Introduction (C. Frischkorn): While femtochemistry in the gas and solution phase has led to enormous progress in the understanding and even control of chemical reactions over the past decades, acomparable level of sophistication in the analysis of surface chemical reactions has not been achieved. In part, this originates from the additional complexity of energy dissipation channels introduced by the presence of a solid interface interacting with the reactants. Reactive processes at surfaces are off undamental importance for technological applications such as heterogeneous catalysis. Here,metals are often investigated as model substrates since the interaction of the adsorbed reaction partners with the substrate may cause a favorable energy landscape, e.g. a reduced reaction barrier compared to the gas phase. Moreover, the ability to bring the reactants on a surface like on a template into close proximity provides additional control of the reaction dynamics. On the otherhand, coherent control schemes which exploit the phase of the exciting laser light field mostly fail in photoinduced surface processes due to ultrafast dephasing caused by coupling to the underlying substrate. Inaddition, femtochemistry at surfaces and interfaces is closely related to the break-down of the Born-Oppenheimer approximation,
a key concept in chemical reaction dynamics where by electrons are assumed to follow the nuclear motion instantaneously. Thus being of general importance, nonadiabatic coupling between nuclear motion and electronic degrees of freedom is one of the fundamental ingredients to the dynamics of ultrafast processes at surfaces and interfaces. Although non adiabatic phenomena are crucial also in various other surfaces cience fields , e.g.in the detection of chemicurrents, chemiluminescence or exoelectron emission  as well as in chemical reactions induced by scanning tunneling microscopy (STM) , ultrafast laser initiated surface chemistry has the mainadvantage of direct access to non adiabatic coupling affects in the time domain.
The present Chapter comprises two fields; (i) ultrafast chemical reactions on metal surfaces and (ii) charge transfer processes across molecule semiconductor interfaces. For both types of interfaces (with a metal surface being a gas-solid interface), electronic coupling andc harge transfer between the adsorbate and substrate play key roles. On metal surfaces, absorption of af emtosecond laser pulse results in a continuum of electron-hole paire xcitations in the substrate,which enables chargetransfer to energetically low-lying resonance levels of adsorbed molecules. On the other hand,the existence of a bandgap of several eV in semiconductor substrates allows for intramolecular excitations of the adsorbate followed by largne injection into the conduction band of the substrate.
In the following, this Chapter starts with a brief overview of surfacefemto-chemistry with the description of general concepts common to all femtochemical surface reactions. Subsequently in separate sections, two experimental examples are given; there combination of two adsorbed atoms which form a molecule and desorb into the gas phase and the diffusion of atomic adsorbates which represents one of the most fundamental motions involved in any chemical process where interatomic bonds are formed and cleaved. A profound theoretical account of the se kinds of ultrafast nonadiabatic processes will complete the femtosecond laser induced chemistry on metal surfaces. The second part of this Chapter concentrates on the ultrafast heterogeneous electron transfer in dye-sensitized metal oxide emiconductors. Starting with a presentation of relevant concepts, aqualitative description andd iscussion of basic phenomena follow. Particular systems of organic molecule-sensitized semiconductors, which undergo ultrafast charge transfer after femtosecond-laser excitation, will be discussed both experimentally and theoretically. Finally, abinitio time-domain simulations of the photoinduced electron transfer will complement this field of ultrafast dynamics at interfaces.
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