The electronic excitation of chemisorbed Cs atoms through the coherent photoinduced charge transfer from the Cu(111) surface turns on repulsive forces that initiate the dissociative wave packet motion. Due to unusually slow electronic relaxation, with the interferometric time-resolved two-photon photoemission technique it is possible to follow the characteristic change in the surface electronic structure due to the ensuing nuclear dynamics for similar to 200 fs after excitation. This direct observation of the incipient bond breaking process on a metal surface provides information on a series of complex electronic and nuclear events that previously could be gleaned only through analysis of the reaction products. The metal band structure, the polarizability of the Cs atom, and the mechanical properties of the surface strongly influence the electronic and nuclear relaxation, and therefore, the reaction efficiency. The importance of time-resolved methods in studying of surface reactions is underscored by significant deviations in the electronic and nuclear dynamics of Cs/Cu(111) from the standard models for surface photochemistry.