Showing papers on "Exciton published in 1976"
TL;DR: In this paper, the authors present a critical review of the theoretical developments in this field, focusing on a different aspect of molecular crystals: the transport of electronic excitation from molecule to molecule.
Abstract: In the last ten years, a number of reviews of properties of molecular crystals have appeared in this series (1, 2, 3). The spectral properties of insulating molecular crystals [which are often described within the framework of the Frenkel theory of excitons (4, 5)] have been reviewed by Hochstrasser (I) and by Robinson (2). The properties of charge transfer crystals have been reviewed recently by SODS (3). In addition, the review by El-Sayed (6) on double resonance techniques applied to triplet states of organic molecules concerned itself with organic molecular crystals. In the present review, I focus on a different aspect of molecular crystals: the transport of electronic excitation from molecule to molecule. This process, and its effect on optical spectra, electron paramagnetic resonance spectra, and fluorescence, has been of great interest in the last few years; I present a critical review of the theoretical developments in this field. Specific attention is paid to the theory of exciton transport in molecular crystals, the theory of exciton-molecular vibration and exciton-phonon interactions, and finally, to a quite different topic, the theory of excitation transport across surfaces. Those works dealing with the optical properties of molecular crystals within the coupled oscillator model are not dealt with. It will be assumed that the interaction of light with the system is weak enough so that retardation effects and the formation of polariton states may be neglected. Only work dealing with excitation transport is dealt with herein. We begin by discussing the simplest type of system in which excitation transfer takes p lace in order to define the terms necessary to treat excitation transfer in more complex systems.
205 citations
TL;DR: In this article, it was shown that the catalysis of carbon monoxide oxidation on a thoria surface treated in oxygen and in vacuo is accompanied by a specific luminescence.
161 citations
TL;DR: In this paper, the cyclotron resonance of electrons and of holes in Cu2O has been observed between 1.2 and 20K using frequencies of 34 and 137 GHz, and the authors attribute the electron resonance to the lowest Gamma 1+( Gamma 6+) minimum of the conduction band and the heavier hole to the uppermost Gamma 7+ hole band.
Abstract: The cyclotron resonance of electrons and of holes in Cu2O has been observed between 1.2 and 20K using frequencies of 34 and 137 GHz. Electrons have an isotropic polaron mass of (0.99+or-0.03)m0 where m0 is the mass of the free electron. Holes have isotropic polaron masses of (0.58+or-0.03)m0 and (0.69+or-0.04)m0, the error bars defining the points of half-height on the observed resonance curves. The authors attribute the electron resonance to the lowest Gamma 1+( Gamma 6+) minimum of the conduction band and the heavier hole to the uppermost Gamma 7+ hole band. The lighter hole is attributed tentatively to the Gamma 8+ maximum of the hole band. The exciton spectra of Cu2O are discussed in terms of these masses.
128 citations
TL;DR: In this paper, the authors reported time-resolved measurements of optical emission and absorption in crystals of fluorite, and showed that the lowest triplet states of the self-trapped exciton resemble a metastable nearest-neighbor fluoride vacancy-interstitial pair, i.e., an $F$-$H$-center pair, with probable point symmetry.
Abstract: We report time-resolved measurements of optical emission and absorption in crystals of ${\mathrm{CaF}}_{2}$, ${\mathrm{SrF}}_{2}$, and ${\mathrm{BaF}}_{2}$ following excitation by pulsed ionizing radiation. Prominent transient absorption bands in the visible and ultraviolet are observed, and their decay is resolved into several time components which correspond to those of the emission. The absorption is shown to originate in the same excited electronic states as the intrinsic recombination luminescence. These states have been identified previously as self-trapped excitons, largely on the basis of polarization measurements and optically detected EPR in the excited states. These results, augmented by kinetic data from the present experiments and by new polarization data, suggest a fairly detailed picture of the various localized electronic states involved in the process of electron-hole recombination in fluorites. The ionic relaxation associated with the self-trapped exciton is evidently more complex than in, e.g., the alkali halides; in the fluorites the lowest triplet states of the self-trapped exciton have been found to resemble a metastable nearest-neighbor fluoride vacancy-interstitial pair, i.e., an $F$-$H$-center pair, with probable point symmetry ${C}_{1h}$.
