Analysis of rotational coupling in collisions of Li+ with Ne leading to double excitation of Ne \ud

Electron angular distributions due to autoionization of Ne, doubly excited to the (2p43s2)1D state in collisions with Li+ in the energy range 1.2-2.2 keV, are measured in coincidence with Li+ scattered into a well defined direction ( Phi =0 degrees , Theta cm=10.8 degrees ). The experimental findings are analysed with the help of a collision model proposed earlier. In this model the initial excitation occurs by radial diabatic coupling to a molecular Sigma -state at small distances, followed by rotational coupling to Pi - and Delta -states at intermediate distances in the second half of the collision. The energy splitting between the Sigma -, Pi - and Delta -states is described by a model function. By adapting two parameters of this model function, the experimental findings can be reproduced within the experimental error in numerical calculations involving the relevant set of coupled differential equations. \u

Stacking dependence of carrier transport properties in multilayered black phosphorous

We present the effect of different stacking orders on carrier transport properties of multi-layer black phosphorous. We consider three different stacking orders AAA, ABA and ACA, with increasing number of layers (from 2 to 6 layers). We employ a hierarchical approach in density functional theory (DFT), with structural simulations performed with Generalized Gradient Approximation (GGA) and the bandstructure, carrier effective masses and optical properties evaluated with the Meta-Generalized Gradient Approximation (MGGA). The carrier transmission in the various black phosphorous sheets was carried out with the non-equilibrium Greens function (NEGF) approach. The results show that ACA stacking has the highest electron and hole transmission probabilities. The results show tunability for a wide range of band-gap, carrier effective masses and transmission with a great promise for lattice engineering (stacking order and layers) in black phosphorous.Comment: 18 Pages , 10 figure

Image contrast required at visual resolution thresholds for SO-1166 and Tri-X film

Thesis (M.A.)--Boston UniversityThe objective of the study was to determine how image contrast required in photographic emulsions to resolve tricolumnar bar targets visually varied as the spatial frequency of the targets increased. Threshold contrast for homogeneous targets, arranged in decreasing order of size and viewed under magnification with perfect optics, remains constant if the magnification is chosen so as to keep the apparent image size of the targets the same. However, when the targets are impressed upon photographic emulsions, threshold contrasts increase because of the disturbing effect of the grain clumps in the emulsion. Two films, Kodak SO 1166, now called Plus-~X Airecon, and Kodak Tri-X were analyzed to determine how the threshold contrasts were affected by the graininess of the films when the background densities of the films were .75 ± .1. Contrast in the photographic images was varied by introducing controlled amounts of stray light into a camera system with a beam splitter. Resolution values of approximately 2.5, 5, 10, 20, 30, and 50 lines/mm were obtained. Ten frames for each of the above values were analyzed by two trained observers, free to select magnification and viewing table illumination, and the three-bar groups within each frame called just-resolved by each observer were scanned with a recording microdensitometer to determine the minimum density difference ΔD required to resolve each of the groups. Average values of threshold resolution and the corresponding average minimum density differences thus obtained were then computed for each of the films and for target length to width ratios of five to one and essentially infinity. The data was plotted with ΔD as a function of resolution R, and third order expressions derived from the curves. It was found that the second and third order terms were of such small magnitude (10^-5R^2 and 10^-7R^3) that they could be disregarded for practical purpose and the resulting linear expressions were of the form ΔD = b+GR where b is a constant, .005, assumed equal to the nominal value of ΔD for visual resolution thresholds of infinitely large targets, and G is a variable dependent primarily upon the grain characteristics of the film. The expressions for the two films, where short (5/1) and long (∞) line targets are employed are: Short Line targets SO 1166 -- ΔD= .005 +.003R Tri-X -- ΔD= .005 + .004R Long Line targets SO 1166 -- ΔD= .005 + .0008R Tri-X -- ΔD= .005+ .0015R It was found that the increase in resolution obtained by using long line targets was about one and one-half times, but that the long line targets were much more difficult to evaluate visually because of the length of the lines

