Showing papers on "Exciton published in 1971"
TL;DR: In this article, ideas from Griffin's exciton model are combined with those from the nucleon-nucleon scattering approach to nuclear transition times to provide a simple closed-form expression for predicting pre-equilibrium decay phenomena.
Abstract: Ideas from Griffin's exciton model are combined with those from the nucleon-nucleon scattering approach to nuclear transition times to provide a simple closed-form expression for predicting pre-equilibrium decay phenomena, including variation of pre-equilibrium emission with target mass, excitation energy, and initial particle and hole numbers. Time estimates for pre-equilibrium emission are given at several excitations.
385 citations
TL;DR: In this paper, a theory for the U-M rule on the low energy tail of the fundamental absorption edge of insulators is proposed, where the exciton propagator is solved for an adiabatic lattice and the self-energy of exciton is obtained.
Abstract: Theory for the Urbach-Martienssen (U-M) rule on the low energy tail of the fundamental absorption edge of insulators is proposed. The exciton propagator solved for an adiabatic lattice is averaged for lattice vibrations at finite temperature, and the self-energy of exciton is obtained. It describes two characters of exciton in the lattice; the one is the mobile nature of exciton in the undeformed lattice, and the other is the localized nature of exciton trapped momentarily by the lattice deformation due to thermal vibrations. The interplay of the two natures results in the U-M tail below the exciton absorption peak. The result can be interpreted in terms of the Franck-Condon principle where the mobile nature is incorporated in the giant oscillator strength of the momentarily trapped exciton. Emission from this trapped state is discussed in connection with the U-M rule.
322 citations
TL;DR: In this article, the authors studied the migration of Frenkel excitons in molecular crystals, starting from the microscopic equations of motion for the exciton density, and found that the migration process is diffusionlike on a long time scale for all temperatures as long as exciton-phonon coupling is present.
Abstract: The migration of Frenkel excitons in molecular crystals is studied theoretically, starting from the microscopic equations of motion for the exciton density. The migration process is found to be diffusionlike on a long time scale for all temperatures as long as the exciton–phonon coupling is present. If there is no exciton–phonon coupling, the migration is wavelike. A discussion of the relevant time scales for the wavelike (coherent) process and the diffusion (incoherent) process is given. Several different approximation techniques, based on the cumulant expansion, projection operator, and functional derivative methods, are shown to be mathematically equivalent as applied to this problem.
288 citations
TL;DR: In this paper, a simple theory of the magnetic field effects of triplet excitons is presented, and applied to the experimental field dependence results yields new information about the mechanisms of these interactions.
Abstract: The rates of many bimolecular reactions involving triplet excitons are magnetic field dependent at room temperature and for modest field strengths. The specific interactions discussed are the fusion of a pair of triplet excitons to yield either a single or triplet exciton, the interaction between a triplet exciton and a trapped triplet, the fission of a singlet exciton into a pair of triplets and the triplet exciton-free radical interaction. The magnetic field effects result from the influence of the field on the spin wavefunctions of the triplet exciton together with the existence of spin selection rules for the interactions. A simple theory of these effects is presented. Application of the theory to the experimental field dependence results yields new information about the mechanisms of these interactions.
157 citations
TL;DR: In this article, the authors measured the singlet-triplet (T1←← S0) excitation spectrum for delayed fluorescence and the prompt fluorescence (S1→S 0) emission spectrum for a tetracene crystal at room temperature.
Abstract: The singlet–triplet (T1 ← S0) excitation spectrum for delayed fluorescence and the prompt fluorescence (S1 → S0) emission spectrum have been measured for a tetracene crystal at room temperature. The origin (0–0 transition) in the two spectra at 10 100 ± 20 and 18 680 ± 30 cm−1, respectively, imply the activation energy for singlet exciton fission, ΔE = 1520 ± 70 cm−1. Raman lines corresponding to the vibrational structure in the above spectra were detected. The fluorescence emission spectra show the need for taking reabsorption effects into account.