120 citations
TL;DR: In this article, the emission spectrum of ZnO crystals in the band edge region was investigated and it was shown that the emission lines at λ = 3696.58 A and λ= 3692.66 A are due to an exciton bound to a neutral Li and Na acceptor, respectively.
111 citations
TL;DR: In this article, upper bounds to the free energy of an exciton interacting with a lattice of phonons with a linear coupling were discussed, and it was shown that the effective number of phonon-closeness changes with exciton bandwidth and temperature, and the possibility of an abrupt transition from a delocalized to localized exciton with an increase in temperature was discussed.
Abstract: We discuss upper bounds to the free energy of an exciton interacting with a lattice of phonons with a linear coupling. From these upper bounds, we find the effective number of phonons clothing an exciton to vary with exciton bandwidth and temperature. The possibility of an abrupt transition from a delocalized to a localized exciton with an increase in temperature is discussed.
103 citations
TL;DR: In this paper, the exciton absorption peak in a III-V ternary alloy was observed and the most accurate determination of both the composition dependence of the Γ band gap and the position of the X crossover was made.
102 citations
TL;DR: In this article, the capacitancevoltage and forward biased current-voltage characteristics, electroluminescence (E.L) and photovoltaic effect of GaSe-SnO2 heterojunction diodes are measured.
Abstract: The capacitance-voltage (C-V) and forward biased current-voltage (I-V) characteristics, electroluminescence (E.L) and photovoltaic effect of GaSe–SnO2 heterojunction diodes are measured. SnO2 layer is deposited on the c-plane of GaSe by using the spray method. The C-V and I-V characteristics of these diodes reveal the existence of a high resistivity layer, probably due to the diffusion of Sn into GaSe. The width of this layer is about 2.6 µm. And the current transport mechanism at low voltage is space-charge-limited. The trap density and the energy level of the trap from the valence band estimated by Lampert theory are about 5×1013~1×1014 cm-3 and 0.4~0.6 eV, respectively. The electroluminescence spectra at 275 K show one emission band due to free exciton recombination.
94 citations
TL;DR: In this article, the A exciton series in the liquid-nitrogen-temperature epsilon 2 spectra of 2H-WSe2 and 3R-WS2, derived from Kramers-Kronig analyses of normal-incidence, basal-plane reflectivity spectra, have been fitted to a Lorentzian oscillator model.
Abstract: The A exciton series in the liquid-nitrogen-temperature epsilon 2 spectra of 2H-WSe2 and 3R-WS2, derived from Kramers-Kronig analyses of normal-incidence, basal-plane reflectivity spectra, have been fitted to a Lorentzian oscillator model. It has been found that the energy levels and oscillator strengths of the excitons obey a three-dimensional theoretical model for 2H-WSe2 and a two-dimensional model for 3R-WS2. The experimental observations are consistent with Toyozawa's (1958) theory for weak exciton-phonon interactions and small exciton effective mass.
91 citations
TL;DR: In this article, the sputtering patterns of NaCl irradiated with low energy electrons were separated into component features from the directional ejection of halogen atoms and a random evaporation of the metal atoms.
Abstract: We have separated the sputtering patterns of NaCl irradiated with low energy electrons into component features from the directional ejection of halogen atoms and a random evaporation of the metal atoms. The sputtering yields anti-correlate with exciton luminescence. Lifetime studies reveal the presence of several exciton states with different diffusion energies. The sputtering yield shows a resonance with the production of surface excitons. The experiments confirm the view that defect formation in the halogen sub-lattice proceeds by a non-radiative exciton decay which initiates a replacement collision sequence along a chain of halogen ions.
