The coupling of molecules to surfaces leads to changes of their reactivity which is the basis of catalysis and many other surface effects. Such modifications do not only take place in the ground states, to become effective in thermally induced reactions. They also operate on electronically excited states, and lead to drastical changes of photochemistry for adsorbates, as compared to similar isolated molecules. These modifications have been investigated for more than 30 years, and a rather consistent picture has emerged. A survey will be given which will emphasize the following aspects:

-         On metal and semiconductor surfaces, the quenching of electronically excited states of adsorbates is extremely fast. Direct spectroscopic measurements yield charge transfer times of some tenths to some femtoseconds for simple excitations. Total quench times are longer, but still very fast.

-         These quench rates are strongly dependent on many parameters, such as type of electronic excitation, bond strength, coverage, lateral ordering, defects, and others. As a general rule, any parameter increasing the tendency to localize the primary excitation (complexity of excitation, decoupling from neighbors, etc.) will make an excitation photochemically more active, while any counteracting, delocalizing parameter (lateral periodicity leading to band formation; strength of adsorbate-surface interaction, etc.) will increase the quench rate.

-         As a result, actual photochemistry at surfaces can be very selective and might even be used to select the bonds to be broken. This can be nicely shown with experiments using primary core excitations which are intrisically atomically localized.

-         At low excitation energies (excited state below the vacuum level), “hot” electrons can be created in the substrate and flow onto the adsorbate. Again backtransfer is fast.

-         On the other hand, insulators can act as collectors of electronic excitations to rather large thicknesses, which can be channelled back to the surface to become effective in adsorbates.