Variable radio-frequency cold atmospheric He + O2 discharges: from electron-heating mechanism to reactive species delivery

Evolution of electron-heating processes with driving frequency and their effects on production reactive oxygen species (ROS) are studied for low-temperature radio-frequency atmospheric helium-oxygen plasma. With increasing frequency from 13.54 to 108.48 MHz at a constant power density of 40 W cm(-3), the space-averaged electron density is found to increase by 35% but this is accompanied by marked decrease in the sheath thickness, the maximum electron temperature, and the electric field in the sheath by 76%, 46% and four-folds, respectively. As a result, plasma species generated via reactions with low threshold electron energy (e.g. O-2*) experience more abundant production and those generated by energetic electrons (e.g. O and O*) undergo a reduction in their number densities as the driving frequency increases. These frequency dependences directly regulate power dissipation in species produced directly by electrons, ROS concentrations, and ultimately plasma chemistry. Delivery of ROS is shown to be effective only from a boundary layer immediately above a sample surface, due to severely compromised diffusion and drift at atmospheric pressure, introducing a further frequency-dependent factor. In general, the increase of the driving frequency affects energetic and low-energy electrons differently and the contrast in frequency effects of different ROS is desirable for manipulating plasma chemistry as experienced by the sample.