83 citations
TL;DR: In this article, the authors measured the giant two-photon absorption spectrum for the direct generation of excitonic molecules using a frequency tunable dye laser, and the absorption peak was found at 3.1870 eV.
Abstract: Giant two-photon absorption spectrum for the direct generation of excitonic molecules is measured with using a frequency tunable dye laser, and absorption peak is found at 3.1870 eV. Emission spectrum with excitation at the giant two-photon absorption band depends on the spectral width, \(\varDelta\varOmega_{1}\), of excitation light. In the case of \(\varDelta\varOmega_{1}{=}0.25\) meV, the two-photon resonance Raman scattering is found to be predominant. The Raman process involves an excitonic molecule and a longitudinal exciton as the intermediate and final states, respectively. On the other hand, for the case of \(\varDelta\varOmega_{1}{=}2.3\) meV, the emission spectrum shows a very sharp line at 3.1649 eV, which has been previously ascribed to be due to the Bose-Einstein condensation of excitonic molecules. Discussions are made on the dependence of emission spectrum upon the energy band-width of excitation light.
TL;DR: The optical constant of synthetic 2HMoS2 has been determined over the range 0.05 to 3.75 eV as mentioned in this paper, where the absorption edge consists of two series of exciton bands due to direct allowed transitions, and the Zeeman splitting and diamagnetic shift of these exciton lines are measured together with the effects of crystal confinement and temperature.
Abstract: The optical constantsof synthetic 2HMoS2 have been determined over the range 0.05 to 3.75 eV. The absorption edge consists of two series of exciton bands due to direct allowed transitions. The Zeeman splitting and diamagnetic shift of these exciton lines are measured together with the effects of crystal confinement and temperature; the derived exciton parameters are different from those of molybdenite. Oscillatory magneto-absorption occurs at the high energy side of each exciton series which gives a measure of the carrier masses.
TL;DR: In this paper, the effect of surface transitions on the bulk reflection spectrum using equations given by a microscopic theory of surface and bulk exciton states was studied and the agreement between the calculated and experimental spectra is excellent.
Abstract: The polarized reflection spectrum of the first singlet transition of crystalline anthracene at 2 °K is reported for light at near normal angles of incidence on the (001) face. The effect on the spectra of condensing small amounts of gases, transparent in the 400 nm region, has been studied and found to be a useful technique for differentiating surface from bulk exciton transitions. The optical properties of the bulk crystal have been calculated by a Kramers–Kronig transformation of the reflectivity data. The complex refractive index so obtained was used to calculate the effect of surface transitions on the bulk reflection spectrum using equations given by a microscopic theory of surface and bulk exciton states. The agreement between the calculated and experimental spectra is excellent. Model calculations show that the two‐dimensional sums of excited state van der Waals interactions are of sufficient range to create a series of excitons localized on surface planes. It is also demonstrated that the reflecti...
TL;DR: In this paper, the authors presented a detailed calculation of the transition temperature of a model filamentary excitonic superconductor, consisting of a linear chain of transition-metal atoms to which is complexed a ligand system of highly polarizable dyelike molecules.
Abstract: We present a detailed calculation of the transition temperature of a model filamentary excitonic superconductor. The proposed structure consists of a linear chain of transition-metal atoms to which is complexed a ligand system of highly polarizable dyelike molecules. Calculations of the electronic properties and experimental data on related materials are used to estimate the strength of the excitonic interaction, Coulomb repulsion, and band structure. From this the superconducting transition temperature was calculated by integration of the gap equation. For the particular structure proposed, transition temperatures of several hundred degrees are calculated. However, we find superconductivity only in those systems where the excitonic medium is within a covalent bond length of, and completely surrounds, the conductive spine. This imposes severe constraints on the structure of any excitonic superconductor. We show that for the structure proposed the momentum dependence of the exciton interaction results in the superconducting state being favored over the Peierls state and in vertex corrections to the electron-exciton interaction which are small.
