We measured quasi-simultaneously the energy distributions of positive
ions at the powered RF and grounded electrode of a parallel plate 13.5
6 MHz discharge using an energy selective mass spectrometer. The resul
ting ion energy distributions show the discharge potential conditions
expected from a capacitive sheath model. Effects of the discharge pres
sure on the collision probability in the sheath, on the tendency of th
e sheath thickness, and on the effective discharge geometry are clearl
y demonstrated. The main ionization region is located in front of the
powered electrode sheath, and the probability of ions reaching the opp
osite grounded electrode is characteristically influenced by discharge
pressure and electrode distance.