The Infrared Imaging Spectrograph (IRIS) for TMT: the atmospheric dispersion corrector

We present a conceptual design for the atmospheric dispersion corrector (ADC) for TMT's Infrared Imaging Spectrograph (IRIS). The severe requirements of this ADC are reviewed, as are limitations to observing caused by uncorrectable atmospheric effects. The requirement of residual dispersion less than 1 milliarcsecond can be met with certain glass combinations. The design decisions are discussed and the performance of the design ADC is described. Alternative options and their performance tradeoffs are also presented.Comment: SPIE Astronomical Instrumentation 201

Dynamical generalization of a solvable family of two-electron model atoms with general interparticle repulsion

Holas, Howard and March [Phys. Lett. A {\bf 310}, 451 (2003)] have obtained analytic solutions for ground-state properties of a whole family of two-electron spin-compensated harmonically confined model atoms whose different members are characterized by a specific interparticle potential energy u($r_{12}$). Here, we make a start on the dynamic generalization of the harmonic external potential, the motivation being the serious criticism levelled recently against the foundations of time-dependent density-functional theory (e.g. [J. Schirmer and A. Dreuw, Phys. Rev. A {\bf 75}, 022513 (2007)]). In this context, we derive a simplified expression for the time-dependent electron density for arbitrary interparticle interaction, which is fully determined by an one-dimensional non-interacting Hamiltonian. Moreover, a closed solution for the momentum space density in the Moshinsky model is obtained.Comment: 5 pages, submitted to J. Phys.

A new type of charged defect in amorphous chalcogenides

We report on density-functional-based tight-binding (DFTB) simulations of a series of amorphous arsenic sulfide models. In addition to the charged coordination defects previously proposed to exist in chalcogenide glasses, a novel defect pair, [As4]--[S3]+, consisting of a four-fold coordinated arsenic site in a seesaw configuration and a three-fold coordinated sulfur site in a planar trigonal configuration, was found in several models. The valence-alternation pairs S3+-S1- are converted into [As4]--[S3]+ pairs under HOMO-to-LUMO electronic excitation. This structural transformation is accompanied by a decrease in the size of the HOMO-LUMO band gap, which suggests that such transformations could contribute to photo-darkening in these materials.Comment: 5 pages, 2 figure

Influence of copper on the electronic properties of amorphous chalcogenides

We have studied the influence of alloying copper with amorphous arsenic sulfide on the electronic properties of this material. In our computer-generated models, copper is found in two-fold near-linear and four-fold square-planar configurations, which apparently correspond to Cu(I) and Cu(II) oxidation states. The number of overcoordinated atoms, both arsenic and sulfur, grows with increasing concentration of copper. Overcoordinated sulfur is found in trigonal planar configuration, and overcoordinated (four-fold) arsenic is in tetrahedral configuration. Addition of copper suppresses the localization of lone-pair electrons on chalcogen atoms, and localized states at the top of the valence band are due to Cu 3d orbitals. Evidently, these additional Cu states, which are positioned at the same energies as the states due to ([As4]-)-([S_3]+) pairs, are responsible for masking photodarkening in Cu chalcogenides

Applicability and fairness of the oral examination in undergraduate psychiatry training in South Africa

