Showing papers on "Exciton published in 1969"
TL;DR: The transition metal dichalcogenides are about 60 in number as discussed by the authors, and two-thirds of these assume layer structures and can be cleaved down to less than 1000 A and are then transparent in the region of direct band-to-band transitions.
Abstract: The transition metal dichalcogenides are about 60 in number. Two-thirds of these assume layer structures. Crystals of such materials can be cleaved down to less than 1000 A and are then transparent in the region of direct band-to-band transitions. The transmission spectra of the family have been correlated group by group with the wide range of electrical and structural data available to yield useful working band models that are in accord with a molecular orbital approach. Several special topics have arisen; these include exciton screening, d-band formation, and the metal/insulator transition; also magnetism and superconductivity in such compounds. High pressure work seems to offer the possibility for testing the recent theory of excitonic insulators.
3,313 citations
TL;DR: In this paper, it was shown that, when steps can be taken to near-neighbor lattice points only, as N → ∞, the required number of steps is 〈n〉∼{N2/6,
Abstract: The following statistical problem arises in the theory of exciton trapping in photosynthetic units: Given an infinite periodic lattice of unit cells, each containing N points of which (N − 1) are chlorophyll molecules and one is a trap; if an exciton is produced with equal probability at any nontrapping point, how many steps on the average are required before the exciton reaches a trapping center for the first time? It is shown that, when steps can be taken to near‐neighbor lattice points only, as N → ∞, our required number of steps is 〈n〉∼{N2/6, linear chain,π−1NlogN, square lattice,1.5164N, single cubic lattice. The correction terms for medium and relatively small N are obtained for a number of lattices.
488 citations
TL;DR: In this paper, the processes involved in the laser effect in CdS excited by electron bombardment have been studied with the aid of an experimental technique allowing the spectral dependence of the optical gain to be obtained.
Abstract: The processes involved in the laser effect in CdS excited by electron bombardment have been studied with the aid of an experimental technique allowing the spectral dependence of the optical gain to be obtained. It is shown that at least three different processes can lead to a laser effect in CdS. The first, which corresponds to a low gain, is due to the annihilation of a free exciton with the emission of a photon and an LO phonon. The second, yielding a medium gain, is due to an exciton-exciton interaction; and the third, which results in a high gain, involves an exciton-electron interaction. These last two processes have been studied theoretically, and the results are compared with the experimental data.
277 citations
TL;DR: In this article, the energy transfer in amorphous polyvinyl carbazole (PVCA) has been studied by fluorescence quenching experiments using perylene, trinitrofluorenone, and hexachloro-p‐xylene as guest molecules.
Abstract: The energy transfer in amorphous polyvinyl carbazole (PVCA) has been studied by fluorescence quenching experiments using perylene, trinitrofluorenone, and hexachloro‐p‐xylene as guest molecules. The PVCA shows excimer fluorescence. The results are discussed in terms of exciton diffusion and competition between excimer‐forming sites and guest molecules for the trapping of excitons. Single‐step transfer from the excimer state by dipole resonance is considered as an alternative mechanism of energy transfer; in all three cases, however, exciton migration is the more probable energy‐transfer mechanism. The concentration of excimer‐forming sites is estimated to be 10−3 mole/mole basic unit of the polymer. In the pure polymer the exciton covers about a thousand basic units during its lifetime.
245 citations
TL;DR: CaO and MgO reflectance spectra measurements at high resolution and at low temperature including exciton spectra at 25 K, obtaining fine structure was obtained by as mentioned in this paper.
Abstract: CaO and MgO reflectance spectra measurements at high resolution and at low temperature including exciton spectra at 25 K, obtaining fine structure
102 citations
TL;DR: In this article, the reflectance and electroreflectance spectra of single crystals of lead iodide (Pb${\mathrm{I}}_{2}$) were measured between 2.2 and 5.0 eV at 4.5 and 77
Abstract: The reflectance and electroreflectance spectra of single crystals of lead iodide (Pb${\mathrm{I}}_{2}$) were measured between 2.2 and 5.0 eV at 4.5 and 77\ifmmode^\circ\else\textdegree\fi{}K, respectively. A Kramers-Kronig dispersion analysis of the data was made, and the real and imaginary parts of the dielectric constant were calculated together with their field-induced modulation. The first three lines of the hydrogenic exciton series associated with the absorption edge are well resolved in reflectivity. In the same energy range, the electoreflectance spectrum is interpreted as the Stark shift of the first exciton line from which the binding energy of the exciton is deduced. In the region of fundamental absorption, the existence of several metastable exciton states gives rise to a new and large contribution to the electroreflectance spectra.
