  Phase Resolved Optical Emission Spectroscopy (PROES) measurements combined
with 1d3v Particle-in-Cell/Monte Carlo Collision (PIC/MCC) simulations are
performed to investigate the excitation dynamics in low-pressure capacitively
coupled plasmas (CCPs) in argon-oxygen mixtures. The system used for this study
is a geometrically symmetric CCP reactor operated in a fixed mixture gas
composition, at fixed pressure and voltage amplitude, with a wide range of
driving RF frequencies (2$~$MHz$~\le f \le~15~$MHz). The measured and
calculated spatio-temporal distributions of the electron impact excitation
rates from the Ar ground state to the Ar$~\rm{2p_1}$ state (with a wavelength
of 750.4~nm) show good qualitative agreement. The distributions show
significant frequency dependence, which is generally considered to be
predictive of transitions in the dominant discharge operating mode. Three
frequency ranges can be distinguished, showing distinctly different excitation
characteristics: (i) in the low frequency range ($f \le~3~$MHz), excitation is
strong at the sheaths and weak in the bulk region; (ii) at intermediate
frequencies (3.5$~$MHz$~\le f \le~5~$MHz), the excitation rate in the bulk
region is enhanced and shows striation formation; (iii) above 6$~$MHz,
excitation in the bulk gradually decreases with increasing frequency. Boltzmann
term analysis was performed to quantify the frequency dependent contributions
of the Ohmic and ambipolar terms to the electron power absorption.
