Water in atmospheric-pressure helium rf plasmas

Radio frequency (rf) plasmas operated at atmospheric pressure have received great attention in recent years for their potential use in many scientific and industrial applications. The optical emission profile of these atmospheric pressure rf discharges typically presents two bright layers, one above each of the electrodes. Although in the low-pressure regime bright layers near the electrodes are typically associated with the so-called gamma mode, these layers are observed in the alpha mode at atmospheric pressure and only after a significant increase in current, the transition into the gamma mode takes place. In the gamma mode, the bright layers are found to light up in an alternating fashion, corresponding to the excitation of radiative states by avalanches across the sheaths. On the other hand, in the alpha mode, the bright layers light up simultaneously mostly as a result of the acceleration of bulk electrons in the expanding and retreating sheathbulk boundaries.