81 citations
TL;DR: In this article, a method was developed and employed for an LCAO-MO semiquantitative determination of valence bands, which seems generally advantageous for complex solids with important covalent bonding effects.
80 citations
TL;DR: In this paper, the problem of resonant scattering of polaritons is solved for short-range exciton-impurity interactions in a natural fashion, and the giant oscillator strengths of bound-exciton transitions are also automatically obtained.
Abstract: To calculate correctly the scattering of light by phonons or impurities in a crystal, the true asymptotic scattering states of the coupled system of crystal plus light (polaritons) should be used. When the light frequency is close to one exciton or optical-phonon frequency, the polariton is entirely excitonlike, and the polariton scattering can, in the Born approximation, be related to exciton scattering properties. If the exciton itself interacts strongly with an imperfection in the crystal, it is not permissible to treat either the exciton scattering or the exciton-photon interaction as perturbations. This problem of resonant scattering of polaritons is solved for short-range exciton-impurity interactions. Radiative damping and spatial dispersion appear in this solution in a natural fashion. Giant oscillator strengths of bound-exciton transitions are likewise automatically obtained. The proper inclusion of radiative damping and spatial dispersion keeps all cross sections finite. The relation between the theory and experiments is briefly discussed.
80 citations
TL;DR: In this article, the bound exciton transitions are described mainly on AgBr:I - together with related cases of AgCl:Br - and AgCl :I -, mirror image like relations are observed between absorption and emission spectra with zero phonon line at the center.
Abstract: Various correlations are observed between absorption and luminescence spectra of silver halides with isoelectronic impurities at low temperature. In Part I, the bound exciton transitions are described mainly on AgBr:I - together with related cases of AgCl:Br - and AgCl:I - . In AgBr:I - , mirror-image like relations are observed between absorption and emission spectra with zero phonon line at the center. Multiphonon structures are observed in both spectra and the energy of coupled phonon is substantially smaller in absorption that in emission. The difference in phonon energies reflects strong binding of excitons with phonons discussed by Toyozawa and Hermanson. In absorption spectra, additional structures are observed and interpreted as creating excitons in higher excited states. In emission, one phonon sidebands are observed reflecting phonon spectrum in pure AgBr. Discussions are also made on temperature variation of absorption spectrum and on asymmetric intensity distribution between absorption and emi...
76 citations
TL;DR: In this article, the theory of a magnon sideband on an exciton absorption line is compared with the experimental measurements on Rb2MnF4, and a prediction is made for the two-magnon Raman lineshape, and the energies of localized modes associated with impurities calculated.
Abstract: The magnetic structure of K2NiF4, K2MnF4 and Rb2MnF4 can be approximately regarded as a two-dimensional square lattice, which enables the magnetic Green functions to be expressed in terms of elliptic integrals. Using these, the theory of a magnon sideband on an exciton absorption line is compared with the experimental measurements on Rb2MnF4, giving reasonable agreement. A prediction is made for the two-magnon Raman lineshape, and the energies of localized modes associated with impurities calculated.
65 citations
TL;DR: In this paper, the energy of the bottom of the conduction band in liquid and solid Ar, Kr and Xe was estimated from spectroscopic data for Wannier impurity states in the liquids and solids and for X-ray exciton states in solids.
TL;DR: In this paper, two sets of sharp emission lines associated with the photoluminescence spectrum of the isoelectronic nitrogen trap in GaP are unambiguously identified as the recombination of a second exciton bound to this center with an energy of 10 meV.
Abstract: Two sets of sharp emission lines, associated with the photoluminescence spectrum of the isoelectronic nitrogen trap in GaP, are unambiguously identified as the recombination of a second exciton bound to this center with an energy of 10 meV. This is the first observation of an excitonic molecule bound at a defect. The two electrons and two holes within this complex combine to form two antisymmetric states of angular momentum ${J}_{t}=0$ and ${J}_{t}=2$. The ${J}_{t}=2$ state is split 0.16 meV by the cubic crystal field, and the ${J}_{t}=0$ state lies 0.17 meV above the center of gravity of this doublet. Transitions from these states to the $A$ and $B$ states of a single exciton bound to nitrogen are seen. The complex Zeeman splittings predicted by this model agree in detail with experiment. The excitonic molecule is stable at low temperature; at 1.5\ifmmode^\circ\else\textdegree\fi{}K, the excitonic molecule emission lines increase as the square of the single exciton intensity with increase in pumping power. However, nonradiative Auger recombination reduces the over-all nitrogen emission by a factor of 3 below the intensity at 4.2\ifmmode^\circ\else\textdegree\fi{}K. The binding energy of the second exciton at the nitrogen trap is nearly equal to that of the single exciton. This remarkable fact may be possible only for an isoelectronic trap.