155 citations
TL;DR: In this paper, the effect of the internal vibrations of monomers (or molecules) on the electronic absorption spectra of aggregates with either helical or three dimensional translational symmetry is considered using molecular exciton theory.
Abstract: The effect of the internal vibrations of monomers (or molecules) on the electronic absorption spectra of aggregates with either helical or three dimensional translational symmetry is considered using molecular exciton theory. In this treatment the single‐particle excitations (vibronic excitons) are coupled to all those two‐particle manifolds in which vibronic and ground vibrational excitons occupy different lattice sites. This allows for collective coupling among single‐particle levels overlapped by two‐particle continua. The main approximations invoked are (a) the crude Born—Oppenheimer approximation to factorize the wavefunctions of isolated monomers, (b) neglect of electron exchange between monomer wavefunctions (tight binding), and (c) the neglect of any mixing of different electronic states by intermonomer forces. Wave sums of exciton resonance interactions are eliminated in favor of a density of sums function. To test the range of coupling strengths for which the theory is valid calculations are performed for a one‐dimensional polymer model with only nearest‐neighbor interactions and a three‐dimensional crystal model with a simple density function. For intermediate coupling the influence of three‐ and higher‐particle states becomes important and these states are included in the energy calculations by an extended fraction type of technique. Other calculations explore the effect of (a) a change in the vibrational frequency of the monomer after electronic excitation, (b) changes in the energy of the optical k=0 levels with direction of the exciting radiation, and (c) changes in the transition intensity of the isolated monomer.
139 citations
TL;DR: In this paper, the Green's-function formulation of Raman scattering cross sections for photons near electronic resonances has been used to calculate resonance enhancements for ordinary allowed scattering and for intraband Fr\"ohlich scattering, which is of higher order in wave vectors and thus forbidden.
Abstract: Resonant enhancements of Raman scattering cross sections for photons near electronic resonances are calculated for ordinary allowed scattering and for intraband Fr\"ohlich scattering, which is of higher order in the wave vectors and thus forbidden. Exciton effects are included exactly in the hydrogenic approximation via numerical calculations using the Green's-function formulation. Much greater enhancements are found for forbidden than for allowed lines, and it is shown that forbidden lines can become comparable to allowed lines near resonance with large Wannier excitons. It is predicted to be feasible to observe one-LO-phonon lines in crystals (e.g., TlCl and TlBr) in which such transitions are always forbidden. Comparison with experiment for CdS is presented.
129 citations
TL;DR: In this article, the singularities near the edges of X-ray spectra in metals are investigated by a new method, using a description of initial and final states in terms of Slater determinants.
Abstract: The singularities near the edges of X-ray spectra in metals are investigated by a new method, using a description of initial and final states in terms of Slater determinants. The calculation of line and band spectra is reduced to that of a single determinant, for which an asymptotic approximation is developed. Special attention is paid to the case in which the final potential is strong enough to bind an electron : the absorption spectrum then possesses two thresholds, whose characteristics are found. The position of the threshold, as well as the nature of the singularities agree completely with the predictions of Hopfield [3]. When the number of electrons decreases, the spectrum goes continuously into that of an insulator, the infrared catastrophy divergence turning into an Auger broadened exciton line.
129 citations
TL;DR: In this paper, the upper branch of an exciton-polariton curve was measured directly for the first time, and the two-photon spectrum of the first exciton line of CuCl showed a strong dependence on the total $K$ vector of the two beams.
Abstract: The upper branch of an exciton-polariton curve is measured directly for the first time. The two-photon spectrum of the first exciton line of CuCl shows a strong dependence on the total $K$ vector of the two beams. The resulting dispersion curve can be quantitatively explained using one-photon data.
