Tracking thermally driven molecular reaction and fragmentation by fast photoemission: C60 on Si(111)

We followed in real time the thermal reaction of fullerene molecules with the Si(111) surface by means of fast photoemission spectroscopy, The formation of SiC via C-60 fragmentation on Si(111) is used as a key example of the capability of fast photoemission, associated with a fine temperature control, in determining the nature of thermally induced chemical reactions. By monitoring every 13 s the evolution of the C1s core level photoemission spectrum, as a function of temperature and as a function of time at fixed temperature, we were able to identify several steps in the interaction of C60 with Si(111). A model describing the thermal evolution of this interaction, in agreement with these and other experimental observations, considers the initial chemisorption of C-60 in mainly metastable configurations, the evolution toward more stable configurations, allowed by molecular rotations and breaking of Si-Si bonds, the cage deformation to further increase the number of C-Si bonds, the final cage fragmentation and SiC formation only above 1050 +/- 10 K.