Abstract
The yields of negatively charged carbon cluster ions ${\mathrm{C}}_{n}^{\ensuremath{-}}$ $(2<~n<~21)$ and ${\mathrm{C}}_{n}^{2\ensuremath{-}}$ $(n=8$ and 10) emitted from graphite irradiated by $14.5\mathrm{keV}$ ${\mathrm{Cs}}^{+}$ ions were monitored during the initial stages of Cs incorporation in the near-surface region of the specimen. The associated work-function variations \ensuremath{\Delta}\ensuremath{\Phi} were determined in situ from the shifts of the sputtered-ions' emission-energy spectra; the total change amounted to $\ensuremath{\Delta}\ensuremath{\Phi}\ensuremath{\sim}2.7\mathrm{eV}.$ The lowering of \ensuremath{\Phi} induces an exponential increase of the ionization probability ${P}^{\ensuremath{-}}$ of the sputtered cluster ions. For ${\mathrm{C}}_{n}^{\ensuremath{-}}$ with $n<~9,$ these variations of ${P}^{\ensuremath{-}}$ were monitored for several selected emission energies (from 0.5 to 15 eV), in order to investigate a possible dependence of ${P}^{\ensuremath{-}}$ on the ion's emission velocity. For ${\mathrm{C}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{C}}_{2}^{\mathrm{\ensuremath{-}}}$ ions, such an influence of the velocity on the ionization probability was observed, whereas for larger clusters no distinct dependence was found in the velocity range accessible $(<~7\ifmmode\times\else\texttimes\fi{}{10}^{5}\mathrm{c}\mathrm{m}/\mathrm{s}).$ The results indicate that a resonant electron-transfer process may effect the ionization of the sputtered cluster species.
Published Version
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