TL;DR: In this paper, a basis for the theory of electroabsorption by excitons in anisotropic and layered semiconductors is developed, in which the exciton are assumed to be two-dimensional.
Abstract: A basis for the theory of electroabsorption by excitons in anisotropic and layered semiconductors is developed, in which the excitons are assumed to be two dimensional. The effective-mass equation for a two-dimensional exciton in a uniform electric field of arbitrary strength is solved exactly (numerically), and the electroabsorption spectra computed for direct allowed, direct "forbidden" (second class), and indirect allowed transitions. The calculated spectra are compared with corresponding spectra for three-dimensional excitons and with spectra calculated neglecting the electron-hole interaction. The results are tabulated in a form that facilitates comparison with experimental data.
TL;DR: In this paper, the polaritons at the bottleneck determine the zero-phonon line, while Polaritons maintaining approximately the thermal-equilibrium distribution in the energy region above E 0 determine the LO-photon sideband, and the temperature and crystal-thickness dependences of the luminescence spectrum and the quenching of the luminance by impurities are discussed in terms of the polariton bottleneck.
Abstract: The intensity ratio of the LO-phonon sideband to the zero-phonon line and the width of the zero-phonon line in the free-exciton luminescence spectrum can never be understood, especially at low temperatures, by the usual theory with the assumption of the thermal equilibrium for excitons. The theory assuming exciton thermal-equilibrium gives too small values for these quantities compared with the observed ones. These large discrepancies are removed by considering the exciton luminescence from the polariton viewpoint. In this viewpoint, polaritons accumulate around the bottleneck in polariton decay lower a little than the lowest exciton energy E 0. Polaritons at the bottleneck determine the zero-phonon line, while polaritons maintaining approximately the thermal-equilibrium distribution in the energy region above E 0 determine the LO-phonon sideband. The temperature and crystal-thickness dependences of the luminescence spectrum and the quenching of the luminescence by impurities are also discussed in terms of the polariton bottleneck.
TL;DR: In this article, the phase memory time T M in the triplet state of the pair of two translationally inequivalent naphthalene molecules in an external magnetic field as a function of temperature was measured.
TL;DR: In this article, specular reflectance spectra at near-normal incidence were measured in the silicates olivine, pyroxene, spinel and Al-garnet in the photon energy range 4-14eV at 300"K, and the optical and dielectric functions were obtained from the reflectance by Kramers-Kronig analysis.
Abstract: Summary Unpolarized specular reflectance spectra at near-normal incidence were measured in the silicates olivine, pyroxene, spinel and Al-garnet in the photon energy range 4-14eV at 300"K, and the optical and dielectric functions were obtained from the reflectance by Kramers-Kronig analysis. The spectra indicate that these minerals, with a variety of divalent cation substitutions, have wide valence and conduction bands separated by an energy gap (EJ of 7.5-9.5eV that is marked by a prominent absorption peak of an exciton associated with this gap. EB increases with the ratio x = Mg/(Mg+Fe+Mn+Ni+Ca) and in olivine it varies from 7.8 eV in fayalite to 8.8 eV in forsterite. For a given x, Eg increases in the order spinel, olivine, pyroxene, garnet over a range of about 1-5 eV. While the pure Mg-silicates are transparent at photon energies below the exciton peak, transition metal substitutions result in strong absorption beginning about 1-2 eV below Eg, presumably due to electronic transitions between the wide bands and d states localized on the transition metal cations. The large magnitude of Eg rules out the possibility that electronic band conduction due to valence-conduction band excitations is an important conduction mechanism in mantle silicates.
TL;DR: In this paper, the ionization yield in rare-gas-doped liquid Ar excited by $ √ √ Bi$ conversion electrons was investigated and the saturated value of the enhanced ionization was in good agreement with the theoretical value obtained under the assumption that the same electron band structure and the same exciton states as those in solid Ar exist in the liquid, too.
