Showing papers on "Exciton published in 1975"
TL;DR: A comprehensive review of the experimental data which have been obtained on singlet exciton energy transfer in organic crystals is presented in this article, where the usefulness of different types of experimental techniques in elucidating the characteristics of both exciton migration and trapping is discussed.
353 citations
TL;DR: In this paper, a model is proposed to explain the luminescence properties of YVO4 crystals based on transitions within the VO3-4 ion, which predicts the observed temperature and concentration dependences for the fluorescence intensities and lifetimes.
227 citations
TL;DR: In this article, optically pumped laser oscillation from multilayer heterostructures consisting of alternating layers of GaAs and Al0.2Ga0.8As was reported.
Abstract: We report optically pumped laser oscillation from multilayer heterostructures consisting of alternating layers of GaAs and Al0.2Ga0.8As. Very thin GaAs layers (50−500 A) exhibit one−dimensional bound states above the band gap of bulk GaAs. The laser oscillation occurs at energies which are slightly below the exciton associated with the lowest energy n=1 bound state.
180 citations
174 citations
161 citations
TL;DR: This CD effect appears to be a sensitive probe to study changes in protein mobility and in protein-protein interactions and is suggested that DMSO increases the protein mobility in the membrane.
154 citations
TL;DR: Within the frame work of Pekar's model for a spatially dispersive, bounded medium, optical spectra of thin crystals, reported by as discussed by the authors, showed a structure due to the interference of additional waves and the Fabry-Perot structure deviating markedly from the one expected in the classical case in the neighbourhood of exciton resonances.
Abstract: Within the frame work of Pekar's model for a spatially dispersive, bounded medium, optical spectra of thin crystals, reported by us recently, are analyzed, showing a structure due to the interference of additional waves and the Fabry-Perot structure deviating markedly from the one expected in the classical case in the neighbourhood of exciton resonances. A comparison of the experimental reflectivity with that obtained numerically provides values for all physical parameters defining dispersion and damping of the photoexcitons. Some additional experimental results for excitons in thin crystals are reported: evidence for the induced surface wave, an effect of the forbidden exciton, etc.
[Russian Text Ignored]
130 citations
TL;DR: In this article, the optical transmission of 2H-MoS2 and 2HMoSe2 has been measured below the A exciton energy for each material and the absorption coefficients have been calculated, and the low absorption starting at 1·1-1·2 eV has been attributed to an indirect d-d transition for both materials.
Abstract: The optical transmission of thick samples of 2H-MoS2 and 2H-MoSe2 has been measured below the A exciton energy for each material. The absorption coefficients have been calculated, and the low absorption starting at 1·1–1·2 eV has been attributed to an indirect d-d transition for both materials. There appears to be considerable d-p valence band overlap giving rise to further higher absorption before the strong A, B excitons, which have been associated with direct transitions from a p-like valence band. An energy band scheme has been sketched on the basis of these interpretations.
112 citations
TL;DR: In this paper, a core surface exciton associated with the Ga dangling bonds is interpreted as an enhancement of valence-electron emission for initial states at and below the valence band maximum when the photon energy is scanned through the threshold for Ga-$3d$-core to intrinsic surface-state excitations.
Abstract: Synchrotron-radiation photoemission data are obtained from GaAs ($\overline{1}\overline{1}0$) and (110) faces for $s$ and $p$ polarization which show an enhancement of valence-electron emission for initial states at and below the valence-band maximum when the photon energy is scanned through the threshold for Ga-$3d$-core to intrinsic-surface-state excitations. The enhancement is interpreted as a core surface exciton associated with the Ga dangling bonds.
110 citations
TL;DR: In this paper, a series of phonon-coupled emission bands with a no-phonon peak at 1.454 eV was found to be dominant in most samples and grows in intensity with increasing In concentration.
Abstract: Photoluminescence measurements have been performed at 4.2 K between 1.2 and 1.6 eV on compensated high‐resistivity CdTe containing from 1015 to 1018 cm−3 indium. A series of phonon‐coupled emission bands with a no‐phonon peak at 1.454 eV is dominant in most samples and grows in intensity with increasing In concentration. On the basis of the compensation model, the 1.454‐eV emission is attributed to radiative recombination at an [cadmium vacancy‐In] acceptor complex. Comparison of the experimental results with this model suggests that the chemical equilibria governing formation of the acceptor complex are frozen in at approximately 500 K as the sample is cooled from the growth temperature. The existence of significant changes at 500 K in the mobility of defects related to the acceptor complex is confirmed by the observation of a prominent growth in the 1.454‐eV emission at an annealing temperature of 500 K in A+‐implanted n‐type CdTe. Exciton emission in the CdTe : In samples is dominated by a narrow bound...
