Showing papers on "Exciton published in 1968"

The Excitonic State at the Semiconductor-Semimetal Transition

Abstract: Publisher Summary Some years ago, Mott considered the role of the Coulomb attraction for a semimetal with small numbers of electrons and holes and observed that under certain conditions the electrons and holes could bind to form a nonconducting phase. Knox remarked that the ordinary Bloch wave groundstate of the crystal could become unstable against the spontaneous formation of excitons if the exciton binding energy exceeded the energy grip. The exciton instability leads to a second-order transition to a new state having a “condensate” of excitons and in which the translational periodicity of the original lattice is broken by a new “long-range order” in the electronic states. This new state is variously referred to as the “excitonic state” or as the “distorted state,” whereas the normal Bloch-wave state is referred to as the “undistorted state.” This chapter gives a brief review of the previous work on the excitonic state. The bulk of the chapter is devoted to an attempt to elucidate further the physical significance of the distorted state, and to examine more carefully than previously the question of the existence of such a state, and of the location and nature of the transition points.

Bound-Exciton, Free-Exciton, Band-Acceptor, Donor-Acceptor, and Auger Recombination in GaAs

Abstract: The broad-band photoluminescence spectra typical of bulk-grown or low-purity epitaxial GaAs breaks into a number of sharp emission lines in very-high-purity GaAs. At low temperatures (\ensuremath{\sim}2\ifmmode^\circ\else\textdegree\fi{}K), emission peaks due to free excitons (1.5156 eV), excitons bound to neutral (1.5145 eV) and ionized (1.5133 eV) donors, and excitons bound to neutral (1.5125 eV) and ionized (1.4886 eV) acceptors are identified. Also, band-acceptor (1.4926 eV) and donor-acceptor (1.4857 eV) recombination is observed. The assignments are based on dependence of the emission lines with temperature, impurity content, and carrier type ($n$- or $p$-type material), as well as on the relative energy positions. The temperature dependence of the exciton-neutral-donor complex appears to be influenced by nonradiative Auger recombination.

Magnetic Field Effect on Triplet Exciton Quenching in Organic Crystals

Abstract: The rate of quenching of triplet excitons by paramagnetic impurities in anthracene crystals at room temperature is found to be magnetic-field dependent A theory of the effect is presented

Theory of infra-red and optical spectra of antiferromagnets

Abstract: The contributions of magnons to the optical properties of antiferromagnets having the rutile structure are discussed. The properties considered are electric-dipole active two-magnon absorption in the infra-red, and magnon sidebands on sharp-line exciton transitions in the visible. The discussion is based on a thorough treatment of the properties of excitons and magnons in the antiferromagnetically ordered state. The site-group and space-group symmetries of the magnetic excitations are derived and the selection rules for electric and magnetic dipole transitions are determined. The occurrence of magnetic Davydov splittings of the excitons is investigated, and their symmetry properties throughout the Brillouin zone are derived. The functional dependences of exciton energy on wave vector are calculated. Applications of the theory are made to experimental results on excitons and magnons in MnF2, FeF2 and CoF2. The possible mechanisms for two-magnon and magnon-sideband absorption are discussed, and the...

Mechanisms for the Anomalous Properties of Eu-Chalcogenides Alloys. I.

Abstract: Various anomalous magneto-optic and magnetic properties of Eu 1- x Gd x Se are explained on the magnetic exciton and the magnetic impurity state model. It is shown that the optically active magnetic exciton with the lowest energy is of the type of (5 d –5 d ), that is, 5 d character on the central Eu 3+ and also 5 d on the nearest neighbour Eu 2+ . The origin of the anomalous magneto-optic properties is attributed to the d – f exchange interaction on the nearest neighbour Eu 2+ ions. While, for the magnetic impurity state, (6 s –6 s ) type state has the lowest energy. At low temperature, a giant spin molecule ( S =46) composed of the spin of an impurity electron and the 4 f spin on the central Gd 3+ ion and the n.n.Eu 2+ ion is constructed through the s – f exchage interaction among them. However, this giant spin molecule disappears rather quickly as temperature increases exceeding J 1/κ , where J 1 is the s – f exchange constant at n.n. Eu 2+ and is estimated as 25κ. The anomalous magnetic properties of ...

