Excitonic bound state

Quantum dots A semiconductor whose excitons (bound state of an electron and a hole) are confined in all three spatial directions. 

Paramagnetic species formed by reactions of the radiation-produced transient species in ice and frozen aqueous systems have been studied by ESR technique. The radiation-produced electrons have been found to react e.g. with acidic solutes to form H-atoms and with group 11(b) metal ions to give the corresponding univalent radical ions, while the holes can react with anions such as S04 2 and H2P04 giving the radical ions S04 and HP04. Evidence that the electron and hole are coupled to each other, and may in fact exist in irradiated pure ice primarily in an (exciton-like) bound state has been discussed. The present work provides evidence for the reactions of the radiation-produced positive holes apart from the reactions of the electrons. 

It is suggested that in irradiated pure ice the electrons and the positive hole initially are coupled to each other in an exciton-like bound state. 

A Mott-Wannier exciton is a neutral quasi-particle, consisting of an excited bound-state electron and its associated "Coulomb hole" in a high-dielectric constant solid, that can also travel throughout the lattice without transporting net charge since the exciton radius is several lattice constants, its binding energy is as low as 0.01 eV it thus tends to be more "delocalized" than the Frenkel exciton. 

In extreme cases a multiple-scattering, sharp resonant structure can result in which the electron is in a quasi-bound state (155). One example is the white line, which is among the most spectacular features in X-ray absorption and is seen in spectra of covalently bonded materials as sharp ( 2eV wide) peaks in absorption immediately above threshold (i.e., the near continuum). The cause of white lines has qualitatively been understood as being due to a high density of final states or due to exciton effects (56, 203). Their description depends upon the physical approach to the problem for example, the LiUii white lines of the transition metals are interpreted as a density-of-states effect in band-structure calculations but as a matrix-element effect in scattering language. 

The Kossel model (146) of single-electron transitions to unoccupied states has been applied to the interpretation of the absorption-edge structure of isolated atoms (inert gases) as well as to molecules and solids, in which case use is made of band-model calculations, including the possible existence of quasi-stationary bound states as exciton states. Parratt (229), who has carried out the first careful analysis of the absorption spectrum of an inert gas, assumed that dipole selection rules govern the transition possibilities, with allowed transitions being Is - np. 

An important defect of all the single electron calculations is the neglect of electron correlation. Various methods have been employed to rectify this problem. Its inclusion using second order perturbation theory has been found to provide much better agreement between theory and experiment in some instances. The inclusion of electron correlation has a profound effect since the electrons and holes in the first excited state are bound by their Coulomb interaction to form a localised state, an exciton. This state is separated from the conduction band by the exciton binding energy. That this model... 

An exciton bound to a shallow neutral donor of interstitial zinc (Fig 1 a) and of interstitial lithium (Fig. lb) is presented, for example, in our spectra. In some instances the radiative recombination of an exciton bound to a neutral defect may not lead to the ground state of the respective defect but to an excited state of the carrier at this occupied center (2 - electron transition). In a hydrogenic model we can calculate an ionization energy of the neutral donor state of interstitial zinc to 0.05 eV and of interstitial lithium to 0.033 eV. 

The values of gh derived for the U and I9 lines are very close to the gh = -1.24 obtained for the hole involved in the exciton bound to ionized donor and to the g factor of the hole in li free exciton state. " On the other hand, the expected g/ values of the holes involved in the acceptor bound exciton transitions differ significantly from the g values of the holes involved into excitons bound to ionized or neutral donors. This is similar to the situation found in CdS. Therefore we conclude that both U and I9 transitions should be assigned to the (Do,2Ci(r7)) complex rather than to the (y4o(T7) A(T7)). 

The inverted Vj A), TeCil), Vj C) ordering of the valence subbands in bulk ZnO was confirmed by the detailed analysis of the Zeeman splitting of the free and bound excitons. The polarization properties and the angular dependence of the transition energies from excitons bound to ionized and neutral impurity centers indicated the T7 character of the upper A valence band. The obtained Tv effective g values are in good with theoretical calculations. We observed no low temperature PL transitions involving the Tg hole states from the B valence subband. 

The interpretation of the interband transition is based on a single particle model, although in the final state two particles, an electron and a hole, exist. In some semiconductors, however, a quasi one-particle state, an exciton, is formed upon excitation [23,24]. Such an exciton represents a bound state, formed by an electron and a hole, as a result of their Coulomb attraction, i.e. it is a neutral quasi-particle, which can move through the crystal. Its energy state is close to the conduction band (transition 3 in Fig. 2), and it can be split into an independent electron and a hole by thermal excitation. Therefore, usually... 

Despite the potential, experimental spectra of ELNES and XANES have not been fully utilized in order to monitor the local structural and chemical environment. One of the major reasons is the presence of core-hole effects which leads to a redistribution of the PDOS features [10]. In other words, the presence of this effect has been considered as a bottleneck for the full interpretation of the experimental spectra. For example, O Brien et al. compared their XANES spectra of MgO, o -Al203 and MgAl204 at cation L2,3-edge with theoretical DOS obtained by band calculations, but their unoccupied DOS did not reproduce the experimental spectra [11]. Thus, the origin of the major spectral features was concluded to be the formation of a core exciton, i. e., a bound state of the excited electron due to the presence of a core hole. 

The band description ignores correlations between electrons that result from the electron-electron repulsive interaction. Alternatively stated, the band description ignores the attractive Coulomb interaction between electrons in the TT -band and holes in the ir-band. This attraction causes the formation of excitons i.e. neutral electron-hole bound states. One of the fundamental unresolved issues of the physics of semiconducting polymers is the magnitude of the exciton binding energy (see Section V-B). 

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