Showing papers on "Exciton published in 1972"
TL;DR: In this article, it was shown that the triplet exciton state of 1,4-dibromonaphthalene behaves as a linear chain characterized by nearest neighbor interactions along the c crystallographic axis.
Abstract: An experimental study is presented that leads to the elucidation of the whole band structure for the triplet exciton state of 1,4‐dibromonaphthalene. We show that the band states are essentially those of a linear chain characterized by nearest neighbor interactions along the c crystallographic axis. Although the crystal is topologically a three‐dimensional network, for the practical purpose of energy transfer and trapping the crystal behaves as a set of linear chains. Heavy doping of DBN‐h6 (host) with up to 18% DBN‐d6 (guest) yielded the expected discrete spectra of a random linear array corresponding to a nearest neighbor interaction of 7.4± 0.1 cm−1 and total bandwidth of 29.6± 0.4 cm−1 for the zero‐zero transition. A symmetric mode at 0+520 cm−1 was shown to have an exciton bandwidth of 15 cm−1, and an unsymmetric vibrational level at 0+250 cm−1 was shown to have a vanishing bandwidth in accordance with expectations from weak coupling theory. The quasiresonance interactions of host and guest were significant and were included in establishing the agreement between experiment and calculation. The total bandwidth is estimated to be 90± 20 cm−1. This type of doping has an effect on line shape even at very low concentration and even in undoped crystals the line shape is asymmetrical, perhaps due to impurities and imperfections that have small zero‐order shifts from the band center. The intensity of transitions to the carbon‐13 isotopic traps is shown to be distributed over the band states in the origin and is clearly visible in the narrow band nontotally symmetric vibrational state. The vibrational analysis of the crystal spectrum (electronic or vibrational states) is shown to be dependent on exciton effects even in the case where no Davydov splitting is observed. Spectra of DBN‐h6 (guest, H) in DBN‐d6 (host, D) were also studied and resonance multiplets were seen and identified in absorption and emission. We identified ··· DHD···, ··· DHHD···, ··· DHHHD···, and probably ··· DHDHD··· in the phosphorescence spectra at 2°K. We present a brief discussion of the effect of dimensionality on the dynamical features of energy transfer and trapping, and we show experimentally, that heavy doping (d6 in h6) enhances the phosphorescence of the crystal about 60‐fold. In addition the existence at 2°K of a Boltzmann distribution over resonance multiplets leads us to propose a long‐range trap‐to‐trap migration as part of the thermalizing process.
159 citations
TL;DR: In this article, the ground state energy of an electron-hole plasma in a Ge type semiconductor is calculated with the help of the improved RPA approximation of Nozieres and Pines (1958).
Abstract: The ground state energy of an electron-hole plasma in a Ge type semiconductor is calculated with the help of the 'improved RPA' approximation of Nozieres and Pines (1958). The detailed band structure is taken into account, except for the warping of the valence band. That is, the effects of the anisotropy, of the valence band coupling and of the multivalley structure are discussed, and are found to stabilize the plasma. The numerical results for Ge and Si are in very good agreement with experiments, which provides a check for the validity of the 'improved RPA' approximation.
150 citations
TL;DR: An approximate theoretical description of phonon participation in the electronic transitions of molecular crystals and of mixed crystals is presented, and it is suggested that the ΔD dispersion shift term is a determining factor of the phonon sideband spectra in triplet and weak singlet exciton spectra as discussed by the authors.
Abstract: An approximate theoretical description of phonon participation in the electronic transitions of molecular crystals and of mixed crystals is presented, and it is suggested that the ΔD dispersion shift term is a determining factor of the phonon‐sideband spectra in triplet and weak singlet exciton spectra. We show that the phonon sidebands map the density of phonon states and do not correspond to discrete transitions to Raman‐active states of the crystal lattice. We also show that the spectra of guest molecules should be accompanied by mainly resonance mode phonon sidebands and that the coupling and the selection rules are different for neat and for mixed crystals. Spectra are presented to illustrate some of the main points of the discussion: in particular, the differences between neat crystal and mixed crystal phonon sidebands; the effect of exciton bandwidth on phonon sidebands; the appearance of local modes; and the differences between phonon sidebands in absorption and luminescence spectra.
