Exciton transitions

The reflectivity of LiF is illustrated in figure Al.3.26. The first large peak corresponds to an excitonic transition. 

Higgins D A and Barbara P F 1995 Excitonic transitions in J-aggregates probed by near-field scanning optical microscopy J. Chem. Phys. 99 3-7... 

Recent tlieoretical [35, 36 and 37] and experimental [38] research has revealed anomalous behaviour of tire dimer anisotropy under certain excitation conditions. If tire dimer is excited by broadband light tliat covers botli excitonic transitions, or by a relatively narrow band properly positioned between tire maxima of tire excitonic transitions, tire... 

PA at l. 48 eV appeal s instantaneously, shows spectral relaxation to the red, and decays on the same timescale of SE, as shown in Figure 8-9. We assign the observed PA to singlet Bu exciton transitions towards higher lying even parity (A ) states. We can speculate on the nature of this state within the proposed model. A possible candidate for the final slate is the inirachain biexciton. However, its energy level is located below the two-exciton stale by an amount equal to the bind-... 

Katsikas L, Eychmuller A, Giersig M, WeUer H (1990) Discrete excitonic transitions in quantum-sized CdS particles. Chem Phys Lett 172 201-204... 

The arrangement of the individual transition dipoles in an aggregrate or polymer determines the selection rule for exciton transitions. Several possible dipole arrangements are shown in Figure 6.6... 

In this case, the intensity ratio of the excitonic transitions becomes... 

Figure 4.12 The energy levels of an exciton. The arrows indicate the possible optical exciton transitions in respect to the energy gap, Eg. E), is the binding energy.

Colloidal CdS particles 2-7 nm in diameter exhibit a blue shift in their absorption and luminescence characteristics due to quantum confinement effects [45,46]. It is known that particle size has a pronounced effect on semiconductor spectral properties when their size becomes comparable with that of an exciton. This so called quantum size effect occurs when R < as (R = particle radius, ub = Bohr radius see Chapter 4, coinciding with a gradual change in the energy bands of a semiconductor into a set of discrete electronic levels. The observation of a discrete excitonic transition in the absorption and luminescence spectra of such particles, so called Q-particles, requires samples of very narrow size distribution and well-defined crystal structure [47,48]. Semiconductor nanocrystals, or... 

The photoconductivity and absorption spectra of the multilayer polydiacetylene are shown in Fig. 22 [150]. The continuous and dotted line relate to the blue and red polymer forms respectively. Interpretation is given in terms of a valence to conduction band transition which is buried under the vibronic sidebands of the dominant exciton transition. The associated absorption coefficient follows a law which indicates either an indirect transition or a direct transition between non-parabolic bands. The gap energies are 2.5 eV and 2.6 eV for the two different forms. The transition is three dimensional indicating finite valence and conduction band dispersion in the direction perpendicular to the polymer chain. 

The reaction can be followed by monitoring the characteristic absorption due to ZnO band-gap excitations, occurring at energies above 28,000 cm-1. The energy of the first excitonic transition depends on the nanocrystal size, and so provides a probe of nanocrystal growth. For a fixed size, the intensity of the transition provides a measure of the nanocrystal concentration (i.e., the yield of the chemical reaction), and this offers a measure of nucleation yields. 

The electron-phonon interactions also influence the linewidth of the interband and exciton transitions. This temperature-dependent homogeneous... 

Fig. 7.21. CL line scan at 9 K of the intensity of the bound exciton transitions 16 to Ig measured at the cross-section of a 2.2 pm thick ZnO film on a-plane sapphire from the sapphire substrate to the ZnO film surface. The inset shows the corresponding SEM image of the cross section. Measured by J. Lenzner...

In conclusion, on phonon-assisted absorption, the investigation of the phonon modes involved in the broadening of an excitonic transition is greatly... 

The absorption is observed at the poles of t, at the energy oj, shifted relative to coB (4.101) by the local field. If a>, is close to an excitonic transition of the host crystal, the absorption resonates strongly for to/ tuke in fact, the impurity transition is polarized according to the host excitonic transition in its vicinity.16 (In other words, the impurity borrows oscillator strength, and more or less spatial extension, from the host band.184... 

FIGURE 4 Temperature dependence of the recombination lifetimes of the excitonic transitions in Ill-nitride epilayers and MQWs. 

From a comparison of the optical absorption and excitation data for the oxides (Table V), it is clear that the energy decreases with increasing cation size along the series Mg to Ba. The bulk exciton transitions of these oxides also decrease in a similar manner (Table VI). It is possible to make a semi-quantitative calculation of the intrinsic surface energy states using the approach of Levine and Mark (151) where the ions in an ideal surface are considered to be equivalent to bulk ions except for their reduced Madelung... 

Comparison of these calculated exciton transitions with the experimental data in Table V shows that the main features of the results are reproduced. The energies for the <100) surface (5-coordinated ions) are only slightly shifted from the bulk, whereas those transitions corresponding to the higher index planes are much closer to the experimental data. On an atomic scale this means that ions whose coordination numbers are 4 and 3 are involved in the observed transitions, whereas 5-coordinated ions at the surface will absorb at higher energies closer to the bulk band edge. This theoretical treatment is approximate since it considers only an ideal surface and assumes that the electron affinity and ionization potential are constant for the different planes. In fact, the evidence already presented on electron transfer in Section VI,A indicates that the ionization potential varies with the coordination of the ion. 

Figure 2. Level schemes and selection rules for the ionized bound exciton transitions.

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