Heterogeneous Electron Transfer

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Electron transfer (ET) represents a ubiquitous phenomenon in physics, chemistry, and biology which has attracted considerable interest over the last six decades. Heterogeneous electron transfer (HET) is a general phenomenon in catalytic reactions, in electrochemistry, and in photo electrochemistry. Since more than two decades there is a continuing effort towards developing the field of molecular electronics, where HET will play a key role. HET has also been studied in nano-hybrid systems mostly with the primary concern of developing a practical application. Since HET reveals unique properties of the electron transfer process it can be considered also a research topic in its own rights.
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Ultrafast heterogeneous electron transfer (HET) between a molecule attached to a semiconductor surface and the conduction band of the semiconductor is discussed theoretically with emphasis on the perylene TiO_2 system. The used description accounts for the specialty of the molecule i.e. its particular electronic level scheme together with its vibrational degrees of freedom. The band continuum of the semiconductor is included and the approach is ready to describe different optical excitation and detection processes. Using a diabatic-state like separation of the whole system into molecular and semiconductor states femtosecond photoinduced dynamics are studied. Since the HET is ultrafast standard rate theories cannot be applied. Instead, the respective time-dependent Schrödinger equation governing the electron-vibrational wave function is solved. Based on this approach and using a time-dependent formulation the steady state linear absorption is calculated. Parameters of perylene attached to nano-structured TiO_2 via different bridge-anchor groups are adjusted by a comparison with measured spectra.
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A direct charge transfer excitation into the conduction band continuum is included into the description. This time-dependent formulation of the absorbance is confronted with a direct formulation in the frequency domain using the molecular Green's function. It is also explained how to observe the energetic distribution of the injected electron which carries signatures of the molecular vibrations in a two-photon photon emission spectrum.
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Literature