THERMAL FORMATION OF CLUSTERS K-2(-4(+) AT VERY-LOW SURFACE DENSITIES() AND K)

The mechanisms of cluster formation at low densities are of great inte rest. In the present experiments with a weak molecular beam of potassi um atoms directed towards a surface at a temperature of 1100 K, there is no possibility that the cluster formation can be driven by the weak interaction between ground-state atoms. The cluster ions K-2(+) and K -4(+) are observed by time-of-flight measurements with signal fluxes s imilar to K+, after pulsing from the sample surface at 10 V. The emiss ion of K atoms is not collision free as shown previously, since Rydber g species K are formed in the desorption process. The very long-range interaction between the Rydberg species provides the attraction force giving the efficient cluster formation during the present high temper ature, low density conditions. (C) 1998 Elsevier Science B.V.