Spectroscopic evidences of high temperatures and pressures during the cluster ion beam interaction with solid surfaces

The use of nanoparticles, consisting in clusters of thousands atoms or molecules of one or more chemical species, recently spread in industry and research. The mass of these clusters is orders of magnitude bigger than the mass of the corresponding monomers so, once ionized and accelerated, they act on the surface of the materials in a very peculiar way. While the monomer interacts with a limited number of atoms of the surface, penetrating into the material and creating defects in its structure, the nanoparticle interacts with a large number of atoms of the surface of the target, causing a disturbance on an area much greater than its cross-section and promoting collective effects. According to molecular dynamics simulations, temperatures of tens of thousands of degrees and pressures of tens of GPa should appear in the area of interaction. Moreover microscopic analysis shows the creation of craters and a removal action of the surface roughness, revealing a mechanical effect of the nanoparticles on the target. Recently a very low intensity luminescence, emitted during the ion cluster beam impact on surfaces has been measured. This luminescence could constitute the basis for a non-invasive technique that allows analyzing the interaction between nanoparticles and surfaces in real time. In this paper a spectroscopic study about the impact of a beam of single-ionized Ar clusters, composed, on average, of 4000 Ar atoms and accelerated by a voltage of 9 kV and 6 kV, on some solid surfaces, performed by a very sensitive ad hoc equipment, able to reveal few photons/mm(2)/s, is reported. The analysis of cluster ion beam induced luminescence allowed us to calculate that the temperature achieved by faster Ar clusters during the impact is of the order of 20,000 K, confirming the forecast of the molecular dynamics calculations. Nevertheless experiments show that there is a strong dependence of luminescence from the target and that a part of the total luminescence cannot be ascribed to the high temperature of Ar clusters. These findings could be connected to luminescence of the target induced by the action of the clusters on its surface. This work demonstrates the possibility to use the cluster ion beam induced luminescence to explore the very peculiar environment created during the cluster-target collision, characterized by unusual values of pressure and temperature, difficult to obtain by other means.