15,995 research outputs found
Ultrafast Intramolecular Charge Transfer of Formyl Perylene Observed Using Femtosecond Transient Absorption Spectroscopy
The excited-state photophysics of formylperylene (FPe) have been investigated in a series of nonpolar, polar
aprotic, and polar protic solvents. A variety of experimental and theoretical methods were employed including
femtosecond transient absorption (fs-TA) spectroscopy with 130 fs temporal resolution. We report that the
ultrafast intramolecular charge transfer from the perylene unit to the formyl (CHO) group can be facilitated
drastically by hydrogen-bonding interactions between the carbonyl group oxygen of FPe and hydrogen-donating
solvents in the electronically excited state. The excited-state absorption of FPe in methanol (MeOH) is close
to the reported perylene radical cation produced by bimolecular quenching by an electron acceptor. This is
a strong indication for a substantial charge transfer in the S1 state in protic solvents. The larger increase of
the dipole moment change in the protic solvents than that in aprotic ones strongly supports this observation.
Relaxation mechanisms including vibrational cooling and solvation coupled to the charge-transfer state are
also discussed
Soliton-radiation trapping in gas-filled photonic crystal fibers
We propose an optical trapping technique in which a fundamental soliton traps an ultrashort small amplitude radiation in a symmetric hollow-core photonic crystal fiber filled with a noble gas, preventing its dispersion. The system is Raman- and plasma-free. Trapping is due to the cross phase modulation effect between the two pulses. The trapped radiation inside the soliton-induced potential will oscillate periodically due to the shock effect, similar to the motion of a mechanical pendulum. DOI: 10.1103/PhysRevA.87.04380
Soliton dynamics in gas-filled hollow-core photonic crystal fibers
Gas-filled hollow-core photonic crystal fibers offer unprecedented
opportunities to observe novel nonlinear phenomena. The various properties of
gases that can be used to fill these fibers give additional degrees of freedom
for investigating nonlinear pulse propagation in a wide range of different
media. In this review, we will consider some of the the new nonlinear
interactions that have been discovered in recent years, in particular those
which are based on soliton dynamics
Trapped Bose-Einstein condensates in the presence of a current nonlinearity
We investigate the effect of a current nonlinearity on the evolution of a
trapped atomic Bose-Einstein condensate. We have implemented techniques from
the field of nonlinear optics to provide new insights into the irregular
dynamics associated with chiral superfluids. We have found that the current
nonlinearity can be treated as a Kerr-like nonlinearity modulated by a
spatiotemporal function that can lead to a number of processes such as
broadening and compression of the wave function. In the long time scale limit,
the wave function is drastically deformed and delocalised compared to its
initial state. However, localised modes which oscillate with the period of the
inverse trap frequency can still be observed.Comment: A short note on the links between nonlinear gauge potentials and
nonlinear optics. Comments are welcom
Slimness of graphs
Slimness of a graph measures the local deviation of its metric from a tree
metric. In a graph , a geodesic triangle with
is the union of three shortest
paths connecting these vertices. A geodesic triangle is
called -slim if for any vertex on any side the
distance from to is at most , i.e. each path
is contained in the union of the -neighborhoods of two others. A graph
is called -slim, if all geodesic triangles in are
-slim. The smallest value for which is -slim is
called the slimness of . In this paper, using the layering partition
technique, we obtain sharp bounds on slimness of such families of graphs as (1)
graphs with cluster-diameter of a layering partition of , (2)
graphs with tree-length , (3) graphs with tree-breadth , (4)
-chordal graphs, AT-free graphs and HHD-free graphs. Additionally, we show
that the slimness of every 4-chordal graph is at most 2 and characterize those
4-chordal graphs for which the slimness of every of its induced subgraph is at
most 1
Assessment of Newspaper Circulation and Readership in Northern Ghana
