Exdton band

In compounds with weak interionic exchange the magnetic exdton bands are quite flat and one observes essentially local exdtations between CEF states in... 

Emission spectra at these points are shown in Figure 8.2d. The band shapes were independent of the excitation intensity from 0.1 to 2.0 nJ pulse . The spectrum of the anthracene crystal with vibronic structures is ascribed to the fluorescence originating from the free exdton in the crystalline phase [1, 2], while the broad emission spectra of the pyrene microcrystal centered at 470 nm and that of the perylene microcrystal centered at 605 nm are, respectively, ascribed to the self-trapped exciton in the crystalline phase of pyrene and that of the a-type perylene crystal. These spectra clearly show that the femtosecond NIR pulse can produce excited singlet states in these microcrystals. 

On a somewhat larger scale, there has been considerable activity in the area of nanocrystals, quantum dots, and systems in the tens of nanometers scale. Interesting questions have arisen regarding electronic properties such as the semiconductor energy band gap dependence on nanocrystal size and the nature of the electronic states in these small systems. Application [31] of the approaches described here, with the appropriate boundary conditions [32] to assure that electron confinement effects are properly addressed, have been successful. Questions regarding excitations, such as exdtons and vibrational properties, are among the many that will require considerable scrutiny. It is likely that there will be important input from quantum chemistry as well as condensed matter physics. 

Fluorescence spectra of semiconductor particles originate from the recombination of charge carriers in either a trapped or exdtonic state [578, 579], The former manifests itself in the appearance of a broad and Stokes-shifted band. In contrast, the spectrum, due to exdtonic fluorescence, appears as a sharp band near the absorption onset and is considered to arise from the detrapping of the trapped electrons [579]. The highly environmentally dependent position of the semiconductor emission maxima has been related to semiconductor sizes and size quantization [579]. 

The present author (Mott 1949,1956,1961) first proposed that a crystalline array of one-electron atoms at the absolute zero of temperature should show a sharp transition from metallic to non-metallic behaviour as the distance between the atoms was varied. The method used, described in the Introduction, is now only of historical interest. Nearer to present ideas was the prediction (Knox 1963) that when a conduction and valence band in a semiconductor are caused to overlap by a change in composition or specific volume, a discontinuous change in the number of current carriers is to be expected a very small number of free electrons and holes is not possible, because they would form exdtons. 

Here, we briefly describe the laser ablation dynamics of phthalocyanine film where the primary species was confirmed, by femtosecond transient absorption spectroscopy [25], to be its electronically excited state. The exdton absorption band was replaced in 20 ps by a hot band of the ground electronic state. This rapid decay was ascribed to mutual interactions between densely formed excitons leading to sudden temperature elevation. The elevated temperature was estimated by comparing transient absorption spectra with the temperature difference ones. It is worth noting that quite normal dynamics of the excited states are detected, even under ablation conditions, which is the reason why we do not accept the plasma mechanism. Then we considered how to correlate these electronic processes of phthalocyanine films with their fragmentation. 

H. Bassler in Primary Photoexdtations in Conjugated Polymers Molecular Exdton versus Semiconductor Band Model,... 

Sariciftd NS (ed). (1997) Primary photoexcitations in conjugated polymers molecular exdton vs semiconductor band model. World Sdentific, Singapore. 

Under an applied voltage, holes from the anode and electrons from the cathode are injected into the bands of the active layer forming exdtons, which give rise to a light emission after their dissociation as explained previously. The color of the emitted light depends on the bandgap of the organic layer. 

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