On the internal energy of sputtered clusters

We have used laser ionization and time-of-flight mass spectrometry to investigate the internal excitation of neutral clusters that were generated by sputtering from a solid indium surface under bombardment with 15 keV Xe + ions.More specifically, single photon ionization of the clusters is accomplished by a tunable, frequency doubled laser and the photoionization efficiency (PIE) curves are collected with the photon energy varied around the ionization energy in a range between 4.2-6 eV.The results are compared with the PIE curves of supposedly cold indium clusters which were produced by a supersonic nozzle expansion using a laser vaporization source and investigated under otherwise similar conditions.As a result, the sputtered clusters show a distinctive broadening and shift of the PIE curve in the threshold region, thus illustrating the influence of the sputtering process in increasing the internal energy of the ejected clusters.The experimental PIE curves are interpreted in terms of a simple model using the internal temperature and ionization energy of the cluster as fit parameters.Depending on the cluster size, temperatures between 3850 and about 1000 K are found for sputtered In n clusters.