There is a widespread concern that the print newspaper industry across the world and in Ghana particularly faces an uncertain future and a long-term decline in readership and circulation due to the prevalence of internet mediated News websites, making print newspapers obsolete in their present format. This paper is an assessment of the circulation and readership of printed newspapers in the northern regional capital, Tamale. It investigates preferred News sources among newsreaders and examines consumption pattern of newspapers in the metropolis. It discussed the challenges the print media industry faces in the wake of News websites’ proliferation in the Ghanaian media landscape. The paper is based on exploratory research design. It sampled four leading print newspapers in Ghana (Daily Graphic, Daily Guide, Ghanaian Times and Business and Financial Times) as reference points for data collection. Four newspaper vendors who vend these newspapers in the metropolis were sampled through simple random sampling. The paper finds a sharp decline in circulation and readership of printed newspaper in the metropolis in favour of internet powered News websites. The paper concludes that although newsreaders prefer sourcing News online, they still find the traditional printed media as the most credible and reliable sources for News. The paper recommends that the traditional print media take advantage of the reach of internet powered platforms to create online presence and ensure that they innovate to get newsreaders subscribe to their brands since print newspaper readership is gradually declining in the metropolis
Sidelobe Control in Collaborative Beamforming via Node Selection
Collaborative beamforming (CB) is a power efficient method for data
communications in wireless sensor networks (WSNs) which aims at increasing the
transmission range in the network by radiating the power from a cluster of
sensor nodes in the directions of the intended base station(s) or access
point(s) (BSs/APs). The CB average beampattern expresses a deterministic
behavior and can be used for characterizing/controling the transmission at
intended direction(s), since the mainlobe of the CB beampattern is independent
on the particular random node locations. However, the CB for a cluster formed
by a limited number of collaborative nodes results in a sample beampattern with
sidelobes that severely depend on the particular node locations. High level
sidelobes can cause unacceptable interference when they occur at directions of
unintended BSs/APs. Therefore, sidelobe control in CB has a potential to
increase the network capacity and wireless channel availability by decreasing
the interference. Traditional sidelobe control techniques are proposed for
centralized antenna arrays and, therefore, are not suitable for WSNs. In this
paper, we show that distributed, scalable, and low-complexity sidelobe control
techniques suitable for CB in WSNs can be developed based on node selection
technique which make use of the randomness of the node locations. A node
selection algorithm with low-rate feedback is developed to search over
different node combinations. The performance of the proposed algorithm is
analyzed in terms of the average number of trials required to select the
collaborative nodes and the resulting interference. Our simulation results
approve the theoretical analysis and show that the interference is
significantly reduced when node selection is used with CB.Comment: 30 pages, 10 figures, submitted to the IEEE Trans. Signal Processin
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Optimum design of composite prestressed concrete girder railway bridges
This paper deals with the formulation of design optimisation of pretsressed concrete bridges. The bridge is of a slab-on-girder type, hence modeled as an equivalent orthotropic plate. The whole bridge system is considered as a simply supported right angle plate. Following linear elastic behaviour, the governing fourth order differential equation of the plate for patch load is solved in order to find out load distribution on the girders forming the bridge as well as the deflections and internal forces at critical sections of the whole bridge. The optimisation problem is formulated for various cross sectional geometries including rectangular, symmetrical I, unsymmetrical I, box, T and inverted T sections. The design variables are the main cross sectional dimensions, prestressing force and tendon eccentricity. The objective function comprises the cost of concrete material, formwork and prestressing steel tendons. The constraint functions are set to satisfy design requirements as per British Standards for bridges (BS 5400). Nonlinear optimisation method based on sequential unconstrained minimisation technique (SUMT) is employed to achieve optimum bridge configuration for specific design parameters of span length, concrete compressive strength and railway loading patterns. A purpose built computer program is set up to carry out the solution of the design optimisation problem efficiently in terms of time and effort. A typical example of unsymmetrical I-section having a small bottom flange as compared to the top flange width with composite deck effect is presented. The results show that the total cost increases as the span increases due to the increase of the initial prestressing force. Furthermore, the total cost decreases as the concrete compressive strength increases in spite of the increasing of the prestressing force. This is due to decrease of the overall depth, top and bottom flange widths, hence leading to a smaller girder size. Such finding will encourage engineers to adopt high strength concrete for bridges as it will help reducing not only the initial cost but also the life cycle cost of the bridge over its entire life
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