Exciton creation

It is discussed how the primary processes of defect formation during irradiation occur via electronic excitation. This can take the form of either the creation of electron-hole pairs, followed by trapping into localized energy states, or of exciton creation leading to the formation of stable vacancy and interstitial defects. Heating the sample after the irradiation causes the release of this stored energy in the form of phonons or photons. Photon emission, ie. luminescence, results from either electron-hole recombination or from vacancy-interstitial recombination. Several examples of both types are discussed for crystalline CaF and SiC. ... 

In organic cells, however, the steps involved in the generation of photo-current are (1) light absorption, (2) exciton creation, (3) exciton diffusion, (4) exciton dissociation in the bulk or at the surface, (5) field-assisted carrier separation, (6) carrier transport, and (7) carrier delivery to external circuit. Assuming that only the excitons which reach the junction interface produce free carriers, if the blocking contact is illuminated [65],... 

Here Eq is the energy of the molecular excitation where also the gas-condensed matter shift is taken into account, and Pn, Pn are the exciton creation and annihilation Pauli operators on the molecule n, where n labels the unit cells of the... 

If we now pass from the operators Bnf, to the exciton creation and annihila-... 

Here ak a ) is the annihilation (creation) operator of an exciton with the momentum k and energy Ek, operator an(a ) annihilates (creates) an exciton at the n-th site, 6,(6lt,) is the annihilation (creation) operator of a phonon with the momentum q and energy u) q), x q) is the exciton-phonon coupling function, N is the total number of crystal molecules. The exciton energy is Ek = fo + tfcj where eo is the change of the energy of a crystal molecule with excitation, and tk is the Fourier transform of the energy transfer matrix elements. 

In low-dimensional systems, such as quantum-confined. semiconductors and conjugated polymers, the first step of optical absorption is the creation of bound electron-hole pairs, known as excitons [34). Charge photogcncration (CPG) occurs when excitons break into positive and negative carriers. This process is of essential importance both for the understanding of the fundamental physics of these materials and for applications in photovoltaic devices and photodctcctors. Since exciton dissociation can be affected by an external electric field, field-induced spectroscopy is a powerful tool for studying CPG. 

In other white light devices, blue, green and red emitters are combined. Kido et al. [169, 170] designed multilayer systems using 6 (TPD) for blue, metal-chelate complexes for green and red emission, respectively. Similar devices have been developed by other groups, using Forster transfer or exciton confinement for the creation of the three primary colors [171, 172]. Exciplex emission was... 

The resemblance of the photocurrent to the optical adsorption spectrum has suggested the involvement of molecular excited states in the creation of charge carriers. While this resemblance is by no means universally observed, the concept of carrier creation via exciton interactions at or very near the illuminated electrode has become increasingly favored. Many of the data leading to these conclusions have been obtained by the use of pulsed light techniques (6, 7,3). These methods are virtually independent of electrode effects and the subsequent analysis of the transient current has led to considerable advances in the theory of charge transfer in molecular crystals. 

The excitonic state in MgO (bulk) arising from the creation of a hole in the Mg-2p core level appears as an impurity level within the band gap of MgO. Previous cluster model studies provided theoretical evidence of the local character... 

The electronic properties of RGS have been under investigation since seventies [3-7] and now the overall picture of creation and trapping of electronic excitations is basically complete. Because of strong interaction with phonons the excitons and holes in RGS are self-trapped, and a wide range of electronic excitations are created in samples free excitons (FE), atomic-like (A-STE) and molecular-like self-trapped excitons (M-STE), molecular-like self-trapped holes (STH) and electrons trapped at lattice imperfections. The coexistence of free and trapped excitations and, as a result, the presence of a wide range of luminescence bands in the emission spectra enable one to reveal the energy relaxation channels and to detect the elementary steps in lattice rearrangement. 

Atomic cryocrystals which are widely used as inert matrices in the matrix isolated spectroscopy become non-inert after excitation of an electronic subsystem. Local elastic and inelastic lattice deformation around trapped electronic excitations, population of antibonding electronic states during relaxation of the molecular-like centers, and excitation of the Rydberg states of guest species are the moving force of Frenkel-pairs formation in the bulk and desorption of atoms and molecules from the surface of the condensed rare gases. Even a tiny probability of exciton or electron-hole pair creation in the multiphoton processes under, e.g., laser irradiation has to be taken into account as it may considerably alter the energy relaxation pathways. 

Because of these problems, observations of the screening of the core-hole and the creation of excitons in core level excitation has become a favored technique for observing the metal-insulator transition the most utilized here has been core-level X-ray photoelectron spectroscopy (XPS)5. 

Let us assume that the coulombic branch is resolved, for K = 0 and for a given direction K, by the diagonalization of (1.70). This means that we know, for each direction K, the eigenenergies a>e(K) for each excitonic mode, as well as the eigenvectors e>, which are linear functions by the transformation (1.32), (1.51) of the creation and annihilation operators of molecular states. Furthermore, let us define the excitonic dipolar moment by the same linear transformation on the molecular dipoles,... 

This coupling is assumed local, i.e., creation or absorption of vibrations occurs without exciton transfer, as in (2.15). This approximation amounts to considering only the R dependence of the local energy Dnm. We find that the xs do not depend on q. 

Figure 2.10. Detail of the 0-0 a-polarized reflectivity at 5 K (cf. Fig. 2.8). The arrow indicates the setup of coherent superposition of reflectivity amplitudes from front and back faces. This structure is located at 140 cm 1 above the bottom of the excitonic band it indicates the threshold of a- ft relaxation with creation of 140cm" Bg phonons and ft excitons.

If the exciton-phonon interaction Hep is strong compared to the emission probability, high-order terms in Hep contribute to P (2.131), providing strong luminescence at the expense of the one-phonon (Raman) process. In contrast, if the emission probability dominates the phonon creation probability, the peak (2.133) dominates the secondary emission at the expense of the luminescence.77 Examples of this competition will be discussed for the surface-state secondary emission, where the picosecond emission of the surface states, and its possible modulation, allow very illustrating insights into the competition of the various channels modulated by static or thermal disorder, or by interface effects. 

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