92 citations
TL;DR: In this paper, a double-beam, single-detector wavelength-modulation system was used to obtain unambiguous line shapes for the real and imaginary part of the dielectric constants, permitting the identification of the types of critical points involved in an optical transition, and the determination of the existence of hyperbolic exciton interactions.
Abstract: The band structures of Ge, Si, GaAs, GaP, GaSb, InAs, InP, InSb, and AlSb have been studied in reflectivity in the energy region 1.6-5.0 eV at temperatures ranging from 80 to 300 \ifmmode^\circ\else\textdegree\fi{}K. Utilizing a double-beam, single-detector wavelength-modulation system, and ensuing Kramers-Kr\"onig analyses, experimentally unambiguous line shapes have been obtained for the real and imaginary part of the dielectric constants, permitting the identification of the types of critical points involved in an optical transition, and the determination of the existence of hyperbolic exciton interactions. Such an interaction has been verified in all materials, except Si, as an ${M}_{1}$ critical point located at $\ensuremath{\Lambda}$ in the Brillouin zone. The location and energy of the interband transition in these semiconductors correlates with existing band calculations. The interband transitions in Si are dominated by structure from a large region of the Brillouin zone. The high-energy ${E}_{2}$ transitions in all materials give evidence of a multiplicity of critical-point structure.
79 citations
TL;DR: In this paper, the room temperature electron paramagnetic resonance spectrum of triplet excitons in single-crystal tetracene is reported, and the exciton spin hamiltonian is fit to the measured angular dependence in the a'b plane.
77 citations
TL;DR: In this paper, the exciton gap in indium phosphide and gallium arsenide was estimated to be approximately 10-3 in certain regions of typical epitaxial layers.
Abstract: Photoluminescence spectra of refined epitaxial indium phosphide and gallium arsenide show clearly a close correspondence of bound exciton transitions. Classified in both materials are emissions due to excitons bound to neutral acceptors and donors, free exciton peaks and various types of phonon coupling. Excitons bound to neutral acceptors in both materials give rise to a sharp doublet emission which readily reveals the presence of strain in the sample. The magnitude of the strain is estimated to be approximately 10-3 in certain regions of typical epitaxial layers. Reflectivity experiments provide new estimates of the exciton gaps-1.4182 eV in indium phosphide and 1.5150 eV in gallium arsenide.
74 citations
74 citations
TL;DR: In this article, it was shown that the exciton spectra in Pb${\mathrm{I}}_{2}$ can only be described in terms of a single Wannier series with a large ground state anomaly.
Abstract: It is shown that the exciton spectra in Pb${\mathrm{I}}_{2}$ can only be described in terms of a single Wannier series with a large ground-state anomaly. This anomaly is explained by a repulsive central-cell correction due to the cationic character of the exciton.
73 citations
Abstract: The quenching kinetics of tightly bound excitons for two different one‐dimensional models are compared. The quenching of fully incoherent, or Forster—Dexter, excitons is described by a standard master equation, and that of fully coherent, or Frenkel, excitons by an ad hoc linear differential equation whose eigenvalues are complex. Moments of the chain excitation function (probability that excitation remains at time t) are calculated on each model for finite chains with either localized or uniform initial conditions, free‐end or periodic boundary conditions, one disruptive or one nondisruptive quencher, and various quencher locations, but with only nearest‐neighbor interactions included. The ad hoc equation is treated only in the limit that the quenching is slow enough not to affect the exciton wave functions in first order. In that limit of Frenkel exciton quenching, an analytic expression is given for the mean de‐excitation time in the presence of uniform decay processes such as fluorescence. The master ...
66 citations
TL;DR: In this article, the coherent potential approximation of exciton in the phonon field was used to describe the change of character of the exciton near the band edge, and the absorption line shape of the optical spectrum was obtained over the whole range of coupling strength.
