Optical absorption photoinduced

Let us now return to MMCT effects in semiconductors. In this class of compounds MMCT may be followed by charge separation, i.e. the excited MMCT state may be stabilized. This is the case if the M species involved act as traps. A beautiful example is the color change of SrTiOj Fe,Mo upon irradiation [111]. In the dark, iron and molybdenum are present as Fe(III) and Mo(VI). The material is eolorless. After irradiation with 400 nm radiation Fe(IV) and Mo(V) are created. These ions have optical absorption in the visible. The Mo(VI) species plays the role of a deep electron trap. The thermal decay time of the color at room temperature is several minutes. Note that the MMCT transition Fe(III) + Mo(VI) -> Fe(IV) -I- Mo(V) belongs to the type which was treated above. In the semiconductor the iron and molybdenum species are far apart and the conduction band takes the role of electron transporter. A similar phenomenon has been reported for ZnS Eu, Cr [112]. There is a photoinduced charge separation Eu(II) -I- Cr(II) -> Eu(III) - - Cr(I) via the conduction band (see Fig. 18). 

The most unique and intriguing features of chalcogenide vitreous semiconductors are the photoinduced changes appearing as a nearly parallel shift of the optical absorption edge to lower energy (so-called photodarkening effect) after irradiation... 

Regardless of the choice of the sample thickness, the total amount of sample particles in the x-ray probe beam under optimized conditions is directly proportional to the x-ray spot size and inversely proportional to the x-ray absorption cross section, whose photoinduced (small) changes we want to measure [12]. Typical x-ray foci at synchrotrons are in the 0.1 - 0.3 mm range. For the examples treated below, this means that we have between 1014 and 1016 molecules in the probed volume. In order to achieve a reasonable photoinduced signal we should excite as many solute molecules as possible. Neglecting the optical absorption cross sections for photoexcitation for the moment, this requires on the order of 1015 laser photons per pulse, or ca. 0.25 mJ of pulse energy (e.g., at 800 nm). In other words, one should aim to... 

The optical absorption, fluorescence and photochemical properties of Ag, provide clear evidence for the occurrence of two distinct forms of Ag in each of the three rare gas matrices (4,8,9). This observation shows that the matrix environment can act to stabilize distinct isomeric forms of this species. It appears that a common form of Ag° is produced during matrix formation in Ar, Kr and Xe and a structurally different yet common form of Ag" is produced by photoinduced aggregation of Ag° atoms in Ar matrices and photoisomerization of Ag, in Kr and Xe matrices. The results available for Ag ,Ag° in rare gas matrices are summarized below ... 

Evidence has been found that the conformation of PATs can be influenced by external pressure. Thus Hess et. al. [103] found from optical absorption that the thermochromic transition of PHT is suppressed by pressure exceeding 1.4 GPa. Also photoluminescence yield and photoinduced absorption as well as electric conductivity are affected [103,104,105]. 

The ODMR technique we have reviewed in this chapter uses spin-depen-dent properties of semiconductors. Therefore, this technique can be extended to other optical phenomena than luminescence and photoinduced absorption such as treated here, e.g., electronic Raman scattering (Romes-tain et al, 1974 Geschwind, 1978), optical absorption (Mollenauer et al, 1969) etc. Since the pioneering work by Lepine (1972) on crystalline silicon. 

The doping- and photoinduced absorption spectra of polythiophene, substituted polythiophenes, and their composites have been reported [145-166]. Let us discuss the subgap absorptions due to charged excitations in unsubstituted polythiophene. The optical absorption spectrum of BF -doped polythiophene [42,46] and the photoinduced absorption spectrum of polythiophene [145] are shown in Figure 20. In the spectrum of BFJ -doped polythiophene (Fig. 20a), doping-induced bands are observed at 0.73 and 1.68 eV, which are located below the gap edge, 1.94 eV. The 0.73-... 

Fig. 20. (a) Optical absorption spectrum of a BF -doped polythiophene film (room temperature) and (b) photoinduced absorption spectrum of a polythiophene film (20 K). 

The wavelength dependence of the photoinduced diffusion coefficient was theoretically compared to the optical absorption spectrum of the trapped electron by Funabashi (1982). It was assumed that the absorption spectrum of the electron is due to electron-transfer transitions with a distribution of transfer distances. At constant light intensity, the photodiffusion coefficient exhibits a peak at shorter wavelengths than the absorption peak. 

The dynamics of the electron spin polarization of the quinone radical-anion generated from photoinduced electron transfer between duroqinone (DQ) and A/, A/ -dimethylaniline (DMA) as well as its p-halogen substituents were investigated by using transient optical absorption and FT EPR spectroscopyThe radical yield decreases as the atomic number of the halogen in the donor rises. The intrinsic enhancement factors of net absorptive polarization due to the... 

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