TL;DR: In this paper, a generalization of the theory of excitons in linear molecular crystals was carried out by Merrifield who constructed the excited states of the crystal by mixing monomolecular states and bimolecular ionized states in which an electron was removed from one molecule in the crystal and placed on a second (charge-transfer states).
Abstract: The optical absorption of linear molecular crystals is investigated theoretically. A generalization of the theory of excitons in linear molecular crystals was carried out by Merrifield who constructed the excited states of the crystal by mixing monomolecular states (Frenkel excitons) and bimolecular ionized states in which an electron is removed from one molecule in the crystal and placed on a second (charge‐transfer states). If the monomolecular optical transition is allowed, we find that the optical absorption consists of a series of bound states and a continuum absorption, which gives rise to photoconductivity; the strength of all the absorption is predominantly borrowed from the Frenkel exciton. The experimental possibilities may be classified into three cases: (1) the Frenkel exciton, the charge‐transfer state with electron and hole at nearest neighbors, and the state with the electron and hole at large separation are well separated in energy. This case gives rise to a strong absorption line, small charge‐transfer sidebands, and very small absorption into the continuum. (2) Near resonance between Frenkel and nearest‐neighbor charge‐transfer states. The strong absorption is now shared by two absorption lines with weak sidebands and very small continuum absorption. (3) Resonance between the Frenkel state and the states with the electron and hole at large separation. This case corresponds to autoionization in a solid, the bound states have very weak absorption, and a strong sharp line will be observed within the continuum absorption. The width of the resonance line is directly proportional to the background absorption.The optical absorption of linear molecular crystals is investigated theoretically. A generalization of the theory of excitons in linear molecular crystals was carried out by Merrifield who constructed the excited states of the crystal by mixing monomolecular states (Frenkel excitons) and bimolecular ionized states in which an electron is removed from one molecule in the crystal and placed on a second (charge‐transfer states). If the monomolecular optical transition is allowed, we find that the optical absorption consists of a series of bound states and a continuum absorption, which gives rise to photoconductivity; the strength of all the absorption is predominantly borrowed from the Frenkel exciton. The experimental possibilities may be classified into three cases: (1) the Frenkel exciton, the charge‐transfer state with electron and hole at nearest neighbors, and the state with the electron and hole at large separation are well separated in energy. This case gives rise to a strong absorption line, small c...
TL;DR: In this paper, the authors calculate the time dependence of the emitted luminescence from an organic phosphor containing exciton traps and for the situation in which the traps themselves also emit a characteristic luminecence, and they show that the results agree qualitatively with recent measurements of the fluorescence of tetracene-doped anthracene.
Abstract: On the basis of a nearest-neighbor random-walk model for exciton propagation, we calculate the timedependent luminescent emission from an organic phosphor containing exciton traps. For the situation in which the traps themselves also emit a characteristic luminescence, we also calculate the time dependence of that luminescence. The results agree qualitatively with recent measurements of the time dependence of the fluorescence of tetracene-doped anthracene; detailed comparison yields reasonable values for the lifetime of each phosphor and for the time the anthracene singlet exciton spends on one site. There is no need to postulate a long-range energy-transfer mechanism.
TL;DR: In this article, the emission spectrum of PbI 2 on the low energy side of the fundamental exciton absorption, was studied in the temperature range 4.2° − 180°K.
TL;DR: In this paper, it was shown that diffusion of singlet excitons cannot explain the time dependence of the fluorescence intensity in tetracene-doped anthracene crystals; however, the predictions of a long-range energy transfer mechanism can be made to agree with experimental results.
Abstract: It is shown that diffusion of singlet excitons cannot explain the time dependence of the fluorescence intensity in tetracene-doped anthracene crystals; however, the predictions of a long-range energy-transfer mechanism can be made to agree with experimental results.
TL;DR: In this paper, an improved theoretical formulation of the triplet exciton bimolecular decay constant in organic crystals is developed, which avoids the ad hoc introduction of a weight factor w used in previous theoretical treatments, allows for explicit calculations of all reaction constants pertinent to mutual exciton annihilation of triplets, and properly accounts for the anisotropy of the interaction responsible for the BIMolecular annihilation.
