VUV PHOTOIONIZATION OF SPUTTERED NEUTRAL SILVER CLUSTERS

We report on a study of neutral Ag atoms and Ag(n) clusters sputtered from a polycrystalline silver sample by keV rare gas ion bombardment. The ejected neutral species were photoionized by a pulsed laser and detected by time-of-flight mass spectrometry. In contrast to previous studies of sputtered neutral silver clusters, the ionization was performed by a VUV laser (lambda = 157 nm) which, due to its high photon energy of 7.9 eV, permits nonresonant single photon ionization (SPI) of all investigated species. It is shown that the corresponding SPI ionization cross sections do not vary dramatically between silver atoms and the different clusters. As a consequence, fragmentation influences encountered in previous studies with longer wavelength lasers are practically eliminated from the determination of yields and kinetic energy distributions of the sputtered clusters. The resulting relative cluster sputtering yields (normalized to the yield of silver atoms) exhibit a power law dependence on the cluster size n according to n-delta with exponents delta ranging from 4.3 to 7.4 depending on the nature and the bombarding energy of the primary ions. The kinetic energy distributions of the sputtered neutral atoms and clusters are evaluated up to clusters containing seven atoms. It is found that the asymptotic decay of the energy distribution towards high emission energies becomes steeper from Ag to Ag3 and remains practically constant for larger clusters.