A calibration method outlined previously As noted above, ethylene TPD studies on bulk Pt surfaces found that intact ethylene desorption dominated at high coverages, but for coverages below ~0.5 ML, decomposition leading to H2 desorption dominated, with the change in branching attributed to decomposition requiring empty sites to bind the decomposition products. For our clusters, the branching between intact dissociation and decomposition generating D2 also appears to follow this pattern, i.e., the initial desorption from C2D4-saturated clusters was entirely of intact C2D4, with D2 observed only at higher temperatures after significant C2D4 desorption had occurred.