Charge transfer excitation energies

For the 4 d and 5 d hexafluoro systems the lowest energy charge-transfer transitions are thus re -<-75 type excitations, except for the t g ions, RhFlowest energy bands are all re - 73. In Table 27 therefore we list the positions of the charge-transfer bands found for the hexafluoro systems of the Ad and 5 d series, together with their assignments. Data... 

The Ea for the dissolution of hematite by mercapto carboxylic acids in acid media in the presence of UV radiation was lower (64 5 kj mol ) than that for dissolution in the absence of radiation (94 8 kJ mol ) (Waite et al. 1986). The reaction in both cases was considered to involve formation of an intermediate organic-Fe surface complex which decomposed as a result of intramolecular electron transfer to release Fe". UV irradiation enhanced the decomposition of the surface complex either through excitation of the ligand field states associated with the free electrons on the S atoms, or through high energy charge transfer states. 

Figure 26. Schematic representation of the nonequilibrium free energy model. LE and CT denote locally excited and charge transfer states respectively. From Ref. 82 with permission, from J. Phys. Chem. 92, 6801 (1988). Copyright 1988, American Chemical Society.

This phenomenon arises from charge-transfer reactions between the semiconductor and excited dye molecules adsorbed on its surface. If the semiconductor band gap is large compared to the dye s excitation energy, electron transfer between the dye and the electrode may involve the highest normally filled level or the excited level of the dye, but usually not both. One of the dye levels will not be electroactive because it will match some energy in the electrode s band-gap region. 

The two processes illustrated here, direct excitation and charge transfer, are distinguishable because the radiative emission from the translationally thermal electronically excited target atom is not shifted in wavelength, whereas that produced by charge transfer exhibits a Doppler shift because of the high kinetic energy of the projectile. 

Electron-hole excitations can be produced in photon absorption or electron energy loss experiments one distinguishes d d excitations, and charge transfer excitations. 

Figure 1. JabJonski-type diagram of the lowest energy levels of electron donor-acceptor molecules formally linked by a single bond which show dual fluorescence phenomenon. D-A, (D A), (D+-A ), (D -A ) and (D-A) denote the ground state, the primary excited and charge-transfer (CT) singlet states, and CT and locally excited triplet states, respectively. The arrows correspond to the radiative (absorption, A, fluorescence, F, and phosphorescence, Ph) and the radiationless (internal conversion, IC, and intersystem crossing, ISC) processes.

Figure 12. Sequence of excitation and charge-transfer steps using a dye-molecule sensitizer. In the first step, the sensitizer (S) is excited by an incident photon of energy hv and an electron is transferred into the conduction band of the semiconductor particle the electron then can be transferred to reduce an organic acceptor molecule (A) adsorbed on the surface.

These reactions can be attributed to the excitation of charge-transfer states. High-energy UV irradiation (A < 200 nm) leads to high-quantum-yield photoreduction of Ru(III) amine complexes. Another photoredox-induced substitution is the result of exciting the intervalence charge-transfer band of the ion pair. ... 

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