An in-depth study of the first steps of electron beam assisted growth of Mo from molybdenum hexacarbonyl on Cu(111) has been carried out exploiting the complementarity of X-ray photoemission and X-ray absorption spectroscopies. Frank van der Merwe (2D) growth mode has been observed for the completion of the two first monolayers of adsorbed molecules through a simple physisorption process. Irradiation of the Mo(CO) 6 deposit by 1keV electron beam induces a modification of molybdenum coordination, the average number of C-neighbors decreasing from 6 to 3. Decomposed molecules remain on the surface after annealing at 520K and organize themselves, the molybdenum atoms moving in Cu(111) surface fcc hollow sites. After annealing at 670K, metallic molybdenum growth begins, if the total amount of adsorbed Mo atoms exceeds 1.2 monolayers.