Interfacial charge transfer excitations

Fig. 10.4 Experimental and simulated Raman excitation profile (top) and electron energy loss spectra (bottom) of a monolayer of PM DA adsorbed on Cu(lll). The Raman enhancement tracks the interfacial charge-transfer excitation for the symmetric surface carboxylate mode. (From Ref. (43), Fig. 5 courtesy of Surface Science, Elsevier.)...

Almost all photoinduced surface electron transfer processes in sensitizer-semiconductor hetero-structures are most naturally characterized as two-step processes with initial excitation of the adsorbate, followed by interfacial electron transfer into a band of an acceptor state, as shown schematically in Figure 3.7. There is, however, also a more unusual case in which the lET is caused by a direct interfacial charge transfer excitation which is described in the... 

Scheme 1 illustration of the interfacial charge-transfer processes in nanocrystalline dye sensitized solar cell. S, S, and S represents the sensitizer in the ground, oxidized, and excited states, respectively. Visible light absorption by the sensitizer (1) leads to an excited state, followed by electron injection (2) onto the conduction band of TiO2. The oxidized sensitizer (3) is reduced by the I /I redox couple (4). The injected electrons into the conduction band may react either with the oxidized redox couple (5) or with oxidized dye molecule (6). 

A complete treatment of interfacial charge transfer in colloidal semiconductor systems with band gap excitation should consider the following factors ... 

Charge injection is fast compared with nuclear relaxation of the excited state (k k,). In this case, interfacial charge transfer would take place from the prepared hot vibronic level (Eq. (34)) and the quantum yield for the primary injection process would be close to unity = 1). For both limiting cases, k[ kr and k[ kr, relation (30) would be relevant, provided electron transfer is nonadiabatic. 

Photocatalysis A catalytic reaction triggered or enhanced by illuminating the system with visible or ultraviolet irradiation. This reaction involves normally the electronic excitation of the catalyst via the absorption of photons and an interfacial charge transfer to an adsorbed species. Typically, the photocatalyst is not consumed in the reaction. 

In this chapter, we describe the excited-state and electron transfer properties of transition metal compounds anchored to nanocrystalline Ti02 (anatase) particles. Emphasis is given to interfacial charge transfer processes relevant to the conversion of light into other forms of energy. We discuss new advances that have enabled practical applications in electrical power generation and the production of useful fuels. The discussion is not meant to be comprehensive but details key advances that represent new opportunities for further research and application. 

In this chapter we describe advances in the femtosecond time-resolved multiphoton photoemission spectroscopy (TR-MPP) as a method for probing electronic structure and ultrafast interfacial charge transfer dynamics of adsorbate-covered solid surfaces. The focus is on surface science-based approaches that combine ultrafast optical pump probe excitation to induce nonlinear multi-photon photoemission (MPP) from clean or adsorbate covered single crystal surfaces. The photoemitted electrons transmit spectroscopic and dynamical information, which is captured by their energy analysis in real or reciprocal space. We examine how photoelectron spectroscopy and microscopy yield information on the unoccupied molecular structure, electron transfer and relaxation processes, light induced chemical and physical transformations and the evolution of coherent single particle and collective excitations at solid surfaces. 

Rapid e / h recombination, the reverse of equation 3, necessitates that D andM be pre-adsorbed prior to light excitation of the Ti02 photocatalyst. In the case of a hydrated and hydroxylated Ti02 anatase surface, hole trapping by interfacial electron transfer occurs via equation 6 to give surface-bound OH radicals (43,44). The necessity for pre-adsorbed D andM for efficient charge carrier trapping calls attention to the importance of adsorption—desorption equihbria in... 

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