Do ultrafast exciton-polaron decoherence dynamics govern photocarrier generation efficiencies in polymer solar cells?

All-organic-based photovoltaic solar cells have attracted considerable attention because of their low-cost processing and short energy payback time. In such systems the primary dissociation of an optical excitation into a pair of photocarriers has been recently shown to be extremely rapid and efficient, but the physical reason for this remains unclear. Here, two-dimensional photocurrent excitation spectroscopy, a novel non-linear optical spectroscopy, is used to probe the ultrafast coherent decay of photoexcitations into charge-producing states in a polymer:fullerene based solar cell. The two-dimensional photocurrent spectra are interpreted by introducing a theoretical model for the description of the coupling of the electronic states of the system to an external environment and to the applied laser fields. The experimental data show no cross-peaks in the two-dimensional photocurrent spectra, as predicted by the model for coherence times between the exciton and the photocurrent producing states of 20\,fs or less

Lessons Learned from the Flight Unit Testing of the Near Earth Asteroid Scout Flight System

The Near Earth Asteroid Scout flight mission is set to launch on the maiden voyage of the Space Launch System as a secondary payload. The spacecraft will be jettisoned in cis-lunar space and embark on an ambitious 2.5 year mission to image an asteroid. The spacecraft is uniquely equipped with an 85m2 solar sail as the main propulsion system. The monolithic sail system is designed to package within a 6U volume for launch and then deploy during flight. The NEA Scout team has presented in the past to the International Symposium on Solar Sailing topics related to the engineering development unit and design efforts to achieve flight hardware build. This paper will focus on the lessons learned from building and testing the NEA Scout flight system. Focus will be on the mechanical, software, and electrical interfaces as well as preparation for subsystem environmental tests, including thermal vacuum. Due to the unique design of the spacecraft, the solar sail subsystem was required to be located in the center of the spacecraft. This requirement lead to design challenges such as designing and accommodating critical cable harnesses to run through the center of the sail subsystem, packaging and deployment design of the sail subsystem, and integrated testing efforts through an avionics test bed to verify and validate a complete system architecture

Numerical modelling of Auriga's Wheel - a new ring galaxy

We model the formation of Auriga's Wheel - a recently discovered collisional ring galaxy. Auriga's Wheel has a number of interesting features including a bridge of stars linking the neighbouring elliptical to the ring galaxy, and evidence for components of expansion and rotation within the ring. Using N-body/SPH modelling, we study collisions between an elliptical galaxy and a late-type disk galaxy. A near direct collision, with a mildy inclined disk, is found to reasonably reproduce the general system morphology ~50 Myr following the collision. The collision must have a relatively low velocity (initially ~150 km s^{-1}) in order to form the observed bridge, and simultaneously match the galaxies separation. Our best-match model suggests the total disk galaxy is ~5 times more massive than the elliptical. We find that the velocity of expansion of the ring is sensitive to the mass of the elliptical, while insensitive to the encounter velocity. We evolve our simulation beyond the current epoch to study the future destiny of the galaxy pair. In our model, the nucleus moves further away from the plane of the ring in the direction of the stellar bridge. The nucleus eventually merges with the elliptical galaxy ~100 Myr after the present time. The ring continues to expand for ~200 Myr before collapsing back. The low initial relative velocity of the two galaxies will eventually result in a complete merger.Comment: Accepted to MNRAS, March 9th, 2012. 17 pages, 16 figures, no table

Changing social contracts in climate-change adaptation

Risks from extreme weather events are mediated through state, civil society and individual action 1 , 2 . We propose evolving social contracts as a primary mechanism by which adaptation to climate change proceeds. We use a natural experiment of policy and social contexts of the UK and Ireland affected by the same meteorological event and resultant flooding in November 2009. We analyse data from policy documents and from household surveys of 356 residents in western Ireland and northwest England. We find significant differences between perceptions of individual responsibility for protection across the jurisdictions and between perceptions of future risk from populations directly affected by flooding events. These explain differences in stated willingness to take individual adaptive actions when state support retrenches. We therefore show that expectations for state protection are critical in mediating impacts and promoting longer-term adaptation. We argue that making social contracts explicit may smooth pathways to effective and legitimate adaptation

Recommendations for dealing with waste contaminated with Ebola virus: a Hazard Analysis of Critical Control Points approach

Objective To assess, within communities experiencing Ebola virus outbreaks, the risks associated with the disposal of human waste and to generate recommendations for mitigating such risks. Methods A team with expertise in the Hazard Analysis of Critical Control Points framework identified waste products from the care of individuals with Ebola virus disease and constructed, tested and confirmed flow diagrams showing the creation of such products. After listing potential hazards associated with each step in each flow diagram, the team conducted a hazard analysis, determined critical control points and made recommendations to mitigate the transmission risks at each control point. Findings The collection, transportation, cleaning and shared use of blood-soiled fomites and the shared use of latrines contaminated with blood or bloodied faeces appeared to be associated with particularly high levels of risk of Ebola virus transmission. More moderate levels of risk were associated with the collection and transportation of material contaminated with bodily fluids other than blood, shared use of latrines soiled with such fluids, the cleaning and shared use of fomites soiled with such fluids, and the contamination of the environment during the collection and transportation of blood-contaminated waste. Conclusion The risk of the waste-related transmission of Ebola virus could be reduced by the use of full personal protective equipment, appropriate hand hygiene and an appropriate disinfectant after careful cleaning. Use of the Hazard Analysis of Critical Control Points framework could facilitate rapid responses to outbreaks of emerging infectious disease

Disc heating: comparing the Milky Way with cosmological simulations

We present an analysis of a suite of simulations run with different particle- and grid-based cosmological hydrodynamical codes and compare them with observational data of the Milky Way. This is the first study to make comparisons of properties of galaxies simulated with particle- and grid-based codes. Our analysis indicates that there is broad agreement between these different modelling techniques. We study the velocity dispersion-age relation for disc stars at z= 0 and find that four of the simulations are more consistent with observations by Holmberg, Nordstroem & Andersen in which the stellar disc appears to undergo continual/secular heating. Two other simulations are in better agreement with the Quillen & Garnett observations that suggest ‘saturation' in the heating profile for young stars in the disc. None of the simulations has thin discs as old as that of the Milky Way. We also analyse the kinematics of disc stars at the time of their birth for different epochs in the galaxies' evolution and find that in some simulations old stars are born cold within the disc and are subsequently heated, while other simulations possess old stellar populations which are born relatively hot. The models which are in better agreement with observations of the Milky Way's stellar disc undergo significantly lower minor-merger/assembly activity after the last major merger, that is, once the disc has formed. All of the simulations are significantly ‘hotter' than the Milky Way disc; on top of the effects of mergers, we find a ‘floor' in the dispersion that is related to the underlying treatment of the heating and cooling of the interstellar medium, and the low density threshold which such codes use for star formation. This finding has important implications for all studies of disc heating that use hydrodynamical code