Abstract: Exciton in the phonon field is treated with the coherent potential approximation which has originally been developed for electron in random lattice Scattering potential at each lattice site is a random variable which takes a Gaussian distribution originating from thermal vibrations of the lattice The approximation describes the change of character of exciton in the phonon field near the band edge In the weak scattering case, the absorption line shape calculated is Lorentz-like in the main part and in the high energy tail of the peak due to the motional narrowing, while it switches over, below the peak, to the Urbach-Martienssen tail representing the localized feature of exciton The emission spectrum changes from the resonance type to the Stokes-shifted type as exciton-phonon coupling increases Overall line shape of optical spectrum is obtainable over the whole range of coupling strength The approximation corresponps to the self-consistent version of the theory by Sumi and Toyozawa (J Phys Soc Jap
65 citations
TL;DR: In this paper, the ground-state energy of an isotropic electron-hole liquid has been calculated using the random-phase approximation of the ground state energy of a single electron.
Abstract: We have calculated the ground-state energy of an electron-hole liquid. The kinetic and exchange energies are included exactly, and the correlation energy is estimated using Hubbard's modification of the random-phase approximation. In an isotropic electron-hole liquid, the metallic state is not bound relative to free excitons. In Ge the anisotropic band structure leads to a substantial binding of the metallic state. Application of a large $〈111〉$ strain to Ge reduces the situation to one resembling the isotropic case.
TL;DR: In this article, the optical spectra of tetragonal HgI 2 single crystals were investigated at 4.2 K and 79 K in 2∼6 eV region of photon energy.
Abstract: The optical spectra of tetragonal HgI 2 single crystals were investigated at 4.2 K and 79 K in 2∼6 eV region of photon energy. The main part of the dichroic optical spectra is well described by the optical transition from three p -like valence bands, which are split due to spin orbit interaction and crystal field perturbation, to an s -like conduction band. The optical structures, which seem to be due to exciton series, were observed in the fundamental absorption edge for an ordinary light. The tail part of the absorption edge was well expressed by Urbach Rule.
TL;DR: In this article, the symmetrical six component Zeeman spectrum of the photoluminescence emission line was observed due to the recombination of excitons bound to neutral Sn acceptors.
TL;DR: In this article, two deep hole traps are identified in n-type GaP grown by the liquid-encapsulated Czochralski (LEC) method, and an isoelectronic hole trap, ≃0.4 eV above the valence band edge, produces red emission (1.72 eV) due to bound-exciton and pair recombination, while infrared luminescence ( 1.51 eV), which apparently results from recombination between a deep acceptorlike center and the donor dopant.
Abstract: Cathodoluminescence and photoluminescence time‐response measurements have been used to study the prominent luminescence bands in n‐type GaP grown by the liquid‐encapsulated Czochralski (LEC) method. In as‐grown crystals, two deep hole traps are identified. An isoelectronic hole trap, ≃0.4 eV above the valence band edge, produces red emission (1.72 eV) due to bound‐exciton and pair recombination, while infrared luminescence (1.51 eV) is produced by pair recombination between a deep (≃0.7 eV) acceptorlike center and the donor dopant. In certain heat‐treated samples, an additional structured luminescence band is produced at 2.07 eV, which also apparently results from recombination at an isoelectronic center. Resolution of the no‐phonon lines of this emission suggests that an exciton is bound by 186 meV to the impurity center created by annealing. Although no positive identification of the chemical nature of these defects has been possible, we find that our results are not inconsistent with previously suggest...
TL;DR: In this article, the activation energy of the excition hopping process has been determined to be Ea = 0.055 eV and the lifetime of pyrene excimer fluorescence is decreased by quenchers in accordance with the steady-state measurements of relative quantum efficiencies.
