IN-SITU MEASUREMENTS OF THE LATTICE-PARAMETER OF SUPPORTED PALLADIUM CLUSTERS

Palladium clusters have been epitaxially grown under ultra-high vacuum (UHV) on MgO(100) surfaces cleaved in situ. The lattice parameter was measured in situ by surface electron energy-loss fine-structure spect roscopy (SEELFS) above the palladium N23 edge. The size distribution o f the clusters was obtained from ex-situ transmission electron microsc opy. An expansion of the palladium lattice, increasing on decrease in the mean cluster size, has been observed. From the UHV in-situ measure ments the lattice dilatation is attributed to pseudomorphism. With the SEELFS technique the Debye-Waller factor was also determined. An incr ease in the mean square displacement of the atoms was observed when th e cluster size decreased; the latter was explained by the increase in the proportion of surface atoms and by the effect of the lattice dilat ation.