Atomic and molecular clusters are subjects of intensive study: as models for investigation of fundamental physical aspects of the transition from atomic scale to bulk material, as building blocks for nanodevices and as controllable and versatile tools for modification of surfaces and shallow layers [1-3].

The interaction of energetic (up to 1000 eV/atom) size-selected Co_n⁺ (n = 30, 50 and 63) and Ar_n⁺ (n = 16 and 41) cluster ions with graphite is studied both experimentally and theoretically. Heating induced etching of the radiation damaged areas introduced by cluster impact provides a measure of the depth to which the collision cascades are developed. These data are found to be in good agreement with the molecular dynamics simulations. It is shown that the penetration depth of the cluster constituents can be linearly scaled with the projected momentum (the cluster momentum divided by surface impact area) [4] and for both cluster species a universal scaling law of stopping is developed. Possible effects of neutralisation of cluster charge and cluster fragmentation on the stopping power are under discussion for the argon clusters. For the cobalt clusters, the transition to pinning is observed with a decrease of the cluster impact energies to ca. 10 eV/atom. Pinned size-selected clusters can be utilized for catalysis, formation of sensors or biochips.

[1] P. Jena and A.W. Castleman, Proc. Nat. Acad. Sci. 103 (2006) 10560.

[2] K. Wegner, P. Piseri, H.V. Tafreshi, P. Milani, J. Phys. D: Appl. Phys. 39 (2006) R439.

[3] V.N. Popok, E.E.B. Campbell, Rev. Adv. Mater. Sci. 11 (2006) 19.

[4] V.N. Popok, S. Vučković, J. Samela, T.T. Järvi, K. Nordlund, E.E.B. Campbell, Phys. Rev. B 80 (2009) 205419.