Abstract: Sensitized fluorescence and fluorescence quenching has been studied in pyrene layers, doped with small amounts of coronene, anthraquinone and acenaphthylene. Coronene exhibits strong sensitized fluorescence at room temperature, but not at 77°K. This behaviour is due to decreased exciton mobility. The activation energy of the excition hopping process has been determined to be Ea = 0.055 eV. The lifetime of pyrene excimer fluorescence is decreased by quenchers in accordance with the steady-state measurements of relative quantum efficiencies. Three possible mechanisms of excimer state hopping in jpyrene are discussed
TL;DR: In this paper, the electron energy levels of lead iodide were computed using the tight binding method and screening the exchange crystal potential with the dielectric function, and the energy band structure was related to the optical absorption spectrum and the first exciton and saddle point peaks were interpreted.
TL;DR: In this article, a stochastic model for the exciton motion which comprises both the coherent and the incoherent motion is presented. But the model is restricted to the case of two molecules and does not take into account the influence of the exchange interaction integral.
Abstract: We use a stochastic model for the exciton motion which comprises both the coherent and the incoherent motion. The incoherent part is taken care of by a stochastic process which allows the local excitation energy and the transition matrix element to fluctuate by means of a Markovian process. The interaction between the spins and their surroundings is described by the usual spin-Hamiltonian which is, however, simplified to a spin 1/2 particle (instead of the triplet state). In the present paper we solve exactly the two limiting cases of completely coherent and incoherent motion (for two molecules). In the incoherent case the influence of the exchange interaction integral is taken into account by perturbation theory. We find expressions which are immediately comparable with ESR-experimental data. This comparison and additional information derived from optical absorption measurements allow us to determine all free parameters of our model uniquely. In particular, the fluctuations of the exchange interaction integral (with strength γ1) play an important role. From these parameters we may furthermore calculate the correlation time of the proton spin resonance in agreement with experimental data. The results show clearly that at room temperature in anthracene crystals the exciton undergoes a hopping process.
TL;DR: In this paper, the rate of the biexcitonic quenching process and the exciton-phonon coupling were investigated for the 1,2,4,5-tetracyanobenzene-durene complex with the use of a nitrogen gas laser.
Abstract: Fluorescence quenching due to exciton-exciton interaction was studied for the crystals of fluoranthene and 1,2,4,5-tetracyanobenzene-durene complex with the use of a nitrogen gas laser and the rate constant for the biexcitonic quenching process γ and the rate of the exciton migration λ were obtained for both crystals. The γ value for both crystals was found to increase with increasing temperature. Finally, we obtained the phonon frequency which contributes to the transport of excitons through exciton-phonon coupling.
TL;DR: In this article, the indirect band gap of GaAs1−x P x has been determined accurately over a large alloy composition range, using a wavelength derivative technique in optical absorption on material grown under complete equilibrium conditions.
Abstract: The indirect band gap of GaAs1−x P x has been determined accurately over a large alloy composition range. The results were obtained with a wavelength derivative technique in optical absorption on material grown under complete equilibrium conditions. Zero‐phonon as well as momentum‐conserving phonon‐assisted free‐exciton transitions were observed. A distinct curvature in the direct band gap‐vs‐alloy composition curve is determined. Photoluminescence measurements of the same samples, together with the absorption edge data, give the alloy composition dependences of the energies of donor‐acceptor pairs and bound excitons; thus the intrinsic nature of the transitions observed in absorption is established.
TL;DR: In this paper, an experimental Raman study of lattice modes in neat and mixed (10%-90%) crystals of naphthalene-hs and naphthale-e-ds at 100 K with 1 ern-I. resolution is presented.
Abstract: We present an experimental Raman study of lattice modes in neat and mixed (10%-90%) crystals of naphthalene-hs and naphthale~e-ds at 100~K with 1 ern-I. resolution .. The spectral fe~tures of the ?e~t crystals are preserved in the heaVIly doped mixed crystals, With small shifts and broademngs characterIstic of an amalgamation limit that assumes weakly coupled excitation bands in the restricted Frenkel-Davydov limit. Rotation-translation interaction does not affect the mixed crystal spectra, thus making the Raman technique uniquely suited for the investigation of the librational (rotational) phonon band structure. The evidence is against localized or pseudolocalized phonons in these isotopic mixed crystals.
