23,960 research outputs found
A quantum dynamical comparison of the electronic couplings derived from quantum electrodynamics and Förster theory:Application to 2D molecular aggregates
Get PDFThe objective of this study is to investigate under what circumstances Förster theory of electronic (resonance) energy transfer breaks down in molecular aggregates. This is achieved by simulating the dynamics of exciton diffusion, on the femtosecond timescale, in molecular aggregates using the Liouville–von Neumann equation of motion. Specifically the focus of this work is the investigation of both spatial and temporal deviations between exciton dynamics driven by electronic couplings calculated from Förster theory and those calculated from quantum electrodynamics. The quantum electrodynamics (QED) derived couplings contain medium- and far-zone terms that do not exist in Förster theory. The results of the simulations indicate that Förster coupling is valid when the dipole centres are within a few nanometres of one another. However, as the distance between the dipole centres increases from 2 nm to 10 nm, the intermediate- and far-zone coupling terms play non-negligible roles and Förster theory begins to break down. Interestingly, the simulations illustrate how contributions to the exciton dynamics from the intermediate- and far-zone coupling terms of QED are quickly washed-out by the near-zone mechanism of Förster theory for lattices comprising closely packed molecules. On the other hand, in the case of sparsely packed arrays, the exciton dynamics resulting from the different theories diverge within the 100 fs lifetime of the trajectories. These results could have implications for the application of spectroscopic ruler techniques as well as design principles relating to energy harvesting materials
Forward-scatter radiant mapping
Get PDFForward-scatter systems have been much neglected for the study of meteors and meteor streams. A great deal of this neglect stems from the complicated geometry which has made the interpretation of results difficult in the past. This no longer presents a problem because of the computer power now available. There are practical advantages in using forward-scatter in that low-power transmitters are much easier to handle than the high-power ones used in pulsed back-scatter radars. The data reduction of the CW signals is also significantly simpler. Because the forward-scatter reflection geometry increases the duration of the echoes relative to the back-scatter case, the problem of the underdense ceiling is partially alleviated. We have built a 'short hop' forward-scatter system between Ottawa and London (Ont) for which the transmitter and receiver are separated by about 500 km. With it, we are able to measure unambiguously the directions of arrival of the echoes using a 5-antenna interferometer. Morton and Jones (1982, MN, 198, 737) have shown how the echo direction distribution can be deconvolved to yield the meteor radiant distribution for back-scatter data. We have extended the technique to the forward-scatter case and present some preliminary meteor radiant distribution maps
Effects of acoustic sources
Get PDFAn experiment was conducted to determine the effect of acoustics on the laminar flow on the side of a nacelle. A flight test was designed to meet this goal and a brief review of the purpose is given. A nacelle with a significant length of laminar flow was mounted on the wing of NASA OV-1. Two noise sources are also mounted on the wing: one in the center body of the nacelle; the second in a wing mounted pod outboard of the nacelle. These two noise sources allow for a limited study of the effect of source direction in addition to control of the acoustic level and frequency. To determine the range of Tollmien-Schlichting frequencies, a stability analysis using the pressure coefficient distribution along the side of the nacelle was performed. Then by applying these frequencies and varying the acoustic level, a study of the receptivity of the boundary layer to the acoustic signal, as determined by the shortening of the length of laminar flow, was conducted. Results are briefly discussed
Impacts of stratospheric aerosol geoengineering strategy on Caribbean coral reefs
Get PDFPurpose: Currently, negotiation on global carbon emissions reduction is very difficult due to lack of international willingness. In response, geoengineering (climate engineering) strategy is proposed to artificially cool the planet. Meanwhile, as the harbor around one-third of all described marine species, coral reefs are the most sensitive ecosystem on the planet to climate change. However, until now, there is no any quantitative assessment on impacts of geoengineering on coral reefs. In this study, we model impacts of stratospheric aerosol geoengineering on coral reefs.
Design/methodology/approach: We will use the HadGEM2-ES climate model to model and evaluate impacts of stratospheric aerosol geoengineering on coral reefs.