Abstract: Experimental investigations have been performed on the ionization yield in rare-gas-doped liquid Ar excited by $^{207}\mathrm{Bi}$ conversion electrons. The enhanced ionizaiton yield (13% relative to the ionization yield in pure liquid Ar) is observed for Xe-doped (1.6%) liquid Ar and is attributed to the ionizing excitation transfer process from Ar excitons to doped Xe. The saturated value of the enhanced ionization is in good agreement with the theoretical value obtained under the assumption that the same electron band structure and the same exciton states as those in solid Ar exist in the liquid, too. This provides strong evidence for the existence of the exciton states in liquid Ar. No significant increase in ionization is observed with Kr-doped liquid Ar. The relaxation processes in liquid Ar are discussed.
TL;DR: In this article, it was found that $F$ centers are formed upon promotion of self-trapped excitons from the lowest metastable level to the second excited level in KCl.
Abstract: Metastable relaxed excitons created by pulsed electron irradiation in alkali-halide crystals are excited to higher states by a laser pulse. Consequent changes in excitonic absorption and emission yield data on triplet-singlet conversion and on polarization of the optical transitions. It is found that $F$ centers are formed upon promotion of self-trapped excitons from the lowest metastable level to the second excited level in KCl.
TL;DR: In this paper, the percolation theory and the cluster structure for binary randomly mixed crystals were formulated in terms of percolations and the clustering structure of binary and ternary mixed crystals, and an important limiting case (exciton supertransfer) was derived for long exciton lifetime relative to jumping and trapping time.
Abstract: The exciton transfer, via migration and trapping, in binary and ternary mixed crystals is formulated in terms of percolation theory and the cluster structure for binary randomly mixed crystals. An important limiting case (exciton supertransfer) is derived for long exciton lifetime, relative to jumping and trapping time. The exciton supertransfer case is solved analytically [in terms of the functions derived by J. Hoshen and R. Kopelman, Phys. Rev. B (in press)] and the solutions involve neither physical parameters nor physical constants. Other limiting cases are derived, as well as an algorithm for the general energy transfer case. This algorithm relates the migration and trapping in binary and ternary systems with the trapping‐free migration in binary systems. The algorithm involves the use of empirical information, i.e., the parameters describing the exciton dynamics in a pure crystal. The various formulations are valid for concentrations both above and below the critical (’’percolation’’) concentration...
TL;DR: In this article, the spectroscopic properties of the purple membrane of Halobacterium halobium in the visible region are examined within the framework of an exciton model for the retinal chromophores in bacteriorhodopsin (bR).
Abstract: The spectroscopic properties of the purple membrane of Halobacterium halobium in the visible region are examined within the framework of an exciton model for the retinal chromophores in bacteriorhodopsin (bR). Both earlier x‐ray and electron diffraction studies as well as more recent circular dichroism (CD) measurements indicate that considerable chromophore interaction is likely in the purple membrane. This interaction disappears when the protein is solubilized. The exciton model accounts for the red shift of the visible absorption band in the membrane from the absorption peak seen in solubilized bR. It explains the approximate conservation of absorption intensity as well as the changes in the observed CD spectrum when bR is solubilized. The dipole moments of the Cl− salt of the protonated N‐butyl Schiff base of all‐trans retinal in its electronic ground and first dipole allowed excited state have recently been measured. If these are introduced into the exciton model an estimate of the distance of adjace...
TL;DR: In this paper, the density of states and transition rates for the equidistant spacing model with the limitation on the hole maximum energy were derived and the influence of the finite depth of nuclear potential well on the pre-equilibrium decay within the exciton model was investigated.
Abstract: The density of states and the transition rates are derived for the equidistant spacing model with the limitation on the hole maximum energy. The previous results of Blann are generalized. The influence of the finite depth of nuclear potential well on the pre-equilibrium decay within the exciton model is investigated. The calculations of proton and deuteron spectra from the reaction120Sn+p are presented.
