Unimolecular decomposition in the sputtering of metal clusters

The stability of metal clusters ${\mathit{M}}_{\mathit{n}}$ sputtered from single-crystal surfaces by keV and sub-keV ${\mathrm{Ar}}^{+}$ ions was investigated by a molecular-dynamics (MD) simulation using a many-body embedded-atom potential. It is shown that the clusters identified within the sputtered flux immediately after the ion impact contain an average internal energy of roughly 1 eV per constituent atom. Consequently, most of the sputtered trimers and tetramers and virtually all ejected clusters with n\ensuremath{\ge}5 are found to be unstable and therefore decompose into stable fragments on a time scale of several tens of picoseconds after the ejection. The contribution of fragmentation to the yields, translational and internal energy distributions of stable and, hence, experimentally detectable dimers, trimers, and tetramers were calculated. It is found that this contribution increases strongly with increasing primary-ion energy ${\mathit{E}}_{\mathit{B}}$ and becomes comparable to the contribution of ``intrinsic'' stable molecules (i.e., such molecules that are identified immediately after the sputtering process) at ${\mathit{E}}_{\mathit{B}}$=5 keV. It is shown that the experimental data available on ${\mathrm{Ag}}_{\mathit{n}}$ multimers sputtered from polycrystalline silver can be reproduced very well by the MD simulation if fragmentation processes are included in predicting the yields and distributions.

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