Exciton Scattering Research Articles

Exciton scattering approach for branched conjugated molecules and complexes. IV. Transition dipoles and optical spectra

The electronic excitation energies and transition dipole moments are the essential ingredients to compute an optical spectrum of any molecular system. Here we extend the exciton scattering (ES) approach, originally developed for computing excitation energies in branched conjugated molecules, to the calculation of the transition dipole moments. The ES parameters that characterize contributions of molecular building blocks to the total transition dipole can be extracted from the quantum-chemical calculations of the excited states in simple molecular fragments. Using these extracted parameters, one can then effortlessly calculate the oscillator strengths and optical spectra of various large molecular structures. We illustrate application of this extended ES approach using an example of phenylacetylene-based molecules. Absorption spectra predicted by the ES approach show close agreement with the results of the reference quantum-chemical calculations.

Exciton scattering approach for branched conjugated molecules and complexes. III. Applications

The exciton scattering (ES) approach is an efficient tool to calculate the excited states electronic structure in large branched polymeric molecules. Using the previously extracted parameters, we apply the ES approach to a number of phenylacetylene-based test molecules. Comparison of ES predictions with direct quantum chemistry results for the excitation energies shows an agreement within several meV. The ES framework provides powerful insights into photophysics of macromolecules by revealing the connections between the molecular structure and the properties of the collective electronic states, including spatial localization of excitations controlled by the energy.

Exciton scattering approach for branched conjugated molecules and complexes. I. Formalism

We develop a formalism for the exciton scattering (ES) approach to calculation of the excited state electronic structure of branched conjugated polymers with insignificant numerical expense. The ES approach attributes electronic excitations in quasi-one-dimensional molecules to standing waves formed by the scattering of quantum quasiparticles. We derive the phenomenology from the microscopic description in terms of many-electron excitations. The presented model can be used to compute both excited state frequencies and transition dipoles in large molecules after the ES ingredients are extracted from smaller molecular fragments.

Exciton scattering approach for branched conjugated molecules and complexes. II. Extraction of the exciton scattering parameters from quantum-chemical calculations

We obtain the parameters of the exciton scattering (ES) model from the quantum-chemical calculations of the electronic excitations in simple phenylacetylene-based molecules. We determine the exciton dispersion and the frequency-dependent scattering matrices which describe scattering properties of the molecular ends as well as of meta- and orthoconjugated links. The extracted functions are smooth, which confirms the validity of the ES picture. We find a good agreement between the ES and quantum-chemical results for the excitation energies in simple test molecules.

Multiscale Modeling of Electronic Excitations in Branched Conjugated Molecules Using an Exciton Scattering Approach

The exciton scattering (ES) approach attributes excited electronic states in quasi-1D branched polymer molecules to standing waves of quantum quasiparticles (excitons) scattered at the molecular vertices. We extract their dispersion and frequency-dependent scattering matrices at termini, ortho, and meta joints for pi-conjugated phenylacetylene-based molecules from atomistic time-dependent density-functional theory (TD DFT) calculations. This allows electronic spectra for any structure of arbitrary size within the considered molecular family to be obtained with negligible numerical effort. The agreement is within 10-20 meV for all test cases, when comparing the ES results with the reference TD DFT calculations.

Exciton scattering and localization in branched dendrimeric structures

π-conjugated dendrimers are molecular examples of tree-like structures known in physics as Bethe lattices. Electronic excitations in these systems can be spatially delocalized or localized depending on the branching topology. Without a priori knowledge of the localization pattern, understanding photoexcitation dynamics reflected in experimental optical spectra is difficult. ‘Supramolecular’-like quantum-chemical calculations quickly become intractable as the molecular size increases. Here we develop a reduced exciton-scattering (ES) model, which attributes excited states to standing waves in quasi-one-dimensional structures, assuming a quasiparticle picture of optical excitations. Direct quantum-chemical calculations of branched phenylacetylene chromophores are used to verify our model and to derive relevant parameters. Complex and non-trivial delocalization patterns of photoexcitations throughout the entire molecular tree can then be universally characterized and understood using the proposed ES method, completely bypassing ‘supramolecular’ calculations. This allows accurate modelling of excited-state dynamics in arbitrary branched structures.

