THE MASS-DISTRIBUTION OF SPUTTERED METAL-CLUSTERS .1. EXPERIMENT

The sputtering of silver clusters Ag(n) under 5 keV Ar+ ion bombardmen t was studied by time-of-flight mass spectrometry of secondary cluster ions Ag(n)+ and their neutral counterparts. The sputtered neutral clu sters Ag(n)0 were ionized by a combination of electron impact for n = 1,...,4 and single photon absorption from an ArF excimer laser beam fo r n = 3,...,18. By comparing ion and neutral cluster signals, the ion fraction of sputtered silver clusters was determined for clusters up t o n = 15. It is found that the preferred charge state changes as a fun ction of cluster size from predominantly neutral at n < 6 to predomina ntly ionic at n > 10. From the ion fraction data, partial cluster yiel ds were evaluated which are independent of the charge state of the det ected particles. It is seen that the yield Y(n) of sputtered silver cl usters follows a power law dependence according to Y(n) is-proportiona l-to n-6.5. From the resulting mass distribution of sputtered particle s, it is estimated that around 17% of the sputtered atoms leave the su rface in a bound state.