Excitonic bands

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... 

For 42, the FL spectral profile with an FWHM of 30 nm is not the mirror image of the 340-nm absorption band, and the photoexcitation wavelength depends strongly on the FLA around the 340-nm exciton band, ranging from 0.00 at 300 nm to 0.36 at 350 nm. By contrast, for 43, the FL spectral profile with an FWHM of 15 nm is the mirror image of the 350-nm absorption band the photoexcitation wavelength, however, has a similar FLA dependence around the 350-nm exciton band, ranging from 0.00 at 300 nm to... 

Figure 1.3 Molecular structure of Sulforhodamine B and exciton band energy diagram for molecular dimers with oblique and coplanar inclined transition dipoles. (Reproduced from ref. 10, with permission.)...

The effect of intermolecular interactions can be readily observed when comparing the absorption spectrum of a molecule in solution to that in the solid state. In solution, where the molecules can be considered as isolated, the spectra are characterized by sharp lines corresponding to absorption bands. However, in the solid, intermolecular interactions cause the formation of exciton bands and splitting of the levels. This phenomenon is often referred to as Davydov splitting. This splitting is thus a measure of the strength of the interactions and for MOMs it can amount to 0.2-0.3 eV. 

Deb and Yoffe [134] examined the decomposition of thallous azide under the action of ultra-violet light in the wavelength region 3200-3800 A. Two exciton bands 3415 and 3348 A have been observed in thallous azide by low-temperature spectroscopy (Nikitine and Gross s method). The refractive index has been measured by the Brewster angle method, the electron energy levels have been estimated and the results of the photochemical decomposition have been related to the electron energy level and to measurement of photoconductivity [33],... 

Fig. 8. An overview of the zero order energy levels of the doubly isotopically tagged helix. The isotope shifts are Sa (13C=I80) and Sb (l3C=l80) while Aa and Ab are diagonal anharmonicities. The shaded areas represent the helix one- and two-exciton bands that become perturbed by the isotopes. The solid (bra evolution) and dashed (ket evolution) arrows represent one of the Liouville paths contributing to the echo.

Alternatively, another process called excitation can occur by which a valence band electron is excited to an energy level lower than the conduction band. The electron remains bound to the hole in the valence band. This neutral electron-hole pair is called an exciton, and it can move through the crystal also. Associated with the exciton is a band of energy levels called the exciton band (see Fig. 18.19). 

A theoretical study by the same author suggests that RDX forms charge transfer complexes upon crystn which are unique because their charge transfer exciton band is of lower energy than the singlet exciton band of their molecular crysts. Static reactivity indices were used to predict the likely primary dissociative products obtainable from each excited state of secondary nitramines theory predicts that the axial and equatorial nitramine groups of the polynitra-mines RDX, alpha- and beta-HMX, may possess quite novel selective decompn paths and hence give different primary dissociative products... 

Figure 19 Ordered packing of guests as a bilayer or as a monolayer due to strong inter-molecular interactions (shown is an idealized picture) can result in strong excitonic bands in the absorption spectra.

The laser beam at 267 nm populates a large number of excited states, each one connected with a particular conformation of the helix and vibrations of the involved chromophores (simulated by the spectral width). Most of these states are delocalized over a few bases. Then, intraband scattering takes place and emission arises from excited states located at the bottom of the exciton band these low-lying states have, in general, different polarization from the initially populated states and lead to a loss of anisotropy. Intraband scattering is obviously faster than 100 fs because, at that time, the anisotropy of the... 

Radiative decay of free excitons from the bottom of the lowest T(3/2), n= 1 excitonic band produces strong lines FE (Fig.la) in spectra of solid Xe,... 

Fig. 6.7. Simulated absorption spectra of mutant RCs of Rb. sphaeroides R26. The experimental data are adopted from Refs. [110,121], The calculated higher excitonic band of the special pair is also shown. Reprinted with permission from [110,121]. Copyright (1997, 2001) American Chemical Society.

The presented theoretical approach to large molecular systems such as bacterial photosynthetic RCs can provide microscopic details of ultrafast radiationless transition taking place faster than 100 fsec. In particular, this approach establishes a standard model for treating such ultrafast processes of RCs. It is possible to analyze and provide similar details for wild-type RCs or other mutant RCs for example, for wild-type RCs of Rb. sphaeroides the electronic coupling of radiationless transition from the B band to the higher excitonic band and that from the higher excitonic band to the lower one are found to be 105.5 and 123 cm-1. For R26.Phe-a mutant RCs, the former coupling is 105 cm-1 and the latter is 123.7 cm-1. 

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