110 citations
TL;DR: In this paper, the energy-momentum relation has been qualitatively studied below the one-phonon continuum, and the following new features have been found in the strong coupling region; (1) complete splitoff of the lowest branch in the whole Brillouin zone, and (2) split-off of second discrete branch.
Abstract: Optical properties of interacting exciton-phonon system have been theoretically studied in terms of a simple model consisting of Frenkel excitons and Einstein oscillators. The problem is characterized by two non-dimensional parameters B and S representing the exciton band width and the interaction energy, respectively. Approximate solutions have been obtained through the diagonalization of the Hamiltonian in special sub-spaces, and they reasonably describe the behavior of the system for all values of B and S . Absorption and emission spectra and the effective mass of the composite particle vary from free exciton type to self-trapped exciton type as we go from B > S to B < S . The energy-momentum relation has been qualitatively studied below the one-phonon continuum, and the following new features have been found in the strong coupling region; (1) complete split-off of the lowest branch in the whole Brillouin zone, and (2) the split-off of the second discrete branch. Furthermore, a clue to the mechanism of...
98 citations
TL;DR: In this paper, it was shown that under certain conditions the polymer band consists of an extremely narrow peak shifted from the monomer line, as has been observed in the spectra of some cyanine dyes.
Abstract: The shape of the vibrational envelope of an absorption band in the solution spectrum of a helical polymer is related to the shape of the corresponding band in the monomer. The approximation used is expected to be valid in both the strong and weak coupling limits. In the strong coupling limit it is shown that under certain conditions the polymer band consists of an extremely narrow peak shifted from the monomer line, as has been observed in the spectra of some cyanine dyes. The identification of this peak with absorption by a propagating exciton mode is supported by a calculation of the effective distance of excitation transfer as a function of coupling strength.
TL;DR: In this article, the authors derived the bandwidth, coherence length and density of states functions from a zero field optically detected microwave experiment for the lowest triplet band in tetrachlorobenzene at temperatures below 4.2°K.
TL;DR: In this article, the binding energies, interparticle distances, oscillator strengths, and exchange corrections for the three-particle complex corresponding to an exciton bound to an ionized donor are calculated as functions of the mass ratio of the electron and hole.
Abstract: Binding energies, interparticle distances, oscillator strengths, and exchange corrections are calculated for the three-particle complex corresponding to an exciton bound to an ionized donor. The results are given as functions of the mass ratio of the electron and hole. Binding of the complex is obtained for mass ratios up to 0.426. The interparticle distances are up to 50 times larger than the corresponding exciton radius. The oscillator strengths are about ${10}^{4}$ times greater than those of free excitons, while the exchange corrections for the complex are comparable to those of free excitions. The results are applied to CdS and ZnO and compared with experimental results on these.
TL;DR: In this paper, a method for the determination of complete exciton band structures in molecular crystals is given, using an exciton "superexchange" approach, which is applicable to nontrivial interchange symmetry systems, within the restricted Frenkel-Davydov theory.
Abstract: A method for the determination of complete exciton band structures in molecular crystals is given. Pairwise exciton interactions are derived from resonance‐pair data using an exciton “superexchange” approach. Koster and Slater's impurity cluster formulation is found to be applicable to nontrivial interchange symmetry systems, within the “restricted Frenkel–Davydov” theory. The derivation starts from the recent general formulation for isotopically mixed crystals of arbitrary concentrations. The resonance pair states are given by the second‐order self‐energy of the mixed crystal Green's function. General symmetry arguments and moment sum rules have been worked out for resonance pairs. It is demonstrated for naphthalene‐h8 resonance pairs in naphthalene‐d8 that superexchange corrections are not only inevitable for the 1B2u pair states but that they can also be utilized to assign experimental pairwise interactions to definite crystal directions, i.e., specific pairs. The naphthalene first singlet excited stat...
TL;DR: In this article, a theory of microwave band-to-band transitions in triplet Frenkel excitons in the coherent migration limit for a one-dimensional exciton associated with translationally equivalent molecules is developed.
