Photochemistry on metal nanoparticles.

The photochemistry of small molecules on well-defined metal surfaces has been the subject of intense research for more than three decades.1 This field is of interest because it rests on the superposition of two influences. On one hand, new reaction channels can become possible by electronic excitation, which are usually not accessible by thermal activation. On the other hand, compared to molecular photochemistry in the gas phase, interactions of molecules with solid substrates open unique pathways of photoexcitation and photoreaction not accessible in homogeneous reactions. This is primarily due to the fact that the bonding interactions with the substrate modify not only the ground state but also the electronically excited states of the adsorbates. In addition, the very rapid exchange of excitation energy between adsorbates and substrate, in particular on metal and semiconductor surfaces, can lead to fast quenching of excited adsorbate states by transfer of charge and/or energy from the adsorbates to the substrate. Furthermore, excitation of adsorbates by hot (excited) electrons produced by photoabsorption in the substrate plays an important role as it induces charge and energy transfer in the opposite direction, from the substrate to adsorbates. Well-defined metal surfaces covered by adsorbate layers under ultrahigh vacuum (UHV) conditions provide systems which are well characterized in all aspects, particularly regarding geometry and electronic structure. The fact that such layers usually consist of molecules which are naturally aligned on the surface2 makes surface photochemistry a powerful alternative to the stereodynamic control of chemical reactions by optically aligned molecular beams.3 Well-defined adsorbate systems thus provide unique playgrounds for surface photochemistry.