In low pressure, radio frequency (RF) discharges of the type used in plasma processing of semiconductor materials, the rate of electron impact excitation and energy transfer processes depends upon both the phase of the RF excitation and position in the discharge. Electron impact collisions create radicals that diffuse or drift to the surfaces of interest where they are adsorbed or otherwise react. To the extent that these radicals have a finite lifetime, their transport time from point of creation to surface of interest is an important parameter. The spatial dependence of the rate of the initial electron impact collisions is therefore also an important parameter. The power that sustains the discharge is coupled into the system by two mechanisms: a high energy “e-beam” component of the electron distribution resulting from electrons falling through or being accelerated by the sheaths, and by “joule heating” in the body of the plasma. In this paper, we will discuss the spatial dependence of excitation rates and the method of power deposition in RF discharges of the type used for plasma processing. The basis of that discussion will be results from a Monte-Carlo plasma simulation code for RF discharges. Preliminary results from a model for the etching of silicon in SF6 /O2 plasmas will be presented and evidence for long term changes in the manner of energy deposition will be discussed.