Magnetic and Electric Field Spectra of Organic Crystals: Optical Measurements of Zero‐Field Splittings

Abstract: The theory of the Zeeman and Stark effects in molecular crystal exciton bands is presented. The development emphasizes the role of the intermolecular potential in determining the spin alignments in the crystal, and it is shown that the interactions V, and the projections of spins from one molecule onto those of another 〈Tα | Tβ〉, determine the crystal spin quantization in the absence of external fields through matrix elements of the type V 〈Tα | Tβ〉. A complete anisotropic Zeeman study in weak and strong fields is presented for the benzophenone crystal. It is shown that relatively low‐resolution optical studies can be used to determine zero‐field splitting parameters to about 5% accuracy. The higher resolution studies have led to the assignments, and relative ordering of the twelve k = 0 exciton components of the 3nπ* state of benzophenone crystals. These results are confirmed by Stark and Stark–Zeeman experiments which demonstrate the applicability of the theory outlined. The final best set of molecular ...

Phonon-Assisted Recombination of Free Excitons in Compound Semiconductors

Abstract: In this paper we study the band-edge optical transitions in "direct" semiconductors involving the interacting exciton-phonon system, especially the longitudinal-optical (LO)-phonon-assisted recombination of free excitons. By introducing an extended Green's-function approach to include propagation in and between the various bands in the exciton spectrum, we obtain rather directly the transition rate to lowest order in the exciton-photon coupling which contains the effects of the interactions between the exciton and lattice vibrations to all orders, and thus includes line broadening, shift, asymmetry, and all renormalizations. Under the appropriate conditions (sufficiently separated bands), this result is equivalent to an expression previously derived by Toyozawa, but has the virtue that all terms appearing in it (in particular, the asymmetry term) are given in precise and general expressions. The relation of the general result to that obtained by conventional perturbation theory is discussed. The radiative decay of free excitons by the one- LO- and two-LO-phonon-assisted processes are formulated in the framework of perturbation theory. The limitation of this approach and its relationship to that in which the exciton-photon interaction is treated more accurately are briefly discussed. It is also shown that the second-order perturbation-theory result for the one-phonon-assisted processes (both for absorption and emission), including the contributions from the full intermediate state (hydrogenic) spectrum, can be evaluated exactly in closed form. The one- and two-LO-phonon-assisted emission spectra for CdS are calculated for several temperatures up to 77\ifmmode^\circ\else\textdegree\fi{}K using only experimentally determined parameters and taking the anisotropy of the valence band into account. Except for the one-LO peak at $T=77\ifmmode^\circ\else\textdegree\fi{}$K, where the polariton effects are important, the calculated line shapes and widths, as well as the intensity ratio of peaks, are found to be in good accord with the observed spectra. The corresponding calculations for ZnO at 77\ifmmode^\circ\else\textdegree\fi{}K are in good agreement with experimental spectrum. The zero-LO exciton peak due to the one-acoustic-phonon-assisted process is calculated in weak exciton-photon coupling approximations for both the deformation potential and piezoelectric couplings. The widths of the calculated lines are orders of magnitude smaller than the observed widths, and we conclude that the difficulty lies in the use of the weak exciton-photon coupling approach.

Direct Observation of the Entire Exciton Band of the First Excited Singlet States of Crystalline Benzene and Naphthalene

Abstract: The density‐of‐states functions for the exciton bands of the first excited singlet states of crystalline benzene and naphthalene have been determined experimentally. The experimental density functions were derived from spectral data involving exciton‐band↔exciton‐band transitions. The experimental results for naphthalene have been compared with calculations based on the octopole model. Using a Frenkel dispersion relation and various sets of coupling constants, density functions have been calculated and compared with the experimental results. For benzene these calculations show that the “optically forbidden” Au component lies between 37 815 and 37 875 cm−1. From the temperature dependence of transitions, phonon contributions have been estimated. It is found that the experimentally derived density function is temperature independent below ∼30°K.

Exciton Luminescence of CuCl, CuBr and CuI Single Crystals

Abstract: Emission, absorption and reflection spectra have been studied on single crystals of cuprous halides. Three types of emission are observed: 1) resonance emission due to the annihilation of the lowest energy intrinsic exciton, 2) narrow lines due to bound excitons at defects or impurities, 3)broad bands and overlapping equidistant narrow bands. In the excitation spectrum for the resonance emission a structure of equal spacing, which corresponds to the longitudinal optical (LO) phonon energy, is found in the exciton absorption band region at low temperatures below 40°K. The resonance emission bands in CuBr and emission lines belonging to the type 2) in CuI are accompanied by the LO phonon assisted satellite bands. The LO phonon energies are obtained as 210±12, 166±7 and 151±8 cm -1 in CuCI, CuBr and CuI crystals, respectively.

Theory of Two-Photon Spectroscopy in Solids

Abstract: The coefficient of absorption of two photons is calculated for a two-band model of a solid. The frequency and polarization dependence is evaluated for the case that one photon (laser) frequency is fixed while the second is varied. Wannier exciton states are explicitly included in both the intermediate and final states. Depending upon the symmetry of the two energy bands, the final state may either be an exciton $s$, $p$, or $d$ state. All three cases have different frequency and polarization dependences. The final formulas for the matrix element are complicated, and approximate formulas are given which are useful for many applications. It is shown that some of the two-photon spectroscopy experiments in semiconductors and alkali halides which have been reported so far can be explained by these calculations.