TL;DR: In this article, the authors show that the nature of relaxed exciton states changes discontinuously from free to self-trapped excition as the composition x exceeds ∼ 0.45, and the transition can be explained by attainment of sufficiently strong exciton-phonon coupling wehn the alloy composition is varried.
TL;DR: In this paper, the rate constant for energy transfer from an anthracene singlet exciton to an aluminium contact through a fatty acid monolayer (thickness d ) was measured and found to be k q ( d ) = (1.0 ± 0.2) × 10 −9 d −3 sec −1.
TL;DR: In this paper, the magneto-absorptions due to excitons have been studied in TlCl and TlBr and the existence of two minima in the conduction band of each halide is suggested.
Abstract: Oscillatory magneto-absorptions due to excitons have been studied in TlCl and TlBr. In zero magnetic field, absorptions due to exciton-phonon quasi-bound state and one and two free-phonon states of exciton quasicontinuum are observed. In magnetic field, members of s exciton series with a subsidiary peak on the high energy side appear. The exciton binding energy is 11.7 meV in TlCl and 9.8 meV in TlBr and the reduced mass is 0.24 4 m 0 in TlCl and 0.23 4 m 0 in TlBr. A direct evidence of exciton polaron effect is obtained by observing the clamping of zero-phonon exciton lines of high quantum number in the vicinity of one free-phonon threshold of 2 s exciton. Above the threshold, absorptions of another series grow as the field increases. They are one-phonon replicas of zero-phonon excitons in magnetic field. The existence of two minima in the conduction band of each halide is suggested.
TL;DR: In this article, the exciton band structure of the 3800 and 2500 A transitions of anthracene has been calculated in the dipole approximation, and it is found that the branches of both transitions are flat for wave vectors k normal to the (001) plane.
Abstract: The exciton band structure of the 3800‐ and 2500‐A transitions of anthracene has been calculated in the dipole approximation. It is found that the branches of both transitions are flat for wave vectors k normal to the (001) plane. In other directions considered the exciton energy exhibits dispersion. The calculations also reveal that for all the directions considered the lowest energy of the 2500‐A exciton band is not a k = 0 state.
TL;DR: In this paper, the electroluminescence, Hall coefficient, and resistivity of high-purity GaP, grown by means of vaporphase epitaxy on bulk single-crystal GaP substrates, have been studied as functions of temperature.
Abstract: The electroluminescence, Hall coefficient, and resistivity of high‐purity GaP, grown by means of vaporphase epitaxy on bulk single‐crystal GaP substrates, have been studied as functions of temperature. Hall mobilities as high as 2730 and 189 cm2/V sec have been obtained at 77° and 300°K, respectively. The electroluminescence (EL) of diodes fabricated by Zn diffusion exhibits intrinsic recombination. At 77°K, in addition to the narrow no‐phonon line due to bound exciton recombination at a neutral donor site, the near‐band‐edge EL exhibits a well‐defined series of peaks corresponding to the phonon‐assisted recombination of free excitons. Peaks are observed associated with both the absorption and emission of TA, LA, and TO phonons with energies of 12.5, 31.0, and 44.0 meV, respectively. The intensities of the peaks associated with phonon emission have the approximate ratio 1:3:1.5 for the TO, LA, and TA phonons, respectively. These energies and relative intensities are in good agreement with absorption and c...
TL;DR: The ground state of the isotropic exciton in a magnetic field has been computed exactly for several values of the magnetic field in the weak, medium and strong regions.
TL;DR: In this paper, the problem of exciton self-trapping in rare-gas solids is investigated from a dynamical point of view, where the vibrational relaxation of a molecule embedded in a solid leads to a fast trapping process in which an exciton becomes localized for all the heavy rare‐gas substances.