Scattering of hot excitons due to optical phonons in quantum wells: Multiphonon resonant Raman process

The hot-exciton scattering rate by optical phonons is calculated in the framework of the envelope-function approximation in narrow quantum wells (QWs). The exciton-phonon Fr\ohlich-type interaction is considered and the contributions of the confined phonons and interface modes to the total scattering rate are discussed in terms of the in-plane exciton kinetic energy and quantum-well width. To take into account the full symmetry of the optical phonons a phenomenological approach is employed in which the mechanical and electrostatic matching boundary conditions are fulfilled at QW interfaces. It is shown that the main contribution to the scattering rate comes from intrasubband transition assisted by phonons with even electrostatic potential state. The experimental observation in short period GaAs/AlAs superlattices on the phonon symmetries and their relative intensities of multiphonon resonant Raman process is qualitatively explained in terms of the relative role of the confined and interface optical phonons on the exciton scattering rate.

Emission Due to Exciton Scattering by LO-Phonons in Gallium Nitride

Emission Due to Exciton Scattering by LO-Phonons in Gallium Nitride

Emission Due to Exciton Scattering by LO-Phonons in Gallium Nitride

Emission Due to Exciton Scattering by LO-Phonons in Gallium Nitride

Scattering and Lifetime of Hot Excitons Interacting with LO-Phonons in InBr and InI Polar Semiconductors

Scattering and Lifetime of Hot Excitons Interacting with LO-Phonons in InBr and InI Polar Semiconductors

Scattering and Lifetime of Hot Excitons Interacting with LO-Phonons in InBr and InI Polar Semiconductors

Scattering probabilities and lifetime for a hot Wannier-Mott exciton which interacts with LO-phonons in InBr and InI polar semiconductors are obtained for T = 0. The calculations were performed for exciton states with principal quantum number n = 2 in the states l = 1, m = 0 and l = 1, m = ±1, when electron and hole effective masses are equal (m e = m h ). The results obtained can be used to explain the special characteristic of secondary radiation emission spectrum measured by Yoshida et al. in 1985. In the latter work, the scattering lines show an alterating intensity which can be accounted for only if the electron and hole effective masses are nearly equal. All computation was performed for different scattering probabilities of the excitons, namely: intraband (W s ), inter-exciton band (W 2→1 ), disintegration into the continuous spectrum (W d ), as well as the lifetime of the exciton in both states. The results show that W s is larger than W 2→1 , even in the case m e = m h , for a wide range of kinetic energy of the excitons. A comparison of these results with those for the case of excitons in the state n = 1 is also made.

Scattering of excitons by free carriers in narrow and wide quantum wells in the presence of a transverse electric field

The effects of an applied transverse electric field on the free carrier-exciton scattering cross sections in semiconducting quantum well structures, are theoretically investigated in the limit of narrow and wide wells, using a finite confining potential model. Quasi 3D features in the scattering cross section in the narrow well limit where the exciton wave function penetrates into the confining barrier region are found to be rather insensitive to changes in electric field. However, the free carrier-exciton elastic cross section in the wide wells shows a trend similar to that due to free carrier-free carrier scattering, in the presence of the transverse field. Comparisons are made with previous models to account for these new effects.

Auger decay of degenerate and Bose-condensed excitons in Cu2O.

We study the non-radiative Auger decay of excitons in Cu$_2$O, in which two excitons scatter to an excited electron and hole. The exciton decay rate for the direct and the phonon-assisted processes is calculated from first principles; incorporating the band structure of the material leads to a relatively shorter lifetime of the triplet state ortho excitons. We compare our results with the Auger decay rate extracted from data on highly degenerate triplet excitons and Bose-condensed singlet excitons in Cu$_2$O.

