The density of F in C4F8/Ar plasma could be reduced by 34% when the Si top plate was bombarded by energetic ions in a parallel-plate-type 500 MHz electron-cyclotron-resonance plasma reactor, but that in CF4/Ar plasma was not reduced. We measured the densities of CFx (x = 1 - 3), Si, and F in both plasmas as a function of ion-bombardment energy and found that F was generated from CF2 in C4F8/Ar plasma but not in CF4/Ar plasma, and that the CF2 density decreased ts a similar extent with increasing ion-bombardment energy in both plasmas. We conclude that the reduction of the F density in C4F8/Ar plasma was caused by the decrease in CF2 density and not by a direct reaction of F with Si when the Si plate was irradiated by energetic ions.