Abstract
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 Inn clusters.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. If a solid surface is bombarded with energetic particles, material is released in a process that closely resembles a pool billiards game. The impinging projectile initiates a cascade of collisions which leads to the ejection ('sputtering') of surface atoms into the gas phase. Not all of these atoms leave the surface as single particles, a fraction of them are emitted as part of a molecule or cluster. In this paper, we investigate the formation and survival of such sputtered clusters during the relatively violent collisional processes that lead to their emission.
Main results. Theoretical models have predicted that sputtered clusters should be internally hot, i.e., contain a relatively large amount of vibrational and rotational excitation energy. The present paper describes an attempt to verify this prediction experimentally. This is not trivial for a cluster containing more than two atoms, since these molecules have many internal degrees of freedom which are all excited to a certain degree, thus making spectroscopic investigations difficult. Here, we use single photon ionization with tunable photon energy to study the photoionization threshold of these particles. By comparing measurements performed on sputtered clusters with those on 'cold' clusters generated in a supersonic nozzle beam, we find a reduced ionization energy which allows to determine the internal temperature of the sputtered clusters.
Wider implications. The sputtering process has wide technological applications both in thin film technology and surface analysis. The formation of clusters is important since it may, for instance, influence the properties of a thin film deposited by sputtering. Moreover, the emission of molecular species is extremely useful for mass spectrometric chemical surface analysis.