Abstract: An improved theoretical formulation of the triplet exciton bimolecular decay constant in organic crystals is developed which avoids the ad hoc introduction of a “weight factor” w used in previous theoretical treatments, allows for explicit calculations of all reaction constants pertinent to mutual exciton annihilation of triplets, and properly accounts for the anisotropy of the interaction responsible for the bimolecular annihilation. A simplified model for the effective density of final electronic states ρ(e) is presented and explicit calculations are performed assuming that only the totally symmetric intramolecular C–C stretching modes are active and by including intermolecular vibrations phenomenologically. Calculations of the bimolecular decay constant (γ) and diffusion tensor, both with and without the inclusion of charge‐transfer interactions, are performed for solid naphthalene, anthracene, and tetracene. We obtain γ (naphthalene) ∼0.1γ (anthracene) and γ (tetracene ∼10−5γ (anthracene); only for an...
TL;DR: In this article, the authors verify that the density function for exciton states is almost symmetric over the entire exciton band of the first singlet state, which is consistent with the entire band structure as observed in band-to-band transitions and disagrees with the band structure calculated from the octopole model for the intermolecular interaction.
Abstract: Perturbation shifts of guest levels in isotopic mixed crystals of naphthalene have been determined. These data verify that the density function for exciton states is almost symmetric over the entire exciton band of the first singlet state. This result agrees with the entire band structure as observed in band‐to‐band transitions and disagrees with the band structure calculated from the octopole model for the inter‐molecular interaction. The transition energies of guest levels also disagree with a model for the exciton splitting involving configuration interaction with charge transfer states. A possible source of the exciton splitting is suggested, and the available data for evaluating theoretical descriptions are outlined.
TL;DR: In this paper, the exciton spectra of CuCl have been investigated in the presence of high transient magnetic fields up to 180 kG and the mixing of the ortho and para-exciton states T15 and T2 in magnetic fields is discussed.
TL;DR: In this paper, the electronic energy transfer from the host material to guest molecules has been studied in the system perylene/Nisopropyl carbazole by measuring the fluorescence intensity ratio guest/host over a great range of guest concentrations.
Abstract: The electronic excitation energy transfer from the host material to guest molecules has been studied in the system perylene/N‐isopropyl carbazole by measuring the fluorescence intensity ratio guest/host over a great range of guest concentrations. Exciton diffusion and single‐step transfer are discussed as possible transfer mechanisms; it is shown that the former process prevails in this system. According to the hopping model, the number of steps during the lifetime of the exciton is calculated to be 4 × 104, and the average displacement from the starting point amounts to 1350 A.
TL;DR: In this article, the creation of H centers is detected as a result of electrons recombining with Vk-type centers, interpreted as evidence favoring the excitonic mechanism for radiation damage in alkali halides.
TL;DR: In this paper, it has been shown that a new band ν B appears in the emission of CuCl when the excitation light becomes very strong (UV laser irradiation), the line was suggested to be a result of a recombination process of biexcitons giving an exciton and a photon.
TL;DR: In this article, the theory of exciton-enhanced Raman scattering is formulated in terms of the scattering of polaritons by optical phonons via the exciton part of the coupled modes.
Abstract: The theory of exciton-enhanced Raman scattering is formulated in terms of the scattering of polaritons by optical phonons via the exciton part of the coupled modes. The expression for the exciton contribution to the scattering tensor is given, within a constant factor, in terms of the same parameters that determine the exciton contribution to the frequency-dependent dielectric constant. The theory also provides a new mechanism for the exciton contribution to the electro-optic effect.
TL;DR: In this article, the doublet structure of these exciton bands was analyzed, according to the theory of Onodera and Toyozawa, by the spin-orbit splitting of Na + L 2,3 levels and the exchange interaction between the electron and the hole of an exciton.
Abstract: The Na + L 2,3 absorption spectra of sodium halides, NaF, NaCl, NaBr and NaI, have been obtained at 300°K using a grazing incidence spectrograph with a concave grating of 2 m radius and a vacuum spark source of Vodar type as a quasi-continuum background. Sharp absorption bands due to X-ray excitons at the \(\varGamma_{1}\) point were found at the long-wavelength edge. The doublet structure of these exciton bands was analysed, according to the theory of Onodera and Toyozawa, by the spin-orbit splitting of Na + L 2,3 levels and the exchange interaction between the electron and the hole of an exciton. The structure of the absorption spectra in the energy region above the exciton doublet was tentatively assigned to the absorptions arising from the formation of excitons at the point other than \(\varGamma_{1}\) and from the transition from the Na + L 2,3 level to the conduction band. The possibility of the presence of the two-electron excitation was suggested.
TL;DR: In this paper, the authors present evidence for the presence of two-dimensional indirect excitions in terms of the absorption spectra in GaSe and the phonon and exciton parameters at the $K$ point are determined.
Abstract: Evidence for the presence of two-dimensional indirect excitions is presented in terms of the absorption spectra in GaSe. The phonon and exciton parameters at the $K$ point are determined.