Low-temperature dynamics of weakly localized Frenkel excitons in disordered linar chains

We calculate the temperature dependence of the fluorescence Stokes shift and the fluorescence decay time in linear Frenkel exciton systems resulting from the thermal redistribution of exciton population over the band states. The following factors, relevant to common experimental conditions, are accounted for in our kinetic model: (weak) localization of the exciton states by static disorder, coupling of the localized excitons to vibrations in the host medium, a possible non-equilibrium of the subsystem of localized Frenkel excitons on the time scale of the emission process, and different excitation conditions (resonant or non resonant). A Pauli master equation, with microscopically calculated transition rates, is used to describe the redistribution of the exciton population over the manifold of localized exciton states. We find a counterintuitive non-monotonic temperature dependence of the Stokes shift. In addition, we show that depending on experimental conditions, the observed fluorescence decay time may be determined by vibration-induced intra-band relaxation, rather than radiative relaxation to the ground state. The model considered has relevance to a wide variety of materials, such as linear molecular aggregates, conjugated polymers, and polysilanes.Comment: 15 pages, 8 figure

HIGHER EDUCATION LAW—THE NEGATIVE EFFECTS OF STUDENT LOANS: A PLEA FOR IMPACTED STUDENTS

Throughout the United States, college enrollment has overwhelmingly increased, reaching its peak in 2010 with approximately twenty million students. Due to the expanded accessibility to attend college through the Department of Education’s higher education programs, the ability to attend college is no longer solely for the elite. This rapid growth, however, has created additional challenges. Despite the evolving higher education demand, government regulations and oversight have mostly remained stagnant. Colleges began to capitalize and take advantage of this market, prompting the rise of for-profit colleges. In 2012, about 12 percent of students attended for-profit colleges, as opposed to the 0.2 percent of students about twenty years prior. Although not all for-profit colleges misuse the system, many colleges have been criticized in recent years for predatory and illegal recruiting tactics. The Department of Education is aware of these problems, but continually enables this conduct through lack of oversight and mismanaged federal funds. The students enrolled in these for-profit colleges have little to no recourse within the court system or otherwise after acquiring burdensome debt based on deceptive tactics and unrealistic promises. This Note will argue that students who are negatively impacted by the predatory tactics of for-profit colleges should have a remedy under the theory of estoppel against the government. Although there is a heightened “affirmative misconduct” standard for this claim, the system will likely go unchanged without a direct effect on the government, given its own interest and benefits based on the current structure of the system

50 Years Later: Women, Work & the Work Ahead (Infographic)

How have things changed for women in the labor force over the last 50 years

The Weapon Focus Effect: Testing an Extension of the Unusualness Hypothesis

The weapon focus effect (WFE) occurs when a weapon distracts eyewitnesses, harming memory for the perpetrator and other details. One explanation is that weapons are unusual in most contexts, and unusual objects distract eyewitnesses. We extended this unusualness hypothesis to include typical objects used in a distinctive manner, as criminals often make use of a typical object as a weapon (e.g., tire iron, beer bottle). Undergraduates (N = 963) viewed a video depicting a man with a handgun, distinctive object, typical object and action, or typical object used as a weapon. Only the handgun reduced eyewitness identification accuracy relative to the typical object and action, replicating the WFE. Importantly, participants who reported high confidence after choosing from a lineup tended to be highly accurate, regardless of condition

Cosmic-Ray Acceleration at Ultrarelativistic Shock Waves: Effects of Downstream Short-Wave Turbulence

The present paper is the last of a series studying the first-order Fermi acceleration processes at relativistic shock waves with the method of Monte Carlo simulations applied to shocks propagating in realistically modeled turbulent magnetic fields. The model of the background magnetic field structure of Niemiec & Ostrowski (2004, 2006) has been augmented here by a large-amplitude short-wave downstream component, imitating that generated by plasma instabilities at the shock front. Following Niemiec & Ostrowski (2006), we have considered ultrarelativistic shocks with the mean magnetic field oriented both oblique and parallel to the shock normal. For both cases simulations have been performed for different choices of magnetic field perturbations, represented by various wave power spectra within a wide wavevector range. The results show that the introduction of the short-wave component downstream of the shock is not sufficient to produce power-law particle spectra with the "universal" spectral index 4.2. On the contrary, concave spectra with cutoffs are preferentially formed, the curvature and cutoff energy being dependent on the properties of turbulence. Our results suggest that the electromagnetic emission observed from astrophysical sites with relativistic jets, e.g. AGN and GRBs, is likely generated by particles accelerated in processes other than the widely invoked first-order Fermi mechanism.Comment: 9 pages, 8 figures, submitted to Ap

Temperature dependent fluorescence in disordered Frenkel chains: interplay of equilibration and local band-edge level structure

We model the optical dynamics in linear Frenkel exciton systems governed by scattering on static disorder and lattice vibrations, and calculate the temperature dependent fluorescence spectrum and lifetime. The fluorescence Stokes shift shows a nonmonotonic behavior with temperature, which derives from the interplay of the local band-edge level structure and thermal equilibration. The model yields excellent fits to experiments performed on linear dye aggregates.Comment: 4 pages, 3 figure

Synthetic X-ray light curves of BL Lacs from relativistic hydrodynamic simulations

We present the results of relativistic hydrodynamic simulations of the collision of two dense shells in a uniform external medium, as envisaged in the internal shock model for BL Lac jets. The non-thermal radiation produced by highly energetic electrons injected at the relativistic shocks is computed following their temporal and spatial evolution. The acceleration of electrons at the relativistic shocks is parametrized using two different models and the corresponding X-ray light curves are computed. We find that the interaction time scale of the two shells is influenced by an interaction with the external medium. For the chosen parameter sets, the efficiency of the collision in converting dissipated kinetic energy into the observed X-ray radiation is of the order of one percent.Comment: 22 pages, 6 figures, accepted to A&

Particle acceleration in ultra-relativistic oblique shock waves

We perform Monte Carlo simulations of diffusive shock acceleration at highly relativistic oblique shock waves. High upstream flow Lorentz gamma factors are used, which are relevant to models of ultra relativistic particle shock acceleration in Active Galactic Nuclei (AGN) central engines and relativistic jets and Gamma Ray Burst (GRB) fireballs. We investigate numerically the acceleration properties -in the ultra relativistic flow regime of $\Gamma \sim 10-10^{3}$- such as angular distribution, acceleration time constant, particle energy gain versus number of crossings and spectral shapes. We perform calculations for sub-luminal and super-luminal shocks, using two different approaches respectively. The $\Gamma^{2}$ energization for the first crossing cycle and the significantly large energy gain for subsequent crossings as well as the high 'speed up' factors found, are important in supporting the Vietri and Waxman models on GRB ultra-high energy cosmic ray, neutrino, and gamma-ray output.Comment: 24 pages, 35 figures, accepted for publication in Astroparticle Physic