Findings: This study shows that a) stratospheric aerosol geoengineering could significantly mitigate future coral bleaching throughout the Caribbean Sea; b) Changes in downward solar irradiation, sea level rise and sea surface temperature caused by geoengineering implementation should have very little impacts on coral reefs; c) although geoengineering would prolong the return period of future hurricanes, this may still be too short to ensure coral recruitment and survival after hurricane damage
Statistical mechanics and thermodynamics of viral evolution
Full text linkThis paper analyzes a simplified model of viral infection and evolution using
the 'grand canonical ensemble' and formalisms from statistical mechanics and
thermodynamics to enumerate all possible viruses and to derive thermodynamic
variables for the system. We model the infection process as a series of energy
barriers determined by the genetic states of the virus and host as a function
of immune response and system temperature. We find a phase transition between a
positive temperature regime of normal replication and a negative temperature
'disordered' phase of the virus. These phases define different regimes in which
different genetic strategies are favored. Perhaps most importantly, it
demonstrates that the system has a real thermodynamic temperature. For normal
replication, this temperature is linearly related to effective temperature. The
strength of immune response rescales temperature but does not change the
observed linear relationship. For all temperatures and immunities studied, we
find a universal curve relating the order parameter to viral evolvability. Real
viruses have finite length RNA segments that encode for proteins which
determine their fitness; hence the methods put forth here could be refined to
apply to real biological systems, perhaps providing insight into immune escape,
the emergence of novel pathogens and other results of viral evolution.Comment: 39 pages (55 pages including supplement), 9 figures, 11 supplemental
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Providing True Opportunity for Opportunity Youth: Promising Practices and Principles for Helping Youth Facing Barriers to Employment
Get PDFMany "opportunity youth" -- youth who are not working or in school -- would benefit substantially from gaining work experience but need help overcoming barriers to employment and accessing the labor market.Those opportunity youth facing the most significant challenges, such as extreme poverty, homelessness, and justice system involvement, often need even more intensive assistance in entering and keeping employment, and are at risk of being left behind even by employment programs that are specifically designed to serve opportunity youth.This paper builds on the research literature with extensive interviews with employment program providers who have had success in helping the most vulnerable opportunity youth succeed in the workforce. Six principles for effectively serving these youth are identified
GRO J1744-28, search for the counterpart: infrared photometry and spectroscopy
Get PDFUsing VLT/ISAAC, we detected 2 candidate counterparts to the bursting pulsar
GRO J1744-28, one bright and one faint, within the X-ray error circles of
XMM-Newton and Chandra. In determining the spectral types of the counterparts
we applied 3 different extinction corrections; one for an all-sky value, one
for a Galactic Bulge value and one for a local value. We find the local value,
with an extinction law of alpha = 3.23 +- 0.01 is the only correction that
results in colours and magnitudes for both bright and faint counterparts
consistent with a small range of spectral types, and for the bright
counterpart, consistent with the spectroscopic identification. Photometry of
the faint candidate indicates it is a K7/M0 V star at a distance of 3.75 +- 1
kpc. This star would require a very low inclination angle (i < 9deg) to satisfy
the mass function constraints; however it cannot be excluded as the counterpart
without follow-up spectroscopy to detect emission signatures of accretion.
Photometry and spectroscopy of the bright candidate indicate it is most likely
a G/K III star. The spectrum does not show Br-gamma emission, a known indicator
of accretion. The bright star's magnitudes are in agreement with the
constraints placed on a probable counterpart by the calculations of Rappaport &
Joss (1997) for an evolved star that has had its envelope stripped. The mass
function indicates the counterpart should have M < 0.3 Msol for an inclination
of i >= 15deg; a stripped giant, or a main sequence M3+ V star are consistent
with this mass-function constraint. In both cases mass-transfer, if present,
will be by wind-accretion as the counterpart will not fill its Roche lobe given
the observed orbital period. The derived magnetic field of 2.4 x 10^{11} G will
inhibit accretion by the propeller effect, hence its quiescent state.Comment: 12 pages, 6 figures, 4 table, MNRAS accepted Changes to the content
and an increased analysis of the Galactic centre extinctio
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