  We investigate the electron heating dynamics in electropositive argon and
helium capacitively coupled RF discharges driven at 13.56 MHz by Particle in
Cell simulations and by an analytical model. The model allows to calculate the
electric field outside the electrode sheaths, space and time resolved within
the RF period. Electrons are found to be heated by strong ambipolar electric
fields outside the sheath during the phase of sheath expansion in addition to
classical sheath expansion heating. By tracing individual electrons we also
show that ionization is primarily caused by electrons that collide with the
expanding sheath edge multiple times during one phase of sheath expansion due
to backscattering towards the sheath by collisions. A synergistic combination
of these different heating events during one phase of sheath expansion is
required to accelerate an electron to energies above the threshold for
ionization. The ambipolar electric field outside the sheath is found to be time
modulated due to a time modulation of the electron mean energy caused by the
presence of sheath expansion heating only during one half of the RF period at a
given electrode. This time modulation results in more electron heating than
cooling inside the region of high electric field outside the sheath on time
average. If an electric field reversal is present during sheath collapse, this
time modulation and, thus, the asymmetry between the phases of sheath expansion
and collapse will be enhanced. We propose that the ambipolar electron heating
should be included in models describing electron heating in capacitive RF
plasmas.
