The dynamics of high energetic electrons (11.7 eV) in a modified industrialconfined dual-frequency capacitively coupled RF discharge (Exelan, LamResearch Inc.), operated at 1.937MHz and 27.118 MHz, is investigated bymeans of phase resolved optical emission spectroscopy. Operating in a He–O2plasma with small rare gas admixtures the emission is measured, withone-dimensional spatial resolution along the discharge axis. Both the low andhigh frequency RF cycle are resolved. The diagnostic is based on timedependent measurements of the population densities of specifically chosenexcited rare gas states. A time dependent model, based on rate equations,describes the dynamics of the population densities of these levels. Based onthis model and the comparison of the excitation of various rare gas states, withdifferent excitation thresholds, time and space resolved electron temperature,propagation velocity and qualitative electron density as well as electron energydistribution functions are determined. This information leads to a betterunderstanding of the dual-frequency sheath dynamics and shows, that separatecontrol of ion energy and electron density is limited.