Showing papers on "Exciton published in 1978"

Dynamical screening and self‐energy of excitons in the electron–hole plasma

Abstract: The dependence of two-particle energies on plasma density is investigated using Green's functions. An effective exciton equation is derived taking into account dynamically screened potential and one-particle self-energies. A variational calculation shows that the bound state levels (exciton energies) remain practically constant whereas the continuum edge (band gap) shifts to lower energies with increasing density, which is in accordance with experimental observations. Die Abhangigkeit der Zwei-Teilchen-Energien von der Plasmadichte wird im Formalismus Greenscher Funktionen untersucht. Es wird eine effektive Exziton-Gleichung abgeleitet, die ein dynamisch abgeschirmtes Potential und Ein-Teilchen-Selbstenergien berucksichtigt. Eine Variationsrechnung zeigt, das Energieniveaus gebundener Zustande (Exzitonenenergien) praktisch konstant bleiben, wahrend sich die Kontinuumskante (die Bandlucke) mit wachsender Dichte zu kleineren Energien verschiebt, was mit experimentellen Ergebnissen ubereinstimmt.

Excitons in rare-gas crystals

Abstract: The paper is a survey of experimental and theoretical results on exciton states in crystals of xenon, krypton, argon and neon. The absorption and luminescence spectra of the crystals and the band structure of electron and exciton states are analysed. Some features of the exciton-phonon interaction in pure and doped crystals are discussed. In contrast with other types of molecular crystal, those of rare gases can simultaneously support free and localized excitons. All features of the exciton spectra, in particular the hydrogen-like series of light absorption, the rich pattern of luminescence and the absorption and emission bands due to impurity centres, have been given a natural explanation in terms of the intermediate-radius exciton strongly bonded to the crystal lattice. These investigations of excitons in atomic cryocrystals have revealed a number of new effects and extended our knowledge of the cooperative excitation states of dielectrics.

Urbach's rule derived from thermal fluctuations in the band-gap energy

Abstract: The exponential absorption edge (known as Urbach's rule) observed in most materials is interpreted in terms of thermal fluctuations in the band-gap energy. The main contribution to the temperature shift of the band-gap energy is due to the temperature-dependent self-energies of the electrons and holes interacting with the phonons. Since the phonon number is fluctuating in thermal equilibrium, the band-gap energy is also fluctuating resulting in an exponential absorption tail below the average band-gap energy. These simple considerations are applied to derive Urbach's rule at high temperatures, while a simplified model with independent, noninteracting atoms is proposed to explain the behavior of Urbach's rule in the whole temperature range. The three parameters entering Urbach's rule are expressed in terms of parameters derived from the temperature shift of the band gap and from the exciton absorption. Comparison with experiments is performed for the II-VI compound ZnO. It is shown that a good agreement is found between the temperature shift of the exciton line observed experimentally and the temperature shift computed from the steepness parameter of Urbach's rule. The agreement with experimental values for the two other parameters is also satisfactory. It is shown that the band-gap shift (and absorption tail) in ZnO is caused by interaction with both acoustical and optical phonons. While the temperature-dependent polaron contribution can account for the optical-phonon contribution, the deformation-potential interaction with LA phonons is not sufficient to account for the acoustical-phonon contribution.

Band-Gap Assignment in Sn O 2 by Two-Photon Spectroscopy

Abstract: We present for the first time two-photon measurements on rutile Sn${\mathrm{O}}_{2}$. The detailed polarization analysis yields ${{\ensuremath{\Gamma}}_{3}}^{+}$ symmetry for the direct forbidden $1S$ exciton and the upper valence band. Our results clear up the long-standing controversy concerning the symmetry of the upper valence band.

Evidence for a triplet state of the self-trapped exciton states in liquid argon, krypton and xenon

Abstract: The lifetimes of luminescence from liquid argon, krypton and xenon excited by energetic electrons have been studied with such a high electric field that all of the observed decay characteristics have to be attributed to the self-trapped exciton luminescence. In all cases the decay shows two exponential components. The lifetimes of the fast components are in the range from 2 to 5 ns, and major second components have lifetimes of 860+or-30 ns for argon, 80+or-3 ns for krypton and 27+or-1 ns for xenon. These lifetimes are interpreted in terms of two self-trapped exciton states, one predominantly singlet in character, the other triplet; this is the first strong evidence for the triplet assignment. The variation in triplet-state lifetimes is attributed to the different spin-orbit couplings.