Scattering of Excitons by Free Carriers in Semiconducting Quantum Well Structures: Finite Potential Well Model

AbstractWe present a theoretical calculation of the elastic and ionization scattering cross sections of excitons due to their scattering by free carriers, in semiconducting quantum well structures of finite width, using the finite confining potential well model. Our results reveal interesting features that differ significantly from the predictions of the infinite confining potential well model. We find that both the elastic and ionization cross sections initially decrease with decreasing well width, but then in‐ crease below a critical well width where the exciton begins to revert to a quasi‐3D behavior. We compare the present results with those obtained using previous models to account for these new effects.

Scattering of Excitons by Phonons in Quantum Wells

A method to describe the effects of the exciton-optical phonon interaction is presented using the cumulant expansion approximation. The functional-integral technique of coherent phonon states is used in order to justify the commonly used model Hamiltonian and generate the proper perturbation series. The influence of the mutual electron—hole screening on the polaronic effects in quantum wells is analyzed. PACS numbers: 73.20.Dx, 71.38.+i, 72.80.Ey

Scattering and relaxation of low-energy excitons in quantum wells

We consider in this work the scattering of low-energy excitons in single quantum wells. In the first part we consider the elastic scattering by a static centre (charged impurity) and discuss the validity of the first Born approximation, which is expected to work well only for particles with large kinetic energy. The second part is devoted to the inelastic scatterings of individual excitons by a phonon reservoir. We discuss in particular the validity of the elastic approximation, which is well known to be very accurate for particles with large kinetic energy.

Scattering of excitons by free carriers in semiconducting quantum well structures: Dependence of scattering cross sections on well width

We have theoretically investigated the scattering of excitons by free electrons and holes in a semiconducting quantum well of finite width. The elastic and ionization scattering cross sections have been calculated as a function of the width of the well using the Born approximation for scattering of the excitons by the free carriers. The behavior of the scattering cross section as a function of the energy of relative motion of the free carriers and the excitons is similar to that previously obtained from 2D calculations for narrow wells. The cross section increases as the well width increases and the relative wave vector of the free carrier at which the total cross section is a maximum shifts to lower values as the well width increases.

Scattering and decay of hot excitons due to acoustic phonons in bulk semiconductors with a degenerate valence band

The scattering and decay probabilities of hot Wannier-Mott excitons, interacting with acoustic phonons in 3D semiconductors with a degenerate valence band are calculated in both cases when temperature is equal to zero and it is finite, the real energy band structure being taken into account. The results show that the scattering probability of the heavy exciton is in all cases greater than that of the light one, and on the contrary, the decay probabilities of the light excitons are greater than that of the heavy ones. The results would be applied to calculate the distribution functions of the heavy and light excitons in semiconductors with a degenerate valence band.

An Approach to the Additional Boundary Condition by a Model of Exciton Scattering by the Surface

A new type of the additional boundary condition(ABC) based on a picture of the surface scattering of exciton is proposed and applied to the oblique incidence reflectance in the case of s- and p-polarized lights. The calculated reflectance is compared with the experiment on CuCl. The spikes appeared in the reflectance spectra of the p-polarized light for the intermediate range of the angle of incidence are well reproduced by the present ABC, but by neither Pekar's ABC, the dielectric approximation, nor the dead layer model.

Scattering of excitons by free carriers in semiconducting quantum well structures

We have theoretically investigated the scattering of excitons by free electrons and holes in a two-dimensional semiconducting quantum well system. The scattering cross-sections have been calculated using the Born approximation for both the elastic and inelastic scattering of the excitons by the free carriers. The threshold for inelastic scattering is increased over the value in a bulk semiconductor because of the enhancement of the exciton binding energy by its confinement. The behavior of the scattering cross-section as a function of the energy of relative motion of the free carriers and the excitons is different than in the bulk and the cross-section is a more sensitive function of the ratio of the electron and hole masses than in the bulk. In fact, the scattering of light hole excitons is suppressed relative to that of heavy hole excitons by several orders of magnitude.