Photoexcitation charge-transfer state recombination

Demonstration of the photorelease has been done in particular with Sr + [46]. This process was monitored on several time scales providing evidence for (1) the delayed formation in 9 ps of the charge transfer state of the merocyanine chromophore following ultrafast photodisruption of the nitrogen - cation interaction, (2) the cation movement away from the excited chromophore into the bulk in 400 ps, (3) recombination of the complex in the ground in about 120 ns. These three steps are respectively illustrated in Fig. 7.17a, b, c (see caption for details). Similar transient absorption studies have been carried out on a PDS-crown-Ca + complex, where PDS is an aza-crown derivative of a substituted stilbene [47]. The spectrodynamics observed on the short time scale are very similar to those found in step (1) of the above description, with in particular a delayed rise of a stimulated emission band attributed to a solvent-separated cation-probe pair. Although the full scenario of the cation photoejection from the DCM-crown-Sr, is complex [46], the spectra shown in Fig. 7.17 demonstrate that at least part of the photoexcited complexes does eject the ion into the bulk. 

One interpretation presumes that the photocurrent onset in the absence of sulfide is determined by electron-hole recombination. The sulfide ions on the surface are then supposed to be bound to these surface recombination levels rendering them unavilable for recombination reactions. The charge transfer reactions could then proceed at lower voltages. In this case the corrosion suppression role of the sulfide ions would be to reduce the oxidized corrosion site before a cadmium ion could go into solution. A variation on this theme is to consider the corrosion site to be the recombination state, i.e., the site on the surface that normally leads to corrosion when oxidized by a photoexcited hole can be... 

Figure 4. HOMO/LUMO scheme for operation of a proposed molecular shift register [48]. a) The clock cycle is initiated by photoexcitation of the donor moiety, resulting in the electronic configuration shown. Decay pathways from this excited state are forward electron transfer within the same monomer unit (solid), back electron transfer to the adjacent monomer unit (dash-dot), and fluorescence (dot), b) Electronic configuration resulting from successive forward electron transfer steps. The charge-separated state [D -Ai-A2 ] can recombine charge within a single monomer unit (dot-dash) or with the adjacent monomer unit (solid).

S is the distance between the semiconductor surface and the reaction plane at OHP, and Np(x = 0) is the number of photoexcited minority carriers per unit volume in the surface region of the semiconductor which arrive from the interior of the semiconductor to this region. f E, hv) is the Fermi distribution of photoexcited minority carrier. This quantity, Np x = 0), depends on the intensity, energy, and absorption coefficient of incident light, diffusion length of electron in the semiconductor, and its band gap, etc. Furthermore, it depends on the charge transfer phenomena and the surface recombination rate at the interface. The surface recombination rate constant depends on the induced density of surface states due to adsorbed anions at the electrodesolution interface. The recombination rate constant can be expressed as... 

CdS and PbS nanocrystals are attached to a metal electrode in a sub-monolayer array [19, 20, 116-120]. Clearly, photoexcitation of the nanocrystals can lead to a long-lived state, which allows one of the charge carriers to be transferred from the nanocrystal before recombination occurs (see Sect. 2.1.1.2). 

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