For as-deposited Pt4/alumina (Fig. ), the Pt ISS intensity initially increased slightly, then slowly declined at long exposures as Pt atoms were slowly sputtered from the surface. The slight increase was attributed to exposure of additional Pt due to sputter removal of a small coverage of adventitious adsorbates, e.g. H2 or CO, which have partial pressures ≤ ~5 x 10 -11 Torr in the UHV system. If an as-deposited Pt4/alumina sample is briefly heated to 750 K to remove the adsorbates prior to ISS analysis (Fig. ), the Pt intensity is substantially higher than in the unheated sample, and simply decreases with exposure time as Pt is sputtered. Note that if the Pt4 clusters had sintered at 750K to form larger, multilayer clusters on the surface, this would have substantially decreased the Pt ISS intensity, thus such extensive sintering is ruled out by the result in Fig. , consistent with the conclusions from the TEM data. The initial Pt ISS intensity for Pt4/alumina after a single C2D4 TPD experiment (Fig. ) was ~18% smaller than that for the 750 K heated Pt4/alumina sample in Fig. , suggesting some attenuation of Pt signal by deposited carbon, and the attenuation increased after 6 TPDs (Fig. ), as expected.