98 citations
TL;DR: In this article, an interpretation is proposed that the absorption structure of alkali halides around the exciton peak originates from a situation where the self-trapped state is energetically close to the lowest edge of the nearly-free state.
Abstract: Absorption and emission spectra of excitons interacting with optical or intramolecular vibrations are calculated with the dynamical coherent-potential approximation proposed previously. Investigations are made for various situations to which limiting concepts of nearly-free, self-trapped and vibronic excitons are not applicable straightforwardly. An interpretation is proposed that the absorption structure of alkali halides around the exciton peak originates from a situation where the self-trapped state is energetically close to the lowest edge of the nearly-free state. In this case, where the exciton-phonon and the excitation-transfer interactions are comparable to each other and strong, configuration mixing between the two types of states is small. When the two types of interaction are weak and comparable to each other, an intermediate state between the nearly-free and the vibronic-exciton states is realized. This situation explains the progressional structure in the exciton absorption band of crystallin...
TL;DR: In this paper, it was shown that up to four excitonic series in Mo$S 2 can be obtained by photoconductivity, photovoltaic effect and wavelength-modulated reflectivity at temperatures of 300 to 4.2 K and in magnetic fields of up to 70 kG.
Abstract: Optical spectra of Mo${\mathrm{S}}_{2}$ in the excitonic region were obtained by photoconductivity, photovoltaic effect and wavelength-modulated reflectivity at temperatures of 300 to 4.2 K and in magnetic fields of up to 70 kG. Cleaved natural crystals and synthetic crystals studied in both the Faraday and Voigt configurations showed nearly the same characteristics. The results indicate the possibility of up to four excitonic series in Mo${\mathrm{S}}_{2}$ as in other Mo dichalcogenides, and support band-structure calculations predicting flat conduction and valence bands originating from Mo orbitals. A ground-state anomaly of the $A$ exciton is explained by a central cell correction of the type used by Harbeke and Tosatti to explain a similiar anomaly in Pb${\mathrm{I}}_{2}$. The reduced mass of the $A$ exciton is found to be ${\ensuremath{\mu}}^{*}\ensuremath{\simeq}0.4{m}_{0}$, both from the study of the series itself, and from its magneto-optical properties, thus removing a large discrepancy found in previous work.
TL;DR: In this paper, an experimental study of the photoelectric yield of doped solid rare gases in the extreme ultraviolet (h/ω = 8−30 eV) spanning the range of impurity excitations, exciton states, and interband transitions was performed.
Abstract: In this paper we report the results of an experimental study of the photoelectric yield of doped solid rare gases in the extreme ultraviolet (h/ω = 8−30 eV) spanning the range of impurity excitations, exciton states, and interband transitions. Results were obtained for Xe in Kr, Xe in Ar, Kr in Ar, and benzene in Ar, Kr, and Xe. For dilute atomic and molecular impurities in solid rare gases three intrinsic photoemission mechanisms are exhibited: (a) direct excitation from the impurity state above the impurity threshold, (b) electronic energy transfer from the host exciton states to the impurity states resulting in exciton induced impurity photoemission, and (c) direct photoemission from the host matrix at energies above the matrix threshold. The photoemission thresholds from impurity states via processes (a) or (b) result in detailed information regarding electron affinities of solid rare gases which are in good agreement with recent data for the pure solids. A detailed study of exciton induced photoemiss...
TL;DR: In this article, the generalized-master-equation (GME) theory of excitation transfer is applied to the standard model of linearly interacting excitons and phonons, the spectral prescription for memory functions given by Kenkre and Knox is verified for the model, peculiarities of memory functions pertinent to isolated versus open systems are analyzed, and exciton transport relevant to representative memory functions is studied.
Abstract: The generalized-master-equation (GME) theory of excitation transfer is applied to the standard model of linearly interacting excitons and phonons, the spectral prescription for memory functions given by Kenkre and Knox is verified for the model, peculiarities of memory functions pertinent to isolated versus open systems are analyzed, and exciton transport relevant to representative memory functions is studied. Physical relations are established between the GME theory and the stochastic-Liouville-equation theories, thus completing the author's earlier formal study of these relations. The influence of spectral features on exciton motion is analyzed in detail and it is shown that the presence of zero-phonon peaks in optical spectra is an indication of long-time coherence in exciton motion. Criteria for the validity of theories in the context of a given real system are developed on the basis of the spectral analysis.
