This work reports the results of an experimental and modelling study on dual-frequency Ar-NH 3 dielectric barrier discharges (DBD) exhibiting the α -γ transition. A combination of space-and timeresolved optical absorption and emission spectroscopy is used to record spatio-temporal mappings of the Ar metastable number density, Ar 750.4 nm line emission intensity, and electron-Ar Bremsstrahlung continuum emission intensity. With the increase of the RF voltage amplitude in a 50 kHz-5 MHz DBD, maximum populations of Ar excited species (1s and 2p states, linked to the population of high-energy electrons) observed in the γ mode decrease and appear earlier in the lowfrequency cycle. On the other hand, the density of the bulk electrons, monitored from the continuum emission intensity, increases, with a more prominent rise in the RF-α mode than in the γ regime.

Such behaviors are consistent with the predictions of 1D fluid model and results from a decrease of the gas voltage required for self-maintenance of the cathode sheath in the γ breakdown.