Interaction of mass selected antimony clusters with HOPG

Abstract

We have investigated the interaction of positively charged antimony clusters Sb n + with HOPG as a function of cluster size (2 < n < 13) and cluster kinetic energy (<600 eV) by tandem time of flight mass spectrometry and scanning tunneling microscopy. The processes taking place when the cluster hits the surface can be classified into three main categories according to its kinetic energy. At very low kinetic energies (⩽13 eV) soft landing should be feasible, i.e. no fragmentation of the cluster is taking place as a result of the impact. With increasing kinetic energy up to about 150–180 eV we find two competing processes. One is the fragmentation of the cluster into the most stable products (unimolecular decay), the other is neutralization. The efficiency of the second process increases with increasing particle energy from about 85% to almost 100% at 150–180 eV kinetic energy. The third category, too, is characterized by two competing processes. At about 150–180 eV collision energy the interaction leads to an ultrafast heating of the projectile. The cluster shatters immediately forming very small fragments. These fragments are ionized in part directly at or near the surface. The second process, namely implantation, starts at about 110 eV for the smallest investigated cluster, and it shows a strong threshold behavior as a function of cluster size. During this process the cluster is presumably strongly deformed and the surface atoms are displaced from their original positions, leaving the impact area in a highly amorphous state.