TL;DR: In this paper, the dispersion relations of band-gap excitons in covalent cubic crystals are investigated and a perturbation method is used following Baldereschi and Lipari.
Abstract: The dispersion relations of band-gap excitons in covalent cubic crystals are investigated. For low momentum a perturbation method is used following Baldereschi and Lipari. At very low momentum the dispersion is strongly affected by a splitting of the fourfold $K=0$ degeneracy for those materials with nonspherical conduction-band energy surfaces. In silicon and germanium this splitting leads to strong nonparabolic effects at energies in the 0.1-meV range. For large momentum a band decoupling scheme which generalizes the center-of-mass transformation method is derived and the concept of "light-mass" and "heavy-mass" excitons naturally results. Numerical values are given for germanium, silicon, and gallium arsenide.
TL;DR: In this article, the authors reported the observation of a second zero-phonon line, induced by a static electric field, lying 10 cm−1 above the first singlet charge-transfer exciton state of crystalline anthracene-PMDA.
Abstract: This paper reports the observation of a second zero‐phonon line, induced by a static electric field, lying 10 cm−1 above the first singlet charge‐transfer exciton state of crystalline anthracene‐PMDA. The splitting of these charge‐transfer exciton states is a quadratic function of the electric field strength for low fields and becomes linear for fields of order 104 V/cm. The origin of these features is discussed on the basis of a one‐dimensional model of the charge‐transfer exciton in the linear donor–acceptor stack. The effect of charge‐transfer admixture to the neutral donor–acceptor ground state and the effect of creating a local charge‐transfer (ionic) state by the absorption of light are considered by use of a second‐quantized Hamiltonian for the charge‐transfer exciton. The splitting of 10 cm−1 observed by use of the static electric field, which removes the inversion invariance of the one‐dimensional donor–acceptor chain along the stack axis arises from the mixing of symmetric and antisymmetric exci...
TL;DR: In this paper, the authors reported two sets of differential reflectivity measurements (electroreflectivity and piezoreflectivity) performed on GaAs under high-resolution conditions.
Abstract: Recent measurements of the lasing energy as a function of temperature in high‐purity GaAs lasers have attempted to investigate the relation between this energy and the one‐electron band gap. Because of a lack of precision in the position of the band gap at high temperature, these measurements show strongly conflicting results. In this work, we report two sets of differential reflectivity measurements (electroreflectivity and piezoreflectivity) performed on GaAs under high‐resolution conditions. Both series of results give for the excitonic absorption edge at room temperature a value E0=1.424±0.002 eV which is about 20 meV higher than the lasing energy reported for the highest‐purity GaAs samples. This result confirms that the lasing energy in GaAs is well below the one electron band gap. In addition, we show that this energy separation is an increasing function of temperature. Finally we discuss a simple model of band‐to‐band recombination including electron‐electron interaction effects and we show that i...
TL;DR: In this article, the optical properties of the excitonic molecule are clarified based on the electronic structure of the molecule derived in the preceding paper, and the luminescence, the optical conversion of a single exciton into the excITonic molecule and the giant two-photon absorption due to the molecules are theoretically discussed and compared with the available experimental data.
Abstract: The optical properties of the excitonic molecule are clarified, basing upon the electronic structure of the excitonic molecule derived in the preceding paper. The luminescence, the optical conversion of a single exciton into the excitonic molecule and the giant two-photon absorption due to the excitonic molecules are theoretically discussed and compared with the available experimental data.
TL;DR: In this article, the authors measured the phosphorescence of betamethylnaphthalene doped into a naphalene−h 8/naphthallene−d 8 mixed crystal and demonstrated that dynamical exciton percolation does occur.
Abstract: The phosphorescence of betamethylnaphthalene doped into a naphthalene−h 8/naphthalene−d 8 mixed crystal has been measured. The results demonstrate that (1) dynamical excitonpercolation does occur (i.e., a transition from an excitoninsulator to an exciton conductor), that (2) it is very useful for the investigation of energy transfer in molecular aggregates, and that (3) it is a critical test of our current knowledge of exciton exchange and superexchange. (AIP)
TL;DR: The dynamics of exciton-polariton recombination in CdS were investigated by measuring time-resolved luminescence spectra between 15 and 80 K.
