4,571 research outputs found
Absorption spectrum of iron in the vacuum ultraviolet 2950 - 1588 angstrom
Absorption spectrum of iron in vacuum ultraviole
Mixed tenure orthodoxy: practitioner reflections on policy effects
This article examines mixed tenure as a policy orthodoxy. It first sets out how mixed tenure may be considered to constitute an orthodoxy within planning, being generally accepted as a theory and practice even in the absence of supporting evidence. Five elements of this orthodoxy are identified, relating to (1) housing and the environment, (2) social change, (3) economic impacts, (4) sustainable communities, (5) and sociospatial integration. Interviews with practitioners involved with three social housing estates that have experienced mixed-tenure policy interventions are reported to consider why the implementation and effects of mixed tenure might not correspond with the orthodox understanding. It is argued that policy ambiguity and weaknesses in policy theory and specification, alongside practical constraints, lie behind incomplete and counterproductive policy implementation, but a belief in pursuing the policy orthodoxy persists nevertheless
Properties of cage rearrangements observed near the colloidal glass transition
We use confocal microscopy to study the motions of particles in concentrated
colloidal systems. Near the glass transition, diffusive motion is inhibited, as
particles spend time trapped in transient ``cages'' formed by neighboring
particles. We measure the cage sizes and lifetimes, which respectively shrink
and grow as the glass transition approaches. Cage rearrangements are more
prevalent in regions with lower local concentrations and higher disorder.
Neighboring rearranging particles typically move in parallel directions,
although a nontrivial fraction move in anti-parallel directions, usually from
pairs of particles with initial separations corresponding to the local maxima
and minima of the pair correlation function , respectively.Comment: 5 pages, 4 figures; text & figures revised in v
Expectations of Field Supervisors in Kenya: Implications for Community-based Human Service Practicums
Community-based learning (CBL), which provides opportunities for undergraduate students to develop disciplinary and work-related knowledge and skills, is increasingly becoming an integral component of higher education. Similar to other countries, there is a widespread belief among employers in Kenya that there is a mismatch between university programs and labour market demands. In order to enhance the employability of graduates, many departments at a Kenyan university have incorporated work-integrated experiential learning opportunities such as practicums in the educational experience for undergraduate students. The aim of this article is to describe the expectations of field supervisors in host organisations participitating in a community-based human services program at a Kenyan University. Fifteen purposively sampled field supervisors participated in individual face-to-face interviews that included questions about their understanding of the department’s expectations of student learning activities during practicums, knowledge of the academic preparation of students in the program and challenges associated with the supervisory role. Six field supervisors exhibited some level of understanding of the expectations of their role in working with practicum students, while nine field supervisors indicated unclear expectations of the students’ practicum experience. Thematic analysis revealed key themes related to: (1) student abilities, learning goals and their contributions to the host organisations, and (2) the academic program of study and academic support available from the university faculty or staff to field supervisors. The results of our study revealed a lack of clarity around practicum expectations for most of the field supervisors interviewed and insufficient preparation of the community-based organisations to host a practicum student. Several recommendations are identified to clarify the expectations of community partner organisations and the staff providing student supervision to ensure benefits for both students and the host organisation. Results from this study can be used to inform the development or improvement of practicum opportunities focused on producing a skilled workforce
Local molecular field theory for the treatment of electrostatics
We examine in detail the theoretical underpinnings of previous successful
applications of local molecular field (LMF) theory to charged systems. LMF
theory generally accounts for the averaged effects of long-ranged components of
the intermolecular interactions by using an effective or restructured external
field. The derivation starts from the exact Yvon-Born-Green hierarchy and shows
that the approximation can be very accurate when the interactions averaged over
are slowly varying at characteristic nearest-neighbor distances. Application of
LMF theory to Coulomb interactions alone allows for great simplifications of
the governing equations. LMF theory then reduces to a single equation for a
restructured electrostatic potential that satisfies Poisson's equation defined
with a smoothed charge density. Because of this charge smoothing by a Gaussian
of width sigma, this equation may be solved more simply than the detailed
simulation geometry might suggest. Proper choice of the smoothing length sigma
plays a major role in ensuring the accuracy of this approximation. We examine
the results of a basic confinement of water between corrugated wall and justify
the simple LMF equation used in a previous publication. We further generalize
these results to confinements that include fixed charges in order to
demonstrate the broader impact of charge smoothing by sigma. The slowly-varying
part of the restructured electrostatic potential will be more symmetric than
the local details of confinements.Comment: To be published in J Phys-Cond Matt; small misprint corrected in Eq.
