Exciton definition

In Science, every concept, question, conclusion, experimental result, method, theory or relationship is always open to reexamination. Molecules do exist Nevertheless, there are serious questions about precise definition. Some of these questions lie at the foundations of modem physics, and some involve states of aggregation or extreme conditions such as intense radiation fields or the region of the continuum. There are some molecular properties that are definable only within limits, for example, the geometrical stmcture of non-rigid molecules, properties consistent with the uncertainty principle, or those limited by the negleet of quantum-field, relativistic or other effects. And there are properties which depend specifically on a state of aggregation, such as superconductivity, ferroelectric (and anti), ferromagnetic (and anti), superfluidity, excitons. polarons, etc. Thus, any molecular definition may need to be extended in a more complex situation. 

Figure 13. Definition of exciton chirality. Summation of the two Cotton effects (broken lines) separated by Davydov splitting A). gives the curves shown in solid lines. Adapted from N. Harada, K. Nakanishi. Circular Dichroic Spectroscopy - Exciton Coupling in Organic Stereochemistry, University Science Books. Mill Valley, California. 1983...

Charge transfer states (CT) are often found in molecular systems side by side with excitonic states. CT states describe polar nonconducting states bound by coulomb interaction of the electron-hole pairs. CT states may be ionized with localization of the charges on definite molecules. 

The interest in semiconductor QD s as NLO materials has resulted from the recent theoretical predictions of strong optical nonlinearities for materials having three dimensional quantum confinement (QC) of electrons (e) and holes (h) (2,29,20). QC whether in one, two or three dimensions increases the stability of the exciton compared to the bulk semiconductor and as a result, the exciton resonances remain well resolved at room temperature. The physics framework in which the optical nonlinearities of QD s are couched involves the third order term of the electrical susceptibility (called X )) for semiconductor nanocrystallites (these particles will be referred to as nanocrystallites because of the perfect uniformity in size and shape that distinguishes them from other clusters where these characteriestics may vary, but these crystallites are definitely of molecular size and character and a cluster description is the most appropriate) exhibiting QC in all three dimensions. (Second order nonlinearites are not considered here since they are generally small in the systems under consideration.)... 

In the crude Born-Oppenheimer approximations, the oscillator strength of the 0-n vibronic transition is proportional to (FJ)2. Furthermore, the Franck-Condon factor is analytically calculated in the harmonic approximation. From the hamiltonian (2.15), it is clear that the exciton coupling to the field of vibrations finds its origin in the fact that we use the same vibration operators in the ground and the excited electronic states. By a new definition of the operators, it becomes possible to eliminate the terms B B(b + b ), BfB(b + hf)2. For that, we apply to the operators the following canonical transformation ... 

In this section we analyze the surface investigation of molecular crystals by the technique of UV spectroscopy, in the linear-response limit of Section I, which allows a selective and sharp definition of the surface excited states as 2D excitons confined in the first monolayer of intrinsic surfaces (surface and subsurfaces) of a molecular crystal of layered structure. The (001) face of the anthracene crystal is the typical sample investigated in this chapter. 

The pre-exponential factor depends on the definition of y and can amount 9/2 if in the triplet—triplet annihilation pathway for decay of triplet excitons a factor of 1/2 is introduced [192],... 

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-... 

The qualitative definition of exciton chirality is very simple (1) if two transition moments constitute a clockwise screw sense, CD shows positive first and negative second Cotton effects. On the other hand, (2) if they describe a counterclockwise screw sense, negative first and positive second Cotton effects are observed. In most cases, intense exciton-coupled CD Cotton effects are observed at the long-axis-polarized transition, and therefore the above results are rephrased as follows ... 

From the quantitative definition of exciton chirality, some important features are derived. 

The /(-value of exciton split CD is the function of the dihedral angle between two transition moments. In the case of vicinal glycol dibenzoates, the sign of the exciton split Cotton effects remains unchanged from 0 to 180°. Therefore, the qualitative definition shown in Table 1 is applicable to a dibenzoate with the dihedral angle of more than 90°. The maximum /(-value is around 70° 8... 

As demonstrated in Section 9.04.4.2, the transition of polyacene chromophores is ideally suitable for observing exciton-coupled CD. The UV data of some polyacenes with D2h-symmetry are shown in Figure 14. In the polyacene systems, there is no ambiguity for determining the long and short axes, and therefore the CD exciton chirality method offers more reliable and definite conclusions of AC. 

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