Abstract: The dynamics of exciton-polariton recombination in CdS are investigated by measuring time-resolved luminescence spectra between 15 and 80 K Decay times of free and bound excitons in $\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}\ensuremath{\perp}\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}$ and $\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}\ensuremath{\parallel}\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}$ polarization are determined for various types of crystals using low-excitation densities The experiments prove the existence of a bottleneck for the relaxation and establish its location Transverse excitons above the "knee" of the dispersion curve are found to be in thermal equilibrium with the lattice at temperatures higher than 30 K At low lattice temperatures, excitons above the "knee" are in partial quasiequilibrium shortly after pulse excitation The effective temperature of this exciton distribution exceeds that of the lattice and decreases rapidly within the observed exciton lifetime of 25-3 nsec
TL;DR: In this article, a theory of electron-hole condensation in germanium and silicon is developed in which recombination, evaporation, and exciton collection are treated as random processes.
Abstract: A theory of electron-hole condensation in germanium and silicon is developed in which recombination, evaporation, and exciton collection are treated as random processes. An expression is derived for the probability distribution of the number of electron-hole pairs bound to a nucleation center. Consideration of the effects of lifetime, surface tension, and nucleation center leads to: (i) the relation of multiexciton complexes to electron-hole droplets; (ii) stable droplet sizes which are strong functions of temperature but only weak functions of pair-generation rate; (iii) the possibility of determining the surface tension from size measurements; and (iv) deviations from the usual phase diagram for a liquid-gas transition at very low temperatures. Detailed numerical calculations are carried out for uniform excitation of germanium and compared to data available from laser excitation experiments.
TL;DR: In this paper, the triplet excitons are generated by the recombination of charge carriers through double injection and the lifetime of the triplets is studied from the decay of the phosphorescence intensity in a transient measurement.
Abstract: The interactions of triplet excitons with trapped charge carriers are studied in anthracene single crystals. The triplet excitons are generated by the recombination of charge carriers through double injection. The lifetime of the triplet excitons is studied from the decay of the phosphorescence intensity in a transient measurement. The interaction rate constant for the trapped charge carriers is of the same order of magnitude as previous published data measured with single injection of electrons. It is further shown that the rate constant varies with different conditions of crystal growth. The simultaneous investigation of double injection delayed fluorescence reveals also a strong quenching of the delayed fluorescence from the excited singlet state by trapped charge carriers. Magnetic field measurements show that this quenching is partly due to an increased dissociation of the triplet pair state through the presence of charge carriers. The possibility of enhanced spin relaxation of the triplets in the pa...
TL;DR: In this paper, upper bounds for the ground-state energy of the exciton-phonon system were derived for all ratios of the polaron radii to the excitons radius.
TL;DR: RefReflection spectra of the layered semiconductor PbI2 have been measured in the band gap region at 1.5 °K and they can be interpreted in terms of a normal ( 1 n 2 ) Wannier exciton series.
TL;DR: In this paper, the reflection spectrum of the first singlet transition of tetracene crystal has been measured for temperatures ranging from 300 to 1.7 ˚ K. At the lowest temperatures two anomalous reflectivity minima are observed inside the b−polarized stopping band that are very similar to structure in the corresponding reflection band of anthracene crystalline.
Abstract: The reflection spectrum of the 0–0 component of the first singlet transition of tetracene crystal has been measured for temperatures ranging from 300 to 1.7 °K. A discontinuous change in the Davydov splitting occurs near 186 °K signifying the occurrence of a phase transition. At the lowest temperatures two anomalous reflectivity minima are observed inside the b‐polarized stopping band that are very similar to structure in the corresponding reflection band of anthracene crystal. The reflectivity minima are tentatively assigned to transitions to surface exciton states lying approximately 5 cm−1 and 160 cm−1 above the b‐polarized exciton transition at 5349 A. The other structure within the reflection band is interpreted as a coupling of the polariton to vibrational modes of intra‐ and intermolecular origins.
TL;DR: In this paper, the conductivity spectrum was fitted using effective mass theory for excitons with asymmetric Lorentzian lineshapes, and the band gap was found to be about 14.5 eV.
TL;DR: In this article, it was shown that perturbed excitons with the molecular configuration of an H center contribute to the fluorescence of alkaline earth fluorides with X-rays.
Abstract: Irradiation of alkaline earth fluorides with X-rays produces a strong fluorescence band in the ultraviolet. This band shows pronounced polarization effects when the crystals are placed in a magnetic field. EPR experiments on excited states using optical detection show that perturbed excitons with the molecular configuration of an H centre contribute to the fluorescence.