Objective: There are several methods of evaluating medical students’ performance, such as written examination, oral examination and objective structured clinical examination (OSCE). Many studies have focused on the reliability and validity of these methods but few studies have explored comparison between these methods. Psychiatry is the only subject at the University of Stellenbosch where the final assessment consists of solely an oral component. The aim of the study was to compare students’ final overall and discipline specific examination marks (i.e. in the other subjects) with the examination marks in psychiatry, and to determine if content or structure (e,g. oral, written or OSCE format) of examination impacts more on the student performance in the examination. Method: 343 final year medical students were included. All undertook their psychiatry rotation at the University of Stellenbosch, South Africa during 2008 and 2009. Data of marks obtained in all the disciplines during 2008 and 2009 were collected and class marks were compared with their final examination marks across all disciplines. Bland-Altman plots were used to assess the level of agreement between the class and examination marks. Cases below the lower threshold were compared to all other cases across all disciplines. The odds ratio for group status was calculated for gender distribution of examiners. Results: The psychiatry class mark and final oral examination mark provided similar measures within a width of 31.5. Cases below the threshold had poorer performance in two other disciplines. The gender distribution of the examiners (female-female) significantly increased the odds ratio for poorer performance in the oral examination.Conclusion: The results suggest that a group of students underperform in their final examination independent of method of evaluation and that the gender of examiners impacts on examination marks. Therefore future research should focus on identifying and modifying factors (including choice of examiner combinations) that contribute to the poor performance of medical students in their final examination, in order to help students perform better. Gender distribution of examiners should also be considered when examinations are structured and designed.Keywords: Oral; Medical; Student; Gender; Examiner; Performance; OSC

Genomic and experimental evidence for multiple metabolic functions in the RidA/YjgF/YER057c/UK114 (Rid) protein family.

BackgroundIt is now recognized that enzymatic or chemical side-reactions can convert normal metabolites to useless or toxic ones and that a suite of enzymes exists to mitigate such metabolite damage. Examples are the reactive imine/enamine intermediates produced by threonine dehydratase, which damage the pyridoxal 5'-phosphate cofactor of various enzymes causing inactivation. This damage is pre-empted by RidA proteins, which hydrolyze the imines before they do harm. RidA proteins belong to the YjgF/YER057c/UK114 family (here renamed the Rid family). Most other members of this diverse and ubiquitous family lack defined functions.ResultsPhylogenetic analysis divided the Rid family into a widely distributed, apparently archetypal RidA subfamily and seven other subfamilies (Rid1 to Rid7) that are largely confined to bacteria and often co-occur in the same organism with RidA and each other. The Rid1 to Rid3 subfamilies, but not the Rid4 to Rid7 subfamilies, have a conserved arginine residue that, in RidA proteins, is essential for imine-hydrolyzing activity. Analysis of the chromosomal context of bacterial RidA genes revealed clustering with genes for threonine dehydratase and other pyridoxal 5'-phosphate-dependent enzymes, which fits with the known RidA imine hydrolase activity. Clustering was also evident between Rid family genes and genes specifying FAD-dependent amine oxidases or enzymes of carbamoyl phosphate metabolism. Biochemical assays showed that Salmonella enterica RidA and Rid2, but not Rid7, can hydrolyze imines generated by amino acid oxidase. Genetic tests indicated that carbamoyl phosphate overproduction is toxic to S. enterica cells lacking RidA, and metabolomic profiling of Rid knockout strains showed ten-fold accumulation of the carbamoyl phosphate-related metabolite dihydroorotate.ConclusionsLike the archetypal RidA subfamily, the Rid2, and probably the Rid1 and Rid3 subfamilies, have imine-hydrolyzing activity and can pre-empt damage from imines formed by amine oxidases as well as by pyridoxal 5'-phosphate enzymes. The RidA subfamily has an additional damage pre-emption role in carbamoyl phosphate metabolism that has yet to be biochemically defined. Finally, the Rid4 to Rid7 subfamilies appear not to hydrolyze imines and thus remain mysterious

Photo induced collisions with laser cooled He* atoms

This paper presents an experimental investigation of cold collisions between metastable Helium atoms in an optical trap at 1mK. Penning (PI) and associative (AI) ionization reactions are distinguished using a mass spectrometer and studied under influence of near resonant laser light. Sensitive behavior of the ion rate is observed when the laser is tuned close to resonance. Experimental ?ndings are well described, on an absolute scale, by a semi-classical model we have developed for optical collisions and by a modi?ed Julienne-Vigu´e model