures, 3 appendice

Noise-free high-efficiency photon-number-resolving detectors

High-efficiency optical detectors that can determine the number of photons in a pulse of monochromatic light have applications in a variety of physics studies, including post-selection-based entanglement protocols for linear optics quantum computing and experiments that simultaneously close the detection and communication loopholes of Bell's inequalities. Here we report on our demonstration of fiber-coupled, noise-free, photon-number-resolving transition-edge sensors with 88% efficiency at 1550 nm. The efficiency of these sensors could be made even higher at any wavelength in the visible and near-infrared spectrum without resulting in a higher dark-count rate or degraded photon-number resolution.Comment: 4 pages, 4 figures Published in Physical Review A, Rapid Communications, 17 June 200

Summary Abstract: The oxidation of carbon monoxide on the Pt(110)-(1×2) surface: The influence of the adlayer composition on the reaction dynamics

We have developed and employed the technique of surface temperature modulation to examine the dynamics of the oxidation of Ca on the Pt(110)-(1×2) surface [2]. This technique permits a straightforward analysis of the energetics of elementary surface reactions even in the presence of strong nonlinearities, providing a decided advantage over competing techniques such as modulated molecular-beam reactive scattering. We shall emphasize here the coverage dependence of the kinetic rate parameters (Er and kr(2)Z)) that describe the elementary surface reaction CO(a) + O(a) → CO2 (g). These data will be compared briefly with results obtained previously concerning the coverage dependence of the energy retained in the translational [3] and internal [4,5] modes of the product CO2 molecule

A facility to remotely assemble radioisotope thermoelectric generators

Radioisotope Thermoelectric Generators (RTGs) are electrical power sources that use heat from decaying radioisotopes to directly generate electrical power. The RTG assembly process is performed in an inert atmosphere inside a large glovebox, which is surrounded by radiation shielding to reduce exposure to neutron and gamma radiation from the radioisotope heat source. In the past, allowable dose rate limits have allowed direct, manual assembly methods; however, current dose rate limits require a thicker radiation shielding that makes direct, manual assembly infeasible. To minimize RTG assembly process modifications, telerobotic systems are being investigated to perform remote assembly tasks. Telerobotic systems duplicate human arm motion and incorporate force feedback sensitivity to handle objects and tools in a human-like manner. A telerobotic system with two arms and a three-dimensional (3-D) vision system can be used to perform remote RTG assembly tasks inside gloveboxes and cells using unmodified, normal hand tools

Summary Abstract: The hydrogenolysis of alkanes over single-crystalline surfaces of iridium: The influence of surface structure on the catalytic selectivity

We have investigated the hydrogenolysis of various short-chain alkanes on the Ir(111) and Ir(110)-(1×2) surfaces. A striking correlation has been discovered between the selectivity for ethane production from the hydrogenolysis of n-butane over Ir catalysts and the concentration of low-coordination-number metal surface atoms. This result is best interpreted as a manifestation of the occurrence of a particular adsorbed reaction intermediate on the Ir(11O)-(1×2) surface, namely, a metallacyc1e pentane. The formation of this intermediate is sterically forbidden on the Ir(111) surface

Characterization of Chimeric Lipopolysaccharides from Escherichia coli Strain JM109 Transformed with Lipooligosaccharide Synthesis Genes (lsg) from Haemophilus influenzae

Previously, we reported the expression of chimeric lipopolysaccharides (LPS) in Escherichia coli strain JM109 (a K-12 strain) transformed with plasmids containing Haemophilus influenzae lipooligosaccharide synthesis genes (lsg) (Abu Kwaik, Y., McLaughlin, R. E., Apicella, M. A., and Spinola, S. M. (1991) Mol. Microbiol. 5, 2475–2480). In this current study, we have analyzed the O-deacylated LPS and free oligosaccharides from three transformants (designated pGEMLOS-4, pGEMLOS- 5, and pGEMLOS-7) by matrix-assisted laser desorption ionization, electrospray ionization, and tandem mass spectrometry techniques, along with composition and linkage analyses. These data show that the chimeric LPS consist of the complete E. coli LPS core structure glycosylated on the 7-position of the non-reducing terminal branch heptose with oligosaccharides from H. influenzae. In pGEMLOS-7, the disaccharide Gal13 3GlcNAc13 is added, and in pGEMLOS-5, the structure is extended to Gal134GlcNAc133Gal133GlcNAc13. PGEMLOS-5 LPS reacts positively with monoclonal antibody 3F11, an antibody that recognizes the terminal disaccharide of lacto-N-neotetraose. In pGEMLOS-4 LPS, the 3F11 epitope is apparently blocked by glycosylation on the 6-position of the terminal Gal with either Gal or GlcNAc. The biosynthesis of these chimeric LPS was found to be dependent on a functional wecA (formerly rfe) gene in E. coli. By using this carbohydrate expression system, we have been able to examine the functions of the lsg genes independent of the effects of other endogenous Haemophilus genes and expressed proteins

Comment: Cultural eutrophication of natural lakes in the United States is real and widespread

This is the published version

Women candidates and councillors in Scottish local government, 1974-2012

While significant attention has been paid to the levels of representation of women in both the Westminster Parliament and the Scottish Parliament, much less considered has been given to the position within local government. This article addresses that deficit for Scotland. It shows that for twenty-five years following the reorganisation of local government in Scotland in 1974 there was a slow but relatively steady increase in the numbers of female candidates and councillors, although more recently this appears to have since plateaued somewhat, together with a similar increase in the number of women councillors taking up more senior roles in Scotland’s councils. The article analyses the representation of women in Scottish local government over the period from 1974 to 2012 against the backdrop of significant change in Scotland, including a further restructuring of local government and the introduction of the Single Transferable Vote for council elections, the creation of the Scottish Parliament, the rise of the SNP and the decline of the Conservative Party

Comparative studies of alkane activation by low-index surfaces of iridium and platinum

The interaction of alkanes with low-index surfaces of iridium at low temperatures and pressures has been studied in our laboratory by thermal desorption mass spectrometry and low-energy electron diffraction. While the reconstructed (110)-(1 × 2) surface of iridium initiates dissociative chemisorption of ethane and all higher alkanes at 125 K in the low coverage limit, similar reactions with the close-packed (111) surface occur readily only at defect sites. Thus, surface geometry influences strongly the activation barriers to C-H bond scission in alkanes on iridium surfaces