TL;DR: In this paper, the emission spectrum of the zero-phonon n = 1A exciton line in CdS crystals has been investigated at temperatures 2-25°K with the high resolution and compared with the polariton dispersion curve, which was measured experimentally.
TL;DR: In this paper, a theoretical derivation of the complete one-phonon density-of-states of a molecular crystal can be mapped out by a carefully chosen exciton sideband involving a localized molecular internal excitation in a dilute isotopic mixed crystal.
Abstract: The complete one-phonon density-of-states of a molecular crystal can be mapped out by a carefully chosen exciton sideband involving a localized molecular internal excitation in a dilute isotopic mixed crystal. A theoretical derivation shows that the necessary criteria are: absence of localized in-band phonons; ap plicability of a phonon amalgamation limit for the mixed crystal; weak exciton-phonon, guest-lattice coupling and weak guest-host exciton interaction. Experimentally, at least two exciton-phonon sidebands should be investigated, so as to exclude trap and defect transitions, and at least one of the guest transi tions should belong to the deep-trap limit and have sidebands weak enough to minimize multiphonon transitions. The above is demonstrated for the naphthalene crystal by high-resolution fluorescence and phosphorescence spectra of 0.25% CloHS in CloDs and by phosphorescence of 0.14% 2-DClOH7 in CloDs at 2°K. The phosphorescence phonon sidebands correlate surprisingly well with Pawley's "atom-atom" calculated phonon density-of-states, and seems to give a much better phonon density map than the very recent high-resolution inelastic, incoherent neutron scattering data. New Raman data are also presented, showing defect (isotopic impurity) induced bands, which are interpreted in terms of the above phonon density-of-states peaks. Applications to reversible and irreversible thermodynamics are mentioned.
TL;DR: In this paper, the bimolecular annihilation of singlet excitons in pyrene, perylene, and anthracene crystals was studied by the aid of a nitrogen gas laser.
Abstract: The bimolecular annihilation of singlet excitons in pyrene, perylene, and anthracene crystals was studied by the aid of a nitrogen gas laser. The rate constant, γ, for bimolecular annihilation was estimated at 293°K to be 9×10−15 and 8×10−14 cm3 sec−1 for pyrene and perylene, respectively. The temperature dependence of γ was measured with the result that it increases with the increasing temperature for the pyrene and perylene crystals, while it is almost independent of temperature for anthracene between 77°K and 293°K. The difference in the temperature dependence of γ was attributed to the difference in the mechanism of the exciton migration of the crystals. The method developed by the present authors for studying the exciton migration based on the exciton bimolecular annihilation was compared with that obtained previously by the use of mixed crystals.
TL;DR: In this article, the absolute value of reaction cross sections, as given by the pre-equilibrium exciton model, was estimated using Fermi gas approximation, and a general agreement with experimental data, particularly in the gross $A$ dependence was obtained.
Abstract: The absolute value of ($n,p$) reaction cross sections, as given by the pre-equilibrium exciton model is estimated using Fermi gas approximation.A general agreement with experimental data, particularly in the gross $A$ dependence is obtained. The energy range considered is 10-20 MeV, and the nuclei are those with $Ag100$: In these ranges evaporation is negligible and the analysis is rather easy. The approximate life-time of one single-particle exciton in nuclear matter is also deduced.
TL;DR: In this paper, the spin polarization and emissive character of the magnetic resonance spectrum of mobile triplet excitons in tetracene single crystals observed by Yarmus, Rosenthal, and Chopp are interpreted in terms of selective population of the Zeeman sublevels by singlet exciton fission.
TL;DR: In this paper, the rotational strengths for the α- and p-band regions calculated from the dipole-velocity procedure in the π-SCF approximation, rather than the point-dipole exciton treatment, are in good agreement with experiment.