Excitonic structure

Chapter 4, presents details of the absorption and reflectivity spectra of pure crystals. The first part of this chapter coimects the optical magnimdes that can be measured by spectrophotometers with the dielectric constant. We then consider how the valence electrons of the solid units (atoms or ions) respond to the electromagnetic field of the optical radiation. This establishes a frequency dependence of the dielectric constant, so that the absorption and reflectivity spectrum (the transparency) of a solid can be predicted. The last part of this chapter focuses on the main features of the spectra associated with metals, insulators, and semiconductors. The absorption edge and excitonic structure of band gap (semiconductors or insulator) materials are also treated. 

Furthermore, surface reconstruction may be viewed, for the variation of certain properties, simply as a response to depression , or to a strain of dilatation, in the surface layer. This approach has been applied with success to account for the surface phonon, and for the global destabilization (blue shift) of the surface exciton. In contrast, the modification of the surface excitonic structure is not amenable to such a simple image of the reconstruction. 

The exact shape of the reconstructed surface (in effect, the nature of the missing stresses ). The information provided by the analysis of the surface excitonic structure seems to indicate displacements only among equivalent molecules. 

The CD of the CT transitions has attracted little direct interest in the literature. Mason (10) has postulated that it arises from an exciton mechanism in which the CT transition is broken up into three degenerate oscillators or chromophores with different origins. Such a procedure is unjustified quantum mechanically, as each chromophore must have negligible electron exchange with any other chromophore considering that a common metal d state is involved in the definition of each such chromophore or oscillator, such a model is, despite its pictorial appeal, theoretically inconsistent. Mason supports his model by noting that the CT CD exhibits the exciton structure characteristic of... 

Perhaps more illuminating is the spatial distribution of the electron and hole wave functions. The hole wave function is concentrated at the central S atom (Sc, 68%) and the electron wave function is mostly delocalized among the 16 Cd atoms (12 M, 4MF, 70%) [36]. This spatial distribution of the wave functions maximizes their overlap and is exactly what is expected of an idealized quantum-confined exciton (Structure 2). To the best of our knowledge, this 10-A, 55-atom CdS cluster is the smallest cluster to exhibit the basic exciton properties. 

Fig. 3 (a) Exciton structure for a typical case of 5 nm CdSe NC. (b) Schematic showing the dark bright splitting for CdSe NCs. (Shown for spherical zincblende structured NCs). 

K. Sakurai, H. Tachibana, N. Shiga, C. Terakura, M. Matsumoto and Y. Tokura. Experimental determination of excitonic structure in polythiophene. Phys. Rev. B Condens. Matter Mater. Phys. 56(15), 9552-9556 (1997). 

T. Renger, A New Spectroscopic Tool for Analyzing Excitonic Structure and Dynamics in Pigment-Protein Complexes , Biophys. J., 2008, 95, 495. 

Figure 3.6 Absorption coefficient and excitonic structure for an annealed (solid lines) and an unannealed (dotted line) sample at room temperature. The inset shows the absorption coefficient for the annealed samples at 77 K. (After Ref [49].) The C-exciton is misinterpreted as the B-exciton and the exciton-LO phonon complex transitions are misinterpreted as the C-exciton.

The function 82(0)) is again given by the integral formula (38) where j is here an unoccupied state above the Fermi level and i is an occupied state below it. In practice, many-body effects again intrude and exciton structure indeed plays a prominent part in 82(a)) for many systems, especially those with large band gaps. All of this will, for present purposes, be swept under the proverbial rug. 

Higgins D A, Reid P J and Barbara P F 1996 Structure and exciton dynamics in J-aggregates studied by polarization-dependent near-field scanning optical microscopy J. Chem. Phys. 100 1174-80... 

Leung K, Pokrant S and Whaley K B 1998 Exciton fine structure in CdSe nanoclusters Phys. Rev. B 57 12 291... 

The NDCPA seems to be a very reasonable way to treat the properties of both electrons and excitons interacting with phonons with dispersion. In principal, the NDCPA can be applied to a system of the Hamiltonian with the electron(exciton)-phonon coupling terms of arbitrary structure. The NDCPA results in an algorithm which can be effectively treated numerically (for example, iteratively). The application of the NDCPA is not restricted to the... 

In a regime of strong interaction between the chains no optical coupling between the ground slate and the lowest excited state occurs. The absence of coupling, however, has a different origin. Indeed, below 7 A, the LCAO coefficients start to delocalize over the two chains and the wavefunclions become entirely symmetric below 5 A due to an efficient exchange of electrons between the chains. This delocalization of the wavcfunclion is not taken into account in the molecular exciton model, which therefore becomes unreliable at short chain separations. Analysis of the one-electron structure of the complexes indicates that the... 

In this article we have reviewed the results of a joint spectroscopic and morphological investigation of -sexilhienyl (T(J. The lowest singlet electronic level, which is assigned to I B , splits in the single crystal into four crystalline levels. The structure of the exciton band is investigated by the combined absorption and... 

We assume that standard Coulomb-correlated models for luminescent polymers [11] properly described the intrachain electronic structure of m-LPPP. In this case intrachain photoexcitation generate singlet excitons with odd parity wavefunctions (Bu), which are responsible for the spontaneous and stimulated emission. Since the pump energy in our experiments is about 0.5 eV larger than the optical ran... 

It has been demonstrated that the whole photoexcitation dynamics in m-LPPP can be described considering the role of ASE in the population depletion process [33], Due to the collective stimulated emission associated with the propagation of spontaneous PL through the excited material, the exciton population decays faster than the natural lifetime, while the electronic structure of the photoexcited material remains unchanged. Based on the observation that time-integrated PL indicates the presence of ASE while SE decay corresponds to population dynamics, a numerical simulation was used to obtain a correlation of SE and PL at different excitation densities and to support the ASE model [33]. The excited state population N(R.i) at position R and time / within the photoexcited material is worked out based on the following equation ... 

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