The yields of negatively charged carbon C-n(-) (n less than or equal to 21) and silicon Si-n(-) (n less than or equal to 9) cluster ions sputtered by 14.5 keV Cs+ ions from graphite and silicon, respectively, were monitored during the initial stages of Cs incorporation in the near-surface regions of the samples. The associated work function (WF) variations DeltaPhi were determined in situ from the shifts of the sputtered ions' emission-energy spectra; the total change DeltaPhi amounted to similar to2.7 eV for graphite and to similar to2.3 eV for silicon. For C-n(-) anions the lowering of Phi induces an exponential increase of the ionization probability P-, whereas for Si-n(-) ions P-is also enhanced by the lowering of the WF, but no distinct exponential scaling of P- with DeltaPhi is observed. (C) 2002 Elsevier Science B.V. All rights reserved.