SPUTTERED NEUTRAL SILVER CLUSTERS UP TO AG18

Neutral silver clusters Ag(n) with n less-than-or-equal-to 18 sputtered from a polycrystalline silver surface under bombardment with 5 keV Ar+ ions were detected by means of energy resolved time-of-flight mass spectrometry. Postionization of the ejected neutral particles was performed by an excimer laser operated with ArF (hv = 6.4 eV). The kinetic energy of the detected particles was determined from their flight time interval between the sputter pulse and the ionizing laser pulse. By saturating the ionization of the sputtered neutrals, relative cluster sputtering yields Y(Ag(n)) were determined as a function of the cluster size. For n less-than-or-equal-to 6, the value of Y(Ag(n)) is found to drop by approximately one order of magnitude if n is increased by one atom, whereas for cluster sizes larger than six atoms the decrease becomes significantly less pronounced and an odd-even intensity alternation similar to that observed for sputtered ion clusters becomes visible. The kinetic energy distributions of silver atoms, dimers, trimers, tetramers and pentamers determined from the time-of-flight method all look very similar and, in particular, show essentially the same high-energy dependence for sputtered atoms and molecules. In comparison, corresponding data taken by electron impact ionization with subsequent electrostatic energy analysis exhibit a significantly different asymptotic behaviour, the high energy slope of the energy spectra becoming steeper as the cluster size increases. From a detailed analysis of the measured signals, it is concluded that due to the concurrence of single- and multiphoton absorption processes the laser postionization results may be corrupted by photo-fragmentation.