Energy Conservation Criterion Research Articles

On the possibility of singlet fission in crystalline quaterrylene.

Singlet fission (SF), the spontaneous down-conversion of a singlet exciton into two triplet excitons residing on neighboring molecules, is a promising route to improve organic photovoltaic (OPV) device efficiencies by harvesting two charge carriers from one photon. However, only a few materials have been discovered that exhibit intermolecular SF in the solid state, most of which are acene derivatives. Recently, there has been a growing interest in rylenes as potential SF materials. We use many-body perturbation theory in the GW approximation and the Bethe-Salpeter equation to investigate the possibility of intermolecular SF in crystalline perylene and quaterrylene. A new method is presented for determining the percent charge transfer (%CT) character of an exciton wave-function from double-Bader analysis. This enables relating exciton probability distributions to crystal packing. Based on comparison to known and predicted SF materials with respect to the energy conservation criterion (ES-2ET) and %CT, crystalline quaterrylene is a promising candidate for intermolecular SF. Furthermore, quaterrylene is attractive for OPV applications, thanks to its high stability and narrow optical gap. Perylene is not expected to exhibit SF; however, it is a promising candidate for harvesting sub-gap photons by triplet-triplet annihilation.

Hubbard nanoclusters far from equilibrium

The Hubbard model is a prototype for strongly correlated many-particle systems, including electrons in condensed matter and molecules, as well as for fermions or bosons in optical lattices. While the equilibrium properties of these systems have been studied in detail, the nonequilibrium dynamics following a strong non-perturbative excitation only recently came into the focus of experiments and theory. It is of particular interest how the dynamics depend on the coupling strength and on the particle number and whether there exist universal features in the time evolution. Here, we present results for the dynamics of finite Hubbard clusters based on a selfconsistent nonequilibrium Green functions (NEGF) approach invoking the generalized Kadanoff--Baym ansatz (GKBA). We discuss the conserving properties of the GKBA with Hartree--Fock propagators in detail and present a generalized form of the energy conservation criterion of Baym and Kadanoff for NEGF. Furthermore, we demonstrate that the HF-GKBA cures some artifacts of prior two-time NEGF simulations. Besides, this approach substantially speeds up the numerical calculations and thus presents the capability to study comparatively large systems and to extend the analysis to long times allowing for an accurate computation of the excitation spectrum via time propagation. Our data obtained within the second Born approximation compares favorably with exact diagonalization results (available for up to 13 particles) and are expected to have predictive capability for substantially larger systems in the weak coupling limit.

Polarized infrared reflectivity of 2D sea surfaces with two surface reflections

Sea surface infrared reflectivity is an important parameter in oceanic remote sensing. Most analytical models consider single surface reflections, and the polarization is usually ignored. However, a loss of energy is reported for large observation angles (θ>50°) because multiple surface reflections are ignored. This article generalizes the infrared reflectivity derivation of Li et al. (Appl. Opt., Vol. 52, 6100–6111, 2013) for 1D surfaces (2D problems) to 2D surfaces (3D problems), so that the cross-polarization effect can be taken into account. The contributions of one and two successive surface reflections are analyzed separately. The bidirectional reflectivity and the hemispherical reflectivity are studied. It is shown that the sea surface infrared reflectivity is significant for large incidence and observation angles. The energy conservation criterion is then checked. The loss of energy is largely reduced after taking into account two surface reflections.

Energy conservation of the scattering from one-dimensional random rough surfaces in the high-frequency limit

Energy conservation of the scattering from one-dimensional strongly rough dielectric surfaces is investigated using the Kirchhoff approximation with single reflection and by taking the shadowing phenomenon into account, both in reflection and transmission. In addition, because no shadowing function in transmission exists in the literature, this function is presented here in detail. The model is reduced to the high-frequency limit (or geometric optics). The energy conservation criterion is investigated versus the incidence angle, the permittivity of the lower medium, and the surface rms slope.

Influence of interfacial slits with elastic slip on the propagation of SH waves

This work examines the influence of various parameters, especially the effect of elastic slip at a single slit or a periodic array of slits situated at the interface between two isotropic elastic media on the propagation of waves. The analysis consists of transforming the formulation of the problem into an integral equation for the jump of the diffracted elastic displacement, at the interface, and solving this numerically after decomposition into Chebyshev polynomials and truncation of the resulting infinite series by use of an energy conservation criterion. The effects are illustrated on two characteristic features, the polar diagram of the amplitude of the far‐field diffracted displacement, and the transmission and reflection coefficients. The features revealed provide an indication of the usefulness of the results in assessing the quality of bonding at interfaces via a wave‐analysis technique.

Solving slab lamellar grating problems by the singular-value-decomposition method

We formulate a slab lamellar grating problem as a linear system and first use the traditional matrix-inversion technique in order to solve the system. The system can readily be solved without difficulties if its dimension is small; however, the solution may not be accurate owing to truncation. As the dimension increases, the system becomes numerically singular, which prevents solution by traditional inversion methods. We then apply the singular-value-decomposition (SVD) method in order to overcome this difficulty and thus obtain the solution. Our results demonstrate that the solution is satisfactory because it meets fundamental energy-conservation criteria and satisfies boundary conditions. Our study concludes that, for grating problems involving numerical singularities, the SVD technique is superior to the inversion technique.

Dissociative excitation of TlI and TlCl by rare-gas metastable atoms and N2 A 3Σ+u

The reactions of TlCl and TlI with rare gas He(3S), Ar(3P2), Kr(3P2), Xe(3P2), and N2(A 3Σ+u) metastable species were studied in a flowing afterglow apparatus. In all cases, emission from the excited states of Tl was observed, and with helium we also observed the 190.8 nm line issued from the 3P1 level of Tl+. We have verified that these excited levels are populated following a first-order process. The rate constants for the different exit channels of dissociative excitation have been determined and the influence of radiative cascade effects is discussed. The largest rate constants, on the order of 10−10 cm3 mol−1 s−1, were obtained for the reactions of TlCl and TlI with xenon, and TlCl with krypton, for which an ionic exit channel is not possible because of energy conservation criteria. The rate constants for the reactions with argon are much smaller, about 10−13 cm3 mol−1 s−1. A large rate constant, 1.3×10−9 cm3 mol−1 s−1 was obtained for the excitation of Tl+(3P1) level by helium metastable atoms.

Acoustic wave scattering from a fluid/solid periodic rough surface

A rigorous theory based on the extended boundary condition method is proposed to solve the problem of elastic wave scattering from a periodic fluid/solid interface. The diffraction efficiencies of the reflected compressional wave in the fluid and the transmitted shear and compressional waves are calculated. The energy conservation criterion is used to check the accuracy of the numerical results. The effect of loss (viscoelasticity) in the solid is also included. The wave diffraction from a water and acrylic rough surface is measured. Good agreement between the theory and the experiment is obtained.

A two-Gaussian model for stabilized filament formation in nonlinear media

We describe the effects of energy conservation on the growth of a Gaussian perturbation superimposed on a Gaussian profile laser beam. Using a simple two-step energy conservation criterion, we obtain a relationship between the self-focal distance zff for the smooth Gaussian profile laser beam and the distance Dt at which the growth in the Gaussian spike terminates. For a very intense laser beam, with appreciable amplitude of the perturbation, the growth of the perturbation is studied using a continuous energy-conservation criterion. The results explain the stabilization of intensity in the perturbation at a certain distance inside the medium. Our model explains formation of bright stabilized filaments and gives reasonable estimates for the distance of the first appearance of filaments.