Heterogeneous formation of aluminum vapor clusters

Small Aln clusters (n &le 6) were found to be present in aluminum vapor effusing through graphite orifices of unit aspect ratio (diameters = 0.25, 0.5, 1.0, 1.5, and 2.0 mm). The aluminum dimer is the largest cluster observed in the saturated vapor above a Knudsen cell. In this case, the dimer is formed as the result of a homogeneous process. The pressure of the Al vapor above the orifices used in this study was found to be substaturated. In addition, the cluster concentration increased with increasing vapor density, maximized in the 1173-1223 K range and then decreased with further increasing temperature. These observations led us to postulate that the clusters formed on the orifice walls and desorbed. To test this postulate, a tungsten orifice (dia. = length = 1.5 mm) was used. Since the desorption energy for Al on tungsten is 5.2 eV, one would not expect desorption of a cluster to occur easily. No clusters were observed effusing from the tungsten orifice. Furthermore, experiments using a graphite orifice with a larger surface area (1.5 mm dia., 48 mm length) produced clusters with n $LESQ 10.