Negative ions extracted from a pulsed rf discharge can have a complex energy dependence as a function of time. This dependence could influence semiconductor processing in pulsed discharges, particularly if precise control of the ion energy is important. In this article, the time‐dependent ion energy distributions of F−from pulsed discharges through CF4are presented for the first time using a mass spectrometer equipped with a Bessel Box energy analyzer. The time‐dependent changes in the ion energy distributions are produced by the manner in which the plasma disintegrates. Negative ions reaching the processing surface very early in the afterglow have lower energies because they experience less acceleration by the dc bias potential before entering the mass spectrometer. These ions appear in a very intense, short duration pulse, suggesting that they are formed in the after glow near the entrance to the mass spectrometer. Ions reaching the surface later in the afterglow have higher energies because they travel further through the dc bias potential. These ions appear with less intensity over a longer period of time. ©1995 American Institute of Physics.