  The effect of driving frequency in the range of 13.56 MHz to 73 MHz on
electron energy distribution and electron heating modes in a 50 mTorr
capacitively coupled argon plasma discharge is studied using 1D-3V
particle-in-cell simulations. Calculated electron energy probability functions
exhibit three distinct ``temperatures'' for low-, mid-, and high-energy
electrons. When compared to published experimental data, the calculated
probability functions show a reasonable agreement for the energy range resolved
in the measurements (about 2 eV to 10 eV). Discrepancies outside this range
lead to differences between computational and experimental values of the
electron number density determined from the distribution functions, but the
predicted effective electron temperature is within 25\% of experimental values.
The impedance of the discharge is interpreted in terms of a homogeneous
equivalent circuit model and the driving frequency dependence of the inferred
combined sheath thickness is found to obey a known, theoretically-derived,
power law. The average power transferred from the field to the electrons
(electron heating) is computed, and a region of negative heating near the
sheath edge, particularly at higher driving frequencies, is identified.
Analysis of the electron momentum equation shows that electron inertia, which
would average to zero in a linear regime, is responsible for negative values of
power deposition near the sheath edge at high driving frequencies due to the
highly nonlinear behavior of the discharge.
