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 de tected by time-of-flight mass spectrometry. In contrast to previous st udies of sputtered neutral silver clusters, the ionization was perform ed by a VUV laser (lambda = 157 nm) which, due to its high photon ener gy of 7.9 eV, permits nonresonant single photon ionization (SPI) of al l investigated species. It is shown that the corresponding SPI ionizat ion cross sections do not vary dramatically between silver atoms and t he different clusters. As a consequence, fragmentation influences enco untered in previous studies with longer wavelength lasers are practica lly eliminated from the determination of yields and kinetic energy dis tributions of the sputtered clusters. The resulting relative cluster s puttering yields (normalized to the yield of silver atoms) exhibit a p ower law dependence on the cluster size n according to n-delta with ex ponents delta ranging from 4.3 to 7.4 depending on the nature and the bombarding energy of the primary ions. The kinetic energy distribution s of the sputtered neutral atoms and clusters are evaluated up to clus ters 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.