Emission of velocity correlated clusters up to Ag₂₁⁺ induced by keV C₆₀⁻ impacting a silver surface

Earlier measurements of the velocity correlated cluster emission (VCCE) effect in keV C₆₀^- impact with a silver target (Ag_<i>n</i>⁺ (<i>n</i> = 1-9), E. Armon, A. Bekkerman, Y. Cohen, J. Bernstein, B. Tsipinyuk, E. Kolodney, <i>Phys. Rev. Lett.</i>, 2014, <b>113</b>, 027604) are extended to much larger clusters up to Ag₂₁⁺. This is the largest range of cluster size demonstrated so far for the impact induced emission of velocity correlated clusters from any target. Measurements of the kinetic energy distributions (KEDs) of all emitted Ag_<i>n</i>⁺ (<i>n</i> = 1-21) cluster ions showed that the VCCE effect as exhibited by a linear increase of most probable energies of the KEDs as a function of cluster size, <i>E</i>_mp(<i>n</i>) is still valid up to Ag₂₁⁺. The extension of the cluster size range was achieved mainly by applying a higher extraction/acceleration voltage for the surface emitted cluster ions than done before for the smaller range (<i>U</i>_ext. = 100 V as compared with 25 V before). The preferential collection and detection of some off-normal lower energy cluster ions resulted in a rather moderate and nearly constant 1-2 eV decrease of <i>E</i>_mp(<i>n</i>) as compared with the earlier (<i>U</i>_ext. = 25 V) results. Namely, the slope of the linear <i>E</i>_mp(<i>n</i>) relation (representing the strength of the VCCE effect) is nearly unaffected by the extraction voltages. It shows that the distortion of the VCCE effect by the extraction field is rather small and specifically, that the effect is getting more pronounced with decrease of the extraction voltage. The observation reported here is important for extending the range of validity of the VCCE effect and for gaining a deeper understanding of the nature and size of the (hypothesized) superhot moving precursor serving as the source for all the emitted clusters.