Exciton-Phonon Bound State: A New Quasiparticle

Abstract: We point out that an exciton and an optical phonon may form a bound state which moves through the crystal. Calculated binding energies and oscillator strengths for this new quasiparticle lend support to recent suggestions that strong exciton-phonon mixing is present in the optical spectra of some ionic crystals.

Band Structures and Optical Properties of Semiconducting Layer Compounds GaS and GaSe

Abstract: The band structures of GaS and GaSe near the Fermi levels are derived in a semiempirical way. In view of the anisotropic structures, the π band structures are mainly studied. The valence band consists of a π band with the heavy effective masses. The conduction band at the center of the Brillouin zone has the light effective masses for the both directions parallel and perpendicular to the layers, while its minimum at the zone edge has a two-dimensional character. The characteristics of the optical properties of these compounds are the existence of the sharp peaks associated with the nearly two-dimensional pair bands at the saddle points and further the limited spatial extension of an exciton along the c -axis. Most of the structures in reflectivities in GaS and GaSe and the binding energy of an exciton in GaSe are explained fairly well.

Electronic and Vibrational Exciton Structure in Crystalline Benzene

Abstract: The purpose of this paper is to lay a consistent theoretical framework for the discussion of a series of forthcoming experimental papers on the absorption and emission of light by benzene crystals. Emphasis will be placed not only on the usual Davydov splittings but also on k≠O states and band‐to‐band transitions in neat (pure) crystals, and on orientational effects, site splittings, shifts, and resonance pair spectra in isotopic mixed crystals. The role of translationally equivalent interactions will be discussed. The interchange‐group concept is used in order to simplify the theoretical analysis. From the four possible interchange groups D2, C2υa, C2υb, C2υc, the D2 group is found to be the most convenient for the classification of benzene exciton functions. A differentiation between static and dynamic interactions is made in the limit of Frenkel excitons, and the concepts of site distortion energy and the ideal mixed crystal are introduced to aid in this distinction. Data pertaining to site shifts and ...

Energy Transfer in Organic Molecular Crystals: A Survey of Experiments

Abstract: Publisher Summary This chapter deals with some current problems of luminescence and energy transfer in organic molecular crystals. The quenching of energy transfer in anthracene crystals at low temperatures is caused by a competition between energy transfer to the guest molecules and to shallow traps that give rise to the emission from impurities or disturbed exciton states at low temperatures. The fluorescence of an anthracene crystal containing small traces of tetracene is measured. The result is that the fluorescence intensity of tetracene is relatively much stronger than the anthracene emission. The tetracene molecules act as traps for the electronic excitation energy, which is traveling in the anthracene crystal. To measure and separate the host and guest quantum yields, one has to measure the total fluorescence spectrum with good resolution. Only in this way is it possible to separate unambiguously the individual contributions from host, guest, and traps to the total emission.

Reflectance Modulation by the Surface Field in GaAs

Abstract: By studying reflectance spectra in GaAs modulated by a second, intense light beam, we have observed oscillatory structures in the spectra near the energy gap as well as near 3 eV. We conclude that these structures result from a neutralization of the builtin surface field by free carriers created by the intense light beam. The experimental line shape near the band gap is qualitatively very similar to the theoretical prediction for the Franz-Keldysh effect but is shifted to lower energies presumably due to exciton effects.

Excimer fluorescence XII The pyrene crystal excimer interaction potential.

Abstract: are determined as functions of the intermolecular separationr (A). V' is shown to be consistent with an exciton resonance state originating from the 'Ba state of the pyrene molecule.

Triplet Exciton Phosphorescence in Crystalline Anthracene

Abstract: Long-lived triplet excitons in anthracene crystals were detected by simultaneous observation of both their weak red phosphorescence and the delayed blue fluorescence which results from exciton-exciton interaction. Absorption and emission spectra at room temperature show that the interacting excitons are free, i.e., they correspond to the electronic excitation levels of the pure crystal. Decay curves obey the anticipated bimolecular-interaction kinetic equations. The triplet radiative lifetime is about 60 sec. In contrast to the results at room temperature, the low-temperature behavior is dominated by exciton-tapping effects. The interaction between free and shallowly trapped excitons is much more efficient in yielding delayed blue fluorescence than is the free-free interaction which is dominant at room temperature.

