  We present an one-dimensional particle-in-cell Monte-Carlo model for
capacitively coupled radio-frequency discharges in oxygen. The model
quantitatively describes the central part of the discharge. For a given voltage
and pressure, it self-consistently determines the electric potential and the
distribution functions for electrons, negatively charged atomic oxygen, and
positively charged molecular oxygen. Previously used collision cross sections
are critically assessed and in some cases modified. Provided associative
detachment due to metastable oxygen molecules is included in the model, the
electro-negativities in the center of the discharge are in excellent agreement
with experiments. Due to lack of empirical data for the cross section of this
process, we propose a simple model and discuss its limitations.
