  We use the one-dimensional object-oriented particle-in-cell Monte Carlo
collision code oopd1 to explore the properties and the origins of both the
electric field and electron power absorption within the plasma bulk for a
capacitively coupled oxygen discharge operated at 10 and 100 mTorr for 45 mm of
gap distance. The properties of the electric field at three different time
slices as well as time averaged have been explored considering the moments of
the Boltzmann equation. The electron power absorption is distinctly different
at these operating pressures. The most relevant contributions to the electric
field at different time steps come from the pressure terms, the ambipolar and
the electron temperature gradient terms, along with the ohmic term. The same
applies for the electron power absorption. At both 10 and 100 mTorr the
relative ohmic contribution to the electron power absorption remains roughly
the same, while the ambipolar term contributes to power absorption and the
temperature gradient term to electron cooling at 100 mTorr, and the opposite
applies at 10 mTorr. At 100 mTorr the discharge is weekly electronegative, and
electron power absorption is mainly due to sheath expansion while at 10 mTorr
it is strongly electronegative, and the electron power absorption occurs mainly
within the electronegative core and drift-ambipolar mode dominates. The
agreement between the calculated values and the simulations is good for both
the electric field and the electron power absorption within the plasma bulk and
in the collapsed sheath region for all the cases considered.