Electronic Properties of Liquid Water in the Vacuum Ultraviolet

Abstract: Reflectance of liquid water has been measured between 1050 and 3000 \AA{} by two methods, one with the water in an open dish in equilibrium with its vapor and one with a Ca${\mathrm{F}}_{2}$ or quartz cell. Real and imaginary parts of the index of refraction and dielectric constant, deduced from the data and Fresnel's equations, suggest an exciton transition at 8.3 eV, an interband transition at 9.6 eV, and a band gap of 9 eV.

Exciton Interpretation of the Vacuum‐Ultraviolet Absorption Spectra of Saturated Organic Polymers

Abstract: A recent application of the molecular exciton theory to the σ bond absorptions of simple gaseous alkanes has prompted the extension of the theory here to saturated organic polymers in the solid phase. The theory explains the experimentally observed insensitivity of the absorption spectra to changes in polymer chain conformation and, in the case of simple alkanes, to changes from gas to solid phase. It also explains the observed similarity between the low‐energy portions of the absorption spectra of such diverse materials as diamond and gaseous alkanes. The exciton energy levels of polyethylene, polypropylene, and diamond are calculated and are found to give very reasonable agreement with the previous experimental data on polyethylene and diamond, and with the absorption spectrum of polypropylene which is presented here between 2500 and 584 A. The polarizations of the optically allowed transitions are also obtained but no experimental data is yet available on these. The possible implications of this excito...

Long wavelength forms of chlorophyll.

Abstract: SPECIES of chlorophyll a absorbing in the red at 673–5, 682–5 and in the range 697–715 nm occur in photosynthetic organisms1. Colloidal dispersions, monolayers spread on water, and various crystalline preparations of chlorophyll are also known to absorb light at wavelengths longer than does monomeric chlorophyll2. The red-shift of chlorophyll in the plant has been explained in terms of an exciton theory3 and is generally considered the result of aggregation or crystallization of the chlorophyll. A large red-shift is taken almost automatically to indicate a high degree of crystallinity4. We have now prepared chlorophyll a solutions and films which absorb in the red at 672–5, 682–5, 710–20 and 740 nm, and have been able to deduce structures for these long wavelength forms of chlorophyll from the infrared spectra.

Optical Studies of NaCl Single Crystals in 10 eV Region II. The Spectra of Conductivity at Low Temperatures, Absorption Constant and Energy Loss

Abstract: Kramers-Kronig analysis was made on the reflectivity spectra of NaCl single crystals in the region 6eV to 12eV for temperature range from 10°K to 573°K. The spectra of absorption constant and energy loss are reported for the temperature range from 10°K to 573°K and compared with those obtained on thin film specimens. The spectra of complex dielectric constant, complex refractive index and conductivity are presented at temperatures of 10°K, 78°K and 298°K. The analysis of σ-spectrum in the region of chlorine doublet and the plateau near the step leads to the conclusion that the spectrum in this region consists of absorptions due to hydrogen like series of exciton. Each of the chlorine doublet standing for n =1 state exciton lines can be fitted with a suitable asymmetric Lorentzian function having a positive degree of asymmetry. The oscillator strengths of (3/2, 1/2)- and (1/2, 1/2)-components are estimated at 0.212 and 0.252 at 10°K, respectively. The exchange interaction between electron and hole of an ex...

Energy levels of nitrogen-nitrogen pairs in gallium phosphide

Abstract: It is proposed that the line spectrum of nitrogen-nitrogen pairs in gallium phosphide arises from the recombination of an exciton situated at one nitrogen atom, its energy being perturbed by the strain field of a second nitrogen atom. The model predicts a series of lines with energies displaced from a series limit by an amount βprimeγNN-3, with βprime a constant and γNN the nitrogen-nitrogen separation.

Delocalized excitons in thin anisotropic crystals

Abstract: The ordinary absorption spectrum of MoS2 contains two series of exciton absorption bands associated with direct optical transitions from a split valence band. In thick crystals the first five members of each exciton series have been detected; these exciton bands move to higher energies and show a diminishing diamagnetic shift with decreasing crystal thickness. The exciton series limit in thin crystals is determined from the oscillatory magneto absorption which occurs to the high energy side of the short wavelength series. Das normale Absorptionsspektrum von MoS2 enthalt zwei Serien von Exzitonenabsorptionsbanden, die mit direkten optischen Ubergangen von einem aufgespaltenen Valenzband verbunden sind. An dicken Kristallen sind die ersten funf Linien beider Serien gemessen worden; bei abnehmender Kristalldicke werden diese Exzitonenbanden zu hoheren Energien hin verschoben und zeigen eine verminderte diamagnetische Verschiebung. Die Grenze der Exzitonenserien wird in dunnen Kristallen von der oszillatorischen Magnetoabsorption bestimmt, die auf der Hochenergieseite der kurzwelligen Serien eintritt.