Electron photoexcitation

The DECP model successfully explained the observed initial phase of the fully symmetric phonons in a number of opaque crystals [24]. The absence of the Eg mode was attributed to an exclusive coupling between the electrons photoexcited near the r point and the fully symmetric phonons. A recent density functional theory (DFT) calculation [23] demonstrated this exclusive coupling as the potential energy surface (Fig. 2.4). The minimum of the potential surface of the excited state shifted significantly along the trigonal (z) axis,... 

Time resolved ultrafast energy transfer from electrons photoexcited in Pt(lll) to adsorbed CO during photodesorption... 

Figure 4. Solvent response following hydrated electron photoexcitation (Eq. l)for HjOandDjO...

By an extensive study by optical, Raman and photomodulation spectroscopy of samples of polyCPDA and polyDCHD we have been able to recognize that different electronic photoexcitations occur in these polymers. Long-lived metastable bipolaronic states are photogenerated in polyCPDA as inferred from the behaviour of the two PA bands exhibited by this polymer. On the contrary, the PA spectrum of polyDCHD... 

Figure 25.1 Simplified processes involved In the electronic photoexcitation and fluorescence of a molecular system. Reproduced with permission from Ref [5] 2001, John Wiley Sons, Inc.

Co -", Ni "", Cu -", and Zn ) [73], GR-Pd-CdS [76], and hierarchical CdS-ZnO-GR hybrids [108]. It is found that their photoactivities for reduction of nitro compounds to amino compounds with ammonium formate (HCOONH4) for hole scavenger in N2 atmosphere (Scheme 8.11) under visible light irradiation are all remarkably enhanced as compared to the blank semiconductors [73,100-102,106, 107]. It has been concluded that the selective reduction efficiency can be driven by appropriate introduction of GR into the matrix of pure semiconductor, which can boost the transfer and prolong the lifetime of the electrons photoexcited from the semiconductor due to the tighter connection between GR and the semiconductor, as well as the optimization of the atomic charge carrier transfer pathway across the interface between GR and the semiconductor. 

Hwang K C and Mauzerall D C 1992 Vectorial electron transfer from an interfacial photoexcited porphyrin to ground-state Cgg and C g and from ascorbate to triplet Cgg and C g in a lipid bilayer J. Am. Chem. Soc. 114 9705-6... 

Bawendi M G ef a/1990 Electronic structure and photoexcited carrier dynamics in nanometre size CdSe clusters Phys. Rev. Lett. 65 1623... 

Fig. 1. Photoexcitation modes iu a semiconductor having band gap energy, E, and impurity states, E. The photon energy must be sufficient to release an electron (° ) iato the conduction band (CB) or a hole (o) iato the valence band (VB) (a) an intrinsic detector (b) and (c) extrinsic donor and acceptor...

Another common loss process results from electron—hole recombination. In this process, the photoexcited electron in the LUMO falls back into the HOMO rather than transferring into the conduction band. This inefficiency can be mitigated by using supersensitizing molecules which donate an electron to the HOMO of the excited sensitizing dye, thereby precluding electron—hole recombination. In optimally sensitized commercial products, dyes... 

In perfect semiconductors, there are no mobile charges at low temperatures. Temperatures or photon energies high enough to excite electrons across the band gap, leaving mobile holes in the Fermi distribution, produce plasmas in semiconductors. Thermal or photoexcitation produces equal... 

Dicarbocyanine and trie arbo cyanine laser dyes such as stmcture (1) (n = 2 and n = 3, X = oxygen) and stmcture (34) (n = 3) are photoexcited in ethanol solution to produce relatively long-Hved photoisomers (lO " -10 s), and the absorption spectra are shifted to longer wavelength by several tens of nanometers (41,42). In polar media like ethanol, the excited state relaxation times for trie arbo cyanine (34) (n = 3) are independent of the anion, but in less polar solvent (dichloroethane) significant dependence on the anion occurs (43). The carbocyanine from stmcture (34) (n = 1) exists as a tight ion pair with borate anions, represented RB(CgH5 )g, in benzene solution photoexcitation of this dye—anion pair yields a new, transient species, presumably due to intra-ion pair electron transfer from the borate to yield the neutral dye radical (ie, the reduced state of the dye) (44). 

It has generally been concluded that the photoinitiation of polymerization by the transition metal carbonyls/ halide system may occur by three routes (1) electron transfer to an organic halide with rupture of C—Cl bond, (2) electron transfer to a strong-attracting monomer such as C2F4, probably with scission of-bond, and (3) halogen atom transfer from monomer molecule or solvent to a photoexcited metal carbonyl species. Of these, (1) is the most frequently encountered. 

Neutron activation analysis of a polymer suggests that when Py is used as the electron doner (D), the initiation proceeds through the Cl atom, but when D = DMSO, both Cr and DMSO residues are the primary radicals produced from the photoexcited ion-pair complex. The following reaction scheme is proposed ... 

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