Plasma liquid interactions are important for a range of applications. For
these, H2O2 and OH represent two key reactive species, whose concentrations in liquids
need to be controlled for effective application outcomes. Here, a combination of gas
and liquid simulations is used to study the concentration profiles of H2O2 and OH in
water treated by a radio-frequency-driven plasma jet, with a glass capillary between the
electrodes, operated in He with admixtures of water vapour. Simulations are compared
with measured H2O2 concentrations and found to be in good qualitative agreement
as plasma power and water admixture are varied. Simulation results show that the
concentration profiles of H2O2 in the liquid are mainly determined by transport,
while those of OH are limited by reactions with H2O2, which consumes OH. For a
given plasma operating condition, the concentration and penetration depth of H2O2
increase with plasma treatment time, while those of OH tend to decrease because
of the increasing H2O2 concentration. Plasma power, water vapour admixture, and
the distance between the jet and the liquid surface all allow for the concentrations of
H2O2 and OH to be controlled. The OH delivered from the gas phase to the liquid, and