TL;DR: In this article, the temperature dependence of sensitized delayed fluorescence in naphthalene-anthracene mixed crystals has been analyzed, and a direct trap to trap energy transfer already at relatively low concentrations is observed.
TL;DR: RefReflectance spectra of SrO and BaO powders outgassed at temperatures up to 1073 K have been measured over the range from textdecoration:overlineν= 20 000 cm-1 to text-decoration= 52 000 cm -1.
Abstract: Reflectance spectra of SrO and BaO powders outgassed at temperatures up to 1073 K have been measured over the range from text-decoration:overlineν= 20 000 cm–1 to text-decoration:overlineν= 52 000 cm–1. Spectral features attributable to exciton states are described, and the effects of O2, N2O, CO2 and H2O on the spectra are reported. Fluorescence occurs when spectra are measured in vacuo. SrO-coated MgO and BaO-coated MgO have also been studied. The spectra show two regions separated by an absorption edge which is centred at 44 000 cm–1 for SrO and 31 500 cm–1 for BaO. On the low text-decoration:overlineν side of the edge the spectra are sensitive to the adsorption of the gases, but exposure to gases causes little or no change in the spectra on the high text-decoration:overlineν side (except for H2O vapour which reacts in depth). The absorption bands at low text-decoration:overlineν are ascribed to excitons bound at surface ions and those at high text-decoration:overlineν to excitons bound at ions in the bulk.The chemical interaction of the gases with the surface states increases in intensity in the order O2 < N2O < CO2 < H2O for SrO and N2O < O2 < CO2 < H2O for BaO. The results are correlated with those for MgO and CaO reported previously.
TL;DR: In this article, a model Hamiltonian for a system of interacting electrons, holes and Wannier excitons is derived and the thermodynamic potential is calculated with the aid of Bogolubov's variational principal.
Abstract: A model Hamiltonian for a system of interacting electrons, holes and Wannier excitons is derived. This system of electronic excitations is assumed to be in a quasi-equilibrium state. With the aid of Bogolubov's variational principal the thermodynamic potential is calculated. Using the most general mean-field Hamiltonian as a trial Hamiltonian, a set of coupled integral equations is obtained for the self-energies. These equations are solved numerically for equal effective masses of the electrons and holes. Below a critical temperature ofkBTc≈0.65Eexb whereEexb is the exciton binding energy, we find a first order phase transition from an exciton rich phase into a degenerate electron-hole phase. The mechanical and thermal stability of both phases is proven. Below a critical temperaturekBTc≈0.11Eexb the exciton system becomes degenerate (Bose-Einstein condensation). A complete phase diagram of these three phases is given.
TL;DR: In this paper, a broadening of both the recombination luminescence and absorption spectra of indirect excitons in Ge is observed, and the broadening is found to be density dependent consistent with an exciton-exciton collision process.
Abstract: A broadening of both the recombination luminescence and absorption spectra of indirect excitons in Ge is observed. This broadening is found to be density dependent consistent with an exciton-exciton collision process. An effective exciton collision radius of 220 \ifmmode\pm\else\textpm\fi{} 80 \AA{} is measured in agreement with theoretical estimates. Density-modulated absorption spectra also indicate an apparent red shift of the exciton energy at high carrier densities. A detailed analysis of the exciton line shape leads to a better understanding of the thermodynamics of the coexisting electron-hole liquid and exciton gas, provides improved values of the binding energy, $\ensuremath{\varphi}=1.8\ifmmode\pm\else\textpm\fi{}0.2$ meV, and the ground-state energy per pair, ${E}_{G}=\ensuremath{-}5.9\ifmmode\pm\else\textpm\fi{}0.3$ meV, and gives the most nearly complete description of the gas-liquid phase boundary. The complications due to density gradients which arise from surface excitation with be briefly discussed.
TL;DR: In this paper, the effect of spectral linewidth and splitting on the coherence of excitation undergoing transfer was investigated in both real and model systems, and several examples derived from both real-and model systems were presented.