(12) in V
Impact Broadening, Shifting, and Asymmetry of the D1 and D2 Lines of Alkali-Metal Atoms Colliding With Noble-Gas Atoms
The Anderson Talman theory of spectral line broadening is used together with potential energy curves calculated at the spin-orbit multi-reference configuration interaction level to compute broadening, shifting, and asymmetry coefficients of the D1 and D2 lines of alkali-metal atoms M, as they collide with noble gas atoms N, where M=K, Rb, and Cs, and N=He, Ne, and Ar. Our calculated coefficients are compared to experimental results for a variety of temperatures. In all cases general agreement is observed for the broadening coefficients, while significant disagreement is observed for the shifting coefficients. We also compare our K+He broadening and shifting results with fully quantum-mechanical calculations that employ the Baranger theory of collisional line broadening, and we compare our results with other semiclassical calculations. As with the comparison to experiment, closer agreement is observed for the broadening coefficients while the shifting coefficients exhibit significant disagreement. We use the natural variation between the difference potentials of the nine M+N pairs to explore the relationship between potential and line shape as determined by Anderson-Talman theory and develop a picture for the mechanism that underlies the general agreement between theoretical and experimental results on the broadening coefficient and the general disagreement on shifting coefficients
An ellipsoidal mirror for focusing neutral atomic and molecular beams
Manipulation of atomic and molecular beams is essential to atom optics applications including atom lasers, atom lithography, atom interferometry and neutral atom microscopy. The manipulation of charge-neutral beams of limited polarizability, spin or excitation states remains problematic, but may be overcome by the development of novel diffractive or reflective optical elements. In this paper, we present the first experimental demonstration of atom focusing using an ellipsoidal mirror. The ellipsoidal mirror enables stigmatic off-axis focusing for the first time and we demonstrate focusing of a beam of neutral, ground-state helium atoms down to an approximately circular spot, (26.8±0.5) μm×(31.4±0.8) μm in size. The spot area is two orders of magnitude smaller than previous reflective focusing of atomic beams and is a critical milestone towards the construction of a high-intensity scanning helium microscope
Forced motion of a probe particle near the colloidal glass transition
We use confocal microscopy to study the motion of a magnetic bead in a dense
colloidal suspension, near the colloidal glass transition volume fraction
. For dense liquid-like samples near , below a threshold force
the magnetic bead exhibits only localized caged motion. Above this force, the
bead is pulled with a fluctuating velocity. The relationship between force and
velocity becomes increasingly nonlinear as is approached. The
threshold force and nonlinear drag force vary strongly with the volume
fraction, while the velocity fluctuations do not change near the transition.Comment: 7 pages, 4 figures revised version, accepted for publication in
Europhysics Letter
An ellipsoidal mirror for focusing neutral atomic and molecular beams
Manipulation of atomic and molecular beams is essential to atom optics applications including atom lasers, atom lithography, atom interferometry and neutral atom microscopy. The manipulation of charge-neutral beams of limited polarizability, spin or excitation states remains problematic, but may be overcome by the development of novel diffractive or reflective optical elements. In this paper, we present the first experimental demonstration of atom focusing using an ellipsoidal mirror. The ellipsoidal mirror enables stigmatic off-axis focusing for the first time and we demonstrate focusing of a beam of neutral, ground-state helium atoms down to an approximately circular spot, (26.8±0.5) μm×(31.4±0.8) μm in size. The spot area is two orders of magnitude smaller than previous reflective focusing of atomic beams and is a critical milestone towards the construction of a high-intensity scanning helium microscope
Particle Aggregation in a turbulent Keplerian flow
In the problem of planetary formation one seeks a mechanism to gather small
solid particles together into larger accumulations of solid matter. Here we
describe a scenario in which turbulence mediates this process by aggregating
particles into anticyclonic regions. If, as our simulations suggest,
anticyclonic vortices form as long-lived coherent structures, the process
becomes more powerful because such vortices trap particles effectively. Even if
the turbulence is decaying, following the upheaval that formed the disk, there
is enough time to make the dust distribution quite lumpy.Comment: 16 pages, 9 figure
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