The post-6-TPD initial Pt ISS intensity for the Pt4/alumina sample (Fig. ) was ~44% attenuated, compared to the initial intensity of the 750 K heated Pt4/alumina sample. It is not surprising that there was attenuation, given the carbon deposition observed by TPD and XPS (~6 C/cluster from TPD, ~8 C/cluster from XPS). Indeed, the surprise is that the attenuation was not much larger. For example, 5-and 20-fold attenuations of the Pt ISS signals were found to result from adsorption of just a single layer of H or O atoms, respectively, on sub-nano Ptn/SiO2 23 . Thus, the much smaller Pt ISS attenuation indicates that the C atoms must be bound in sites where they have little effect on He + scattering from Pt -such as cluster periphery or buried in the cluster core. For the Pt4Ge/alumina sample after 6 TPDs (Fig. ), the initial Pt signal was ~31% attenuated, relative to the heated Pt4Ge/alumina (Fig. ), which can be compared to the amount of C deposition seen from TPD (~1.8/cluster) and XPS (~3/cluster). For both Pt4 and Pt4Ge samples, ISS indicates that a significant fraction of the Pt atoms remained accessible to He + scattering after 6 TPD runs.