Intrinsic luminescence excitation spectrum and extended x-ray absorption fine structure above the K edge in Ca F 2

Abstract: Excitation spectra of the 4.44-eV intrinsic luminescence in a single crystal of Ca${\mathrm{F}}_{2}$ at liquid-nitrogen temperature was measured using synchrotron radiation in the x-ray region. The intrinsic luminescence at 4.44 eV is due to self-trapped exciton recombination. The intensity of the emitted uv light was measured as a function of the x-ray photon energy from below the calcium $K$ edge to 200 eV above the edge. Dips were observed in the luminescence excitation spectrum corresponding to peaks in the absorption coefficient both in the core excitation region near the edge and in the region of excitation to the continuum far above the edge. The effect is correlated with modulation of the radiative recombination probability of the core hole. The close relationship between the inverse of the luminescence yield and the extended x-ray absorption fine structure (EXAFS) after background subtraction shows that excitation spectra of uv luminescence can be used to measure EXAFS in thick single crystals and biological samples with suitable luminescence centers.

Effects of structural disorder on the optical properties of molecular crystals

Abstract: In this paper we advance a model for absorption line shapes of Frenkel excitons in molecular crystals, considering the effects of static disorder and of weak exciton–phonon coupling. Our treatment is limited to single‐particle excitations located at the bottom of the exciton band. The effects of disorder scattering at zero temperature are handled by applying the average t matrix, single‐site approximation to a system characterized by a Gaussian distribution of site excitation energies. The simultaneous effects of disorder scattering and of phonon scattering were treated by the introduction of an effective exciton–phonon Hamiltonian, which was subsequently utilized within the framework of the single‐site approximation for disorder scattering at finite temperatures. Model calculations of the optical absorption line shapes were performed for a quasi‐one‐dimensional system. The zero‐temperature line shapes are asymmetrically broadened, the high energy edge being close to a Lorentzian, while the low energy edge decreases fast with decreasing energy. This asymmetry disappears at higher temperatures where phonon scattering effects dominate. We propose that the asymmetric low‐temperature absorption band in 1, 4‐dibromonaphthalene originates from the effects of structural disorder. The asymmetric optical line broadening experimental data for this quasi‐one‐dimensional system are well accounted for in terms of our theory.

Phonon interactions in the luminescence of amorphous silicon

Abstract: Experimental data and a new model of luminescence in glow-discharge-deposited amorphous silicon are described. The shape of the luminescence excitation spectrum rules out the previous model based on zero-phonon transitions. Instead, phonon interactions give a Stokes shift of 0.4–0.5 eV and the recombination is due to self-trapping of excitons bound to band tail localized states. The model explains the luminescence peak position and line width, the Stokes shift, and the thermal quenching of the luminescence intensity.

Core excitons and the dielectric response of polystyrene and poly(2‐vinylpyridine) from 1 to 400 eV

Abstract: The complete spectra of valence and core electronic excitations of atactic polystyrene, PS, and poly(2‐vinylpyridine), PVP, from 1 to 400 eV were measured by electron energy loss spectroscopy. Energy losses of 80 keV electrons transmitted through 1000 A thick films were measured with a resolution of 0.1 eV. The spectrum of PS contains several previously unreported broad peaks between 9 and 20 eV which coincide with structure observed in gas phase benzene spectra and which have been attributed to Rydberg states, but which are described here as valence molecular orbital excitations. The dielectric response function of PS and PVP from 1 to 100 eV is computed from energy loss data. The calculated reflectivity and refractive index are in excellent agreement with recent optical measurements. The spectrum of carbon core electron excitations above 285 eV consists of a series of sharp peaks due to transitions to empty molecular states (core excitons) followed by transitions to a free‐electron‐like continuum of states. The core exciton spectrum is quantitatively described in a model based on a molecular orbital ground state calculation (CNDO/S). The results show that such spectra provide a measurement of the distribution of empty molecular orbitals. In addition, shifted exciton peaks are observed which correspond to inequivalent core electron binding energies due to differences in net atomic charge.

The effects of impurity scattering and transport topology on trapping in quasi-one-dimensional systems: Application to excitons in molecular crystals

Abstract: A model is developed which permits a detailed examination of the effects of impurity scattering and deviations from a strictly one‐dimensional transport topology on the trapping rate and on the mean‐square displacement of mobile species in systems which are nearly one dimensional in their transport characteristics, i.e., quasi‐one‐dimensional. The model is applied specifically to Frenkel excitons in molecular crystals, but may be readily adapted to other types of systems. Both coherent (wavelike) and incoherent (diffusive) microscopic modes of exciton transport are considered. In the strictly one‐dimensional limit following pulsed optical excitation, a time‐dependent trapping rate function is obtained as opposed to the commonly employed trapping rate constant. It is demonstrated that the coherent and incoherent trapping rate functions have identical dependencies on time and on impurity concentration and the macroscopic rate of transport can be calculated. To treat deviations from strictly one‐dimensional ...

