Structure and vibrations of chemically produced Au55 clusters

We performed optical absorption and temperature-dependent extended x-ray-absorption fine-structure (EXAFS) measurements on 55-atom gold clusters made by the Schmid process. The results are consistent with a fcc cuboctahedron structure, but with first-neighbor distances contracted by 0.075 \AA{}. The electronic properties, as detected by optical and x-ray absorption, are as expected for a small chunk of gold. The EXAFS Debye-Waller factor shows that the clusters have 40% less thermal vibration than does the bulk. We present a theory for this effect, based on the change of bulk modulus with volume, the measured contraction of the clusters, and the softening effect of the surface modes.