SPUTTERED NEUTRAL SILVER CLUSTERS UP TO AG18

Neutral silver clusters Ag(n) with n less-than-or-equal-to 18 sputtere d from a polycrystalline silver surface under bombardment with 5 keV A r+ ions were detected by means of energy resolved time-of-flight mass spectrometry. Postionization of the ejected neutral particles was perf ormed by an excimer laser operated with ArF (hv = 6.4 eV). The kinetic energy of the detected particles was determined from their flight tim e 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 cluste r 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 o ne atom, whereas for cluster sizes larger than six atoms the decrease becomes significantly less pronounced and an odd-even intensity altern ation similar to that observed for sputtered ion clusters becomes visi ble. The kinetic energy distributions of silver atoms, dimers, trimers , tetramers and pentamers determined from the time-of-flight method al l look very similar and, in particular, show essentially the same high -energy dependence for sputtered atoms and molecules. In comparison, c orresponding data taken by electron impact ionization with subsequent electrostatic energy analysis exhibit a significantly different asympt otic behaviour, the high energy slope of the energy spectra becoming s teeper as the cluster size increases. From a detailed analysis of the measured signals, it is concluded that due to the concurrence of singl e- and multiphoton absorption processes the laser postionization resul ts may be corrupted by photo-fragmentation.