Abstract: The problem of exciton self‐trapping in rare‐gas solids is investigated from a dynamical point of view. Experimental ultraviolet emission spectra from rare‐gas solids and liquids have been interpreted as arising from the radiative decay of an excited‐state dimer which becomes localized in the substance. This embedded dimer can arise from electron capture by a self‐trapped hole (R2+), or by the dynamical trapping of an exciton into an excimer state (R2*). A study of the vibrational relaxation of a molecule embedded in a solid leads to a fast trapping process in which an exciton becomes localized for all the heavy rare‐gas substances. This localization is achieved within 6 × 10−12 sec by means of a resonant transfer relaxation process in which two neighboring atoms lose sufficient energy (∼ 0.5 eV) to become stabilized in a lower vibrational state of the excimer.
TL;DR: In this paper, the rate constant for dissociation of singlet excitons at an anthracene crystal metal interface is measured as a function of reaction distance, and it is found that tunnelling of the excited electron is the rate-determining step.
TL;DR: In this paper, a number of newly discovered luminescence bands between ≈ 0.7 eV and ≈ 2.2 eV which appear to be characteristic of diamonds possessing various concentrations and configurations of the major impurity, nitrogen, were identified with the radiative recombination of free holes with electrons in the platelet nitrogen donor impurity band.
TL;DR: In this paper, the influence of an external magnetic field on the normal incidence reflectivity of CuCl single crystals has been measured in the Voigt and Faraday configurations, and the results are discussed in terms of the dielectric function calculated from the reflectivity spectrum by a Kramers-Kronig transformation.
Abstract: The influence of an external magnetic field on the normal incidence reflectivity of CuCl single crystals has been measured in the Voigt and Faraday configurations. The results are discussed in terms of the dielectric function calculated from the reflectivity spectrum by a Kramers-Kronig transformation. In the Voigt configuration two new exciton bands can be observed, which are attributed to a triplet and a longitudinal exciton. Their energetic positions are used to determine the parameters of electron-hole exchange interaction in the 1s excitons, and the spin-orbit splitting parameter of the upper valence band. The excitonic g-values are determined from the Faraday configuration measurements and can be interpreted on the basis of the tight binding model.
TL;DR: The extinction coefficients of AgCl and AgBr have been obtained down to liquidhelium temperature by observing the transmission of thin films in the near-uv direct exciton region (3.5-6.7 eV) and in the extreme uv (30-240 eV), using synchrotron radiation as discussed by the authors.
Abstract: The extinction coefficients of AgCl and AgBr have been obtained down to liquid-helium temperature by observing the transmission of thin films in the near-uv direct exciton region (3.5-6.7 eV) and in the extreme uv (30-240 eV) using synchrotron radiation. The index of refraction was determined for both materials by a dispersion relation, and the optical constants were then constructed for AgCl in the range 3-240 eV, using all available data. Some interpretation of the exciton and band-to-band spectra is given. An estimate is made of the effective number of electrons which contribute to the absorption over a wide range of photon energies.
TL;DR: In this article, the density of states functions for heavily doped isotopic binary mixed crystals of naphthalenes with arbitrary compositions and various energy separations (trap depths) were calculated.
Abstract: Using the recently acquired exciton dispersion relations for crystalline naphthalene, we have calculated the density‐of‐states functions for heavily doped isotopic binary mixed crystals of naphthalenes with arbitrary compositions and various energy separations (trap depths). This constitutes the first attempt to extend the negative factor counting (NFC) method, developed originally for lattice phonons, to a real physical system of three‐dimensional molecular excitons. In most calculations, a total of 1280 molecules were included. The exciton interactions, which included both the translationally equivalent and the interchange equivalent ones, involved all 16 neighbors. Calculations based on the coherent potential approximation (CPA) were also performed for comparison. It was concluded that these two sets of calculations compared very well except in the split‐band limit and at low concentrations. Under these conditions the cluster or conglomerate states become important and the computer‐simulated density‐of...