Fundamental insights into the ultrafast dynamics of energy transfer processes at surfaces are of central importance for a microscopic understandin of chemical reactions at solid surfaces, e.g. in heterogeneous catalysis. The detailed investigation of the rates and pathways of energy flow in the adsorbate--substrate system as well as the chemical dynamics has become possible utilizing intense femtosecond laser pulses. Surprisingly, the strong non--equilibrium situation that arises upon irradiation with these pulses results in a new reaction channel caused by hot electron excitation for a number of systems investigated. This electron-mediated reaction mechanism is exemplified here for the model reaction Had + Had -> H2,g on Ru(001) and discussed in the broader context of other simple reactions such as the formation of H2O, CO2 and the CO desorption on the same surface.