Multiphonon processes in YbS

Abstract: The resonance of the Raman scattering by the 1 $\mathrm{LO}(\ensuremath{\Gamma})$ phonon and its overtones has been investigated in YbS. The results are interpreted in terms of a configuration-coordinate model which considers a strongly interacting exciton-LO-phonons system. The model is also able to account for multiphonon structure in the wavelength-modulated reflectance. The phonon energy observed in the reflectance spectrum is, however, somewhat smaller than that observed in Raman scattering, a fact which is attributed to a frequency renormalization in the excited state of the exciton.

Effects of impurity scattering and transport topology on exciton migration and trapping: An experimental study of quasi-one-dimensional molecular crystals

Abstract: A series of experiments in which the time resolved triplet x‐trap emission from single crystals of 1,2,4,5‐tetrachlorobenzene (TCB) at 1.35 °K is presented for various concentrations of the doped‐in scattering impurity, d2‐TCB. It is demonstrated that exciton‐impurity scattering is the dominant process affecting macroscopic exciton transport and trapping. The time‐dependent trapping rate is found to be proportional to the inverse square root of the scattering impurity concentration in agreement with theoretical prediction. This implies that transport is close to strictly one‐dimensional. Excellent agreement between the data and a model involving microscopically incoherent transport is found, but the data also shows generally good agreement with a model employing microscopically coherent transport. From the concentration dependence and time‐dependent trapping curves, an upper bound of ∼5×103 sec−1 can be placed on the frequency of multidimensional steps between one‐dimensional chains. Transport is macrosco...

Luminescence decay in glow-discharge deposited amorphous silicon

Abstract: The photoluminescence decay of glow-discharge-deposited amorphous silicon is reported. The low temperature radiative lifetime of the broad luminescence band near 1·3 eV is found to be about 30 μs, and is independent of luminescence energy. The long lifetime is possibly due to weak electron-hole overlap. A competing non-radiative process dominates in weakly luminescent doped and undoped samples, and is thought to be due to tunnelling of an exciton to a nearby defect.

Theory of exciton transport with quadratic exciton-phonon coupling

Abstract: The coupling of excitons with phonons is an important process determining the mechanism of exciton transport in molecular crystals. Although linear exciton–phonon coupling has been extensively treated before, there has been little work on quadratic coupling. In this paper the theory of exciton transport is extended to take weak quadratic coupling into account, and the diffusion constant computed for a simple model. The results are qualitatively similar to those obtained from other treatments of linear and quadratic coupling, and may show a rather weak temperature dependence.

Exciton line shapes and migration with stochastic exciton lattice coupling

Abstract: In a recent article, Sumi [J. Chem. Phys. 67, 2943 (1977)] has discussed the optical line shape of an exciton interacting with phonons using a Gaussian Markov process for this interaction. By assuming that the correlation time of the process is nonzero, he was able to explore various limits of (motional) narrowing of the line. His analysis used a dynamic coherent potential approximation (CPA) in order to calculate the line shape. In the present paper, we derive these results in closed analytic form, without the CPA, by using standard analysis. Since our results agree with Sumi very closely, the present approach provides a simple way of understanding the underlying physics. In addition, we show how the exciton density of states enters in a simple way, and compare the hemicircular and the Lorentzian forms for this density of states.

Recombination; a survey

Abstract: Auger, optical, multiphonon and cascade are the main recombination processes known. Carriers can recombine from band to band, or after they have formed an exciton, or after one or both are trapped at an impurity or defect. This paper gives attention to recombination by bound and free excitons, and wide-gap materials are discussed as well as semiconductors, because the recombination mechanisms are often better understood. We treat particularly the competition between radiative and multiphonon process, including that of Dexter, Klick and Russell. It is pointed out that cascade trapping by a charged centre does not always give rise to a large (∼ 10−12cm2) cross section. Exciton lifetime and fluorescence are described. Configuration diagrams are illustrated by the cases of Vk centres and similar behaviour in semiconductors where the valence band is of lone pair type and in insulators such as As2Te3 and SiO2. Recombination in some amorphous semiconductors is contrasted with that in crystals; SiO2 presents some special problems. Finally the effect of magnetic field and an ESR signal is described.

Angular dependence of the reflection spectra and directional dispersion of the anisotropic exciton polaritons in ZnO

Abstract: The reflection spectra of the ordinary and extraordinary polaritons near the A1-, B1-exciton resonances of ZnO are presented for two geometries: normal reflection of planes inclined to the c-axis at varying degrees, and oblique reflection of as-grown (10I10) faces. The experimental and theoretical spectra are compared. The calculations are based on a) the dispersion relations derived from Fresnel's equation taking account of spatial dispersion, b) the Pekar additional boundary conditions combined with an exciton free surface layer. There are indications of the correlation between the thickness of this layer and decreasing oscillator strength of the polaritons. Die Reflexionsspektren der ordentlichen und auserordentilichen A1-, B1-Exziton Polaritonen von ZnO werden fur zwei Geometrien untersucht: senkrechte Reflexion an unter verschiedenen Winkeln zur c-Achse geneigten Flachen und Reflexion bei schragem Lichteinfall an gewachsenen (10I10)-Flachen. Die gemessenen Spektren werden mit den theoretischen verglichen. Grundlagen fur die Rechnungen sind a) die von der Fresenlelschen Gleichung abgeleiteten Dispersionsbeziehungen mit raumlicher Dispersion, b) die zusatzlichen Randbedingungen von Pekar mit exzitonfreier Randschicht. Der sich andeutende Zusammenhang zwischen der Dicke der Randschicht und abnehmender Oszillatorstarke der Polaritonen wird diskutiert.

Electronic properties of (100) surfaces of GaSb and InAs and their alloys with GaAs

Abstract: Smooth, monocrystalline (100) surfaces of the alloys In1-xGaxAs and GaSb1-yAsy were prepared by molecular beam epitaxy. Both As-stabilized c(2 × 8) and metal-stabilized c(8 × 2) surface reconstructions were observed for In1-xGax,As over the entire alloy range. GaS1-yAsy exhibited a c(2 × 6) or (2 × 3) structure for y ≤ 0.2, and, after a transition region, the anion-stabilized c(2 × 8) or the Ga-stabilized c(8 × 2) structures for y ≥ 0.5. Electron energy loss spectroscopy revealed the simultaneous presence of two empty, dangling-bond-derived surface states in both alloy systems. For In1-xGaxAs the In-derived empty surface state lies ≅0.4 - 0.5 eV below that of Ga and moves from above the conduction band edge into the band gap for x ≥ 0.6. The overlap between the Ga- and In-derived empty surface states causes the quenching of the Ga(3d) surface exciton. For GaSb1-yAsy the Sb dangling bonds generate an empty, localized surface state which lies 0.2 - 0.3 eV above the empty, Ga-derived surface state. Both levels lie above the conduction band edge throughout the alloy range.

ENDOR of the self-trapped exciton in KCl

Abstract: ENDOR of the self-trapped exciton in KCl is studied using the optical detection technique. The contact interaction with the neighbouring nuclei is very similar to that of the F centre in its ground state which indicates that the radial extent of electronic wavefunctions are similar in the two cases. The quadrupole resonance of the two central halogen nuclei was also detected, observation of two different quadrupole splittings indicates that the self-trapped exciton is slightly displaced from the centrosymmetrical position between two lattice sites.

Photoluminescence in CuGaSe2

Abstract: The photoluminescence spectra of CuGaSe2 have been investigated at 76 K in the range 0.65 3 4 m grating monochromator. The luminescence observed consisted of a rather sharp structure near the energy gap, plus two broad peaks at considerably smaller energies. The broad peaks, centered around 1.52 and 1.30 eV respectively, are attributed to transitions involving deep localized states. The sharper structure near the energy gap is resolved into two peaks, one at 1.72 eV and another at 1.68 eV. This structure is explained by radiative recombination of free excitons (peak at 1.72 eV), and by recombination of free electrons with bound holes (peak at 1.68 eV). A thermal treatment done on the CuGaSe2 crystals changed both their electrical properties and their luminescence spectra. These changes produced by sample treatment, plus measurements of the excitation power dependence and the polarization of the luminescent radiation, are in agreement with the interpretation presented.

Photocarrier thermalization by laser excitation spectroscopy

Abstract: The excitation spectroscopy of photoluminescence lines in semiconductors, i.e. the study of the variation of intensity and lineshape of the lines with exciting photon energy, permits to follow in a detailed manner the energy relaxation path of hot photocreated electrons. Many quantities such as electron and exciton densities, electron and exciton effective temperatures are shown to oscillate as a function of exciting energy. We show that the fundamental oscillating quantity in GaAs at low temperatures is the electron effective temperature, which in turn influences other quantities. The possibility to measure at once the electron density and the electronic temperature permits to explain oscillatory photoconductivity.

Resonant Raman Scattering under Two-Photon Excitation of Excitonic Molecules in CdS

Abstract: Two-photon-resonant Raman scattering under the giant two-photon excitation of excitonic molecule has been observed from various scattering directions, and two types of Raman scattering, M T and M M are found. The transverse and mixed modes of the \(A(\varGamma_{5})\) exciton state are left behind in the M T and M M Raman processes, respectively. These Raman scatterings reflect the characteristic dispersions of their respective exciton states, and conform to the geometrical selection rules derived on the assumption that the excitonic molecule state has the \(\varGamma_{1}\)-symmetry. A non-linear absorption peak due to the giant two-photon absorption is found at 2.5506 eV (λ=486.10 nm) with the use of intense probe light. The binding energy of excitonic molecule is found to be 4.4 meV for two \(A(\varGamma_{6})\) triplet excitons.