Exciton , band, definition

The lowest excited state of the system is by definition the exciton state at the bottom of the exciton band. If this state is not degenerate, when the sample contains N atoms, there will be N states in the exciton band. In the case when this lowest exciton is p-times degenerate, there will be pN states in the exciton band. In general, the degeneracy of the exciton states will be removed when the exciton wave vector is not zero. Similar arguments apply to each of the exciton states so that the total number of exciton states is equal to the number of excited states that exist in a system of N non-interacting atoms. 

Results of photoemission studies of polyethylene have shown definite evidence for wide energy bands among deep valence orbitals ( ), but the nature of the fundamental absorption edge has not been resolved. Band structure calculations predict direct interband excitations to occur above 12.6 eV (.8) whereas the absorption threshold is at 7.2 eV and a strong peak in e occurs at 9.0 eV. The momentum dependence of the absorption threshold indicates that the threshold is of excitonic origin, i.e. the excitation is localized by the strong electron-hole or configuration inter-... 

Figure 10.2 illustrates the electroabsorption spectrum of phenyl-substituted traras-polyacetylene thin film (Liess et al. 1997). The feature at 2.0 eV is the red-shifted l Bu exciton. The feature at 2.5 eV is attributed to a dipole-forbidden state, namely the m Ag state. Unlike polydiacetylene crystals, disordered trans-polyacetylene thin film does not exhibit Pranz-Keldysh oscillations (described in Chapter 8) and therefore a definite assignment of a conduction band edge cannot made. However, because disordered polydiacetylene also does not exhibit Pranz-Keldysh oscillations, but a smeared-out feature similar to the one exhibited at 2.5 eV in Fig. 10.2 it is sometimes assumed that this feature does mark the band edge. Another interpretation is that this feature represents the n = 2 Mott-Hubbard exciton, described in Chapter 6, with the particle-hole continuum lying close in energy (possibly at 2.7 eV, which is three times the THG feature at... 

The decomposition of the excitonically mixed optical absorption spectrum of Rps. viridis in component monomer bands is critically discussed. It is shown that the presence of crossproducts of transition moments in the expression for the dipole strength of the mixed band precludes a decomposition in band areas. In contrast, a vectorial presentation of the component dipole moments is shown to offer insight in the exciton make-up of the mixed band. It is proposed that the vector direct product of a component transition moment and that of the mixed band is an acceptable measure of the "contribution of the component band to the mixed band. With this definition the so-called exciton component P(+) is shown to contribute for 45% to the 848 nm band of Rps. viridis. 

---