The positive and negative ion energy distributions were measured at th
e grounded electrode of a square wave modulated RF discharge in O-2 an
d Ar at between 1.5 and 5 Pa gas pressure. The resulting ion energy di
stributions were strongly affected by the modulation frequency accordi
ng to the existence of different discharge periods during the modulati
on cycle. These discharge periods were directly represented in the ene
rgy distribution of the positive ions which is determined by the plasm
a sheath voltage in front of the grounded electrode. Negative ions gen
erated in the plasma sheath at the RF electrode were accelerated towar
ds the bulk plasma and detected with a high energy at the opposite gro
unded electrode. Additionally, during the afterglow with decaying shea
th potential, low energy negative ions reached the electrodes. The int
ensity ratio between the two groups of negative ions allowed conclusio
ns on the negative ion generation process. (C) 1997 Elsevier Science L
td.