Intermediate exciton theory

Excited States and Correlation Effects in Polymers Intermediate Exciton Theory of Excited States.—It is well known that the HF picture does not permit a reasonable calculation of excited states and optical... 

The intermediate exciton theory of Takeuti [which applied some empirical parameters and assumed parabolic curves e(fc) effective mass approximation] was developed in an ab initio form without any empirical parameters, using not HF but quasi-particle (QP) one-electron energies (see Section 5.3) by Suhai. ... 

APPLICATION OF INTERMEDIATE EXCITON THEORY TO UV SPECTRA OF DIFFERENT POLYMERS... 

As the next example we take the (CH ), polyethylene (PE) chain. In a subsequent calculation Suhai also applied the intermediate exciton theory with QP one-particle energies to this chain. Using again a 6-31G basis he calculated the HF and subsequently the QP band structures of this system. In the latter case he obtained for the fundamental gap a value of 10.3 eV, while the experimental value is 8.8 eV.< The reasons for this 1.5 eV discrepancy have already been discussed (see Sections 5.3 and 8.2.1.1). 

In a subsequent calculation the intermediate exciton theory with correlation was applied to a cytosine stack, the superimposed cytosine molecules possessing the same relative geometry as in the in vivo stable DNA B. In this calculation, only a double-C basis was applied (no polarization functions) owing to the rather large size of the unit cell (a cytosine molecule). ... 

No correlation calculations have been performed until now on biopolymers, but such computations have been successfully executed in the cases of polymers with small unit cells (transpolyacetylene and polydiacetylene see below). The same holds for exciton spectra which have been successfully computed applying intermediate (charge transfer) exciton theory /5/ for the above mentioned two chains /6/. One should mention, however, that only the inclusion of the major part of correlation resulted in results in reasonable agreement with experiment. There is an early calculation on transport properties of periodic DNA models using simple tight binding (Huckel) band structures /7/. In the... 

In Fig. 1 the absorption spectra for a number of values of excitonic bandwidth B are depicted. The phonon energy Uq is chosen as energy unit there. The presented pictures correspond to three cases of relation between values of phonon and excitonic bandwidths - B < ujq, B = u)o, B > ujq- The first picture [B = 0.3) corresponds to the antiadiabatic limit B -C ljq), which can be handled with the small polaron theories [3]. The last picture(B = 10) represents the adiabatic limit (B wo), that fitted for the use of variation approaches [2]. The intermediate cases B=0.8 and B=1 can t be treated with these techniques. The overall behavior of spectra seems to be reasonable and... 

In the intermediate domain of values for the parameters, an exact solution requires the specific inspection of each configuration of the system. It is obvious that such an exact theoretical analysis is impossible, and that it is necessary to dispose of credible procedures for numerical simulation as probes to test the validity of the various inevitable approximations. We summarize, in Section IV.B.l below, the mean-field theories currently used for random binary alloys, and we establish the formalism for them in order to discuss better approximations to the experimental observations. In Section IV.B.2, we apply these theories to the physical systems of our interest 2D excitons in layered crystals, with examples of triplet excitons in the well-known binary system of an isotopically mixed crystal of naphthalene, currently denoted as Nds-Nha. After discussing the drawbacks of treating short-range coulombic excitons in the mean-field scheme at all concentrations (in contrast with the retarded interactions discussed in Section IV.A, which are perfectly adapted to the mean-field treatment), we propose a theory for treating all concentrations, in the scheme of the molecular CPA (MCPA) method using a cell... 

Recently, many researchers have studied fine particles or microcrystals to clarify the intermediate state between bulk crystals and isolated atoms and molecules (12-16). From these studies in the field of nonlinear optics, Hanamura predicted excitonic and surface-state enhancements of third-order optical nonlinearity in microcrystals because of quantum confinement effects, which is one of several size effects theoretically established by Ekimov et al. and Brus (17-19). Following these theories, enhancements of have been reported in semiconductor-microcrystallite-doped glasses and polymers (20,21). Organic microcrystals, however, have attracted very little attention so far (22,23) owing to the difficulties of preparing them. 

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