Reaction pathways for bio-active species in a He/H2O atmospheric pressure capacitive discharge

摘要

Helium/trace gas atmospheric pressure radio-frequency (rf) capacitive discharges have increasing biomedical applications. We have performed a principal pathway analysis for a chemically complex, bounded He/H2O atmospheric pressure, planar capacitive discharge, with a discharge gap of 0.5 mm and a power of 0.85 W cm(-2) at 13.56 MHz (n(e) approximate to 1.6 x 10(17) m(-3)). The discharge is embedded in a larger volume in which the H2O fraction is controlled to be 0.001. The generation and loss pathways for eleven species of interest for discharge maintenance and biomedical applications have been determined. The production and consumption pathways of He*, H2O, H11O5+ and electrons are found to be tightly coupled. The metastable He* generated by electron impact excitation of He is mostly consumed by Penning reactions with H2O, followed by subsequent three-body association reactions with H2O, to form the dominant positive ion, H11O5+. The main loss pathways for H11O5+ are ion cluster fragmentations at the wall, which are important generation pathways for H2O. The generation and loss pathways for electrons are almost the same as for H11O5+. OH and H2O2 generation and loss are strongly coupled, and they are important intermediate species in the generation pathways for the purely O-containing bio-active species: O-2(a), O, O-3 and O*. The generation and loss pathways for the latter four species were found to be strongly coupled by volume and surface processes, with O-2 as an important precursor. The generation of O-2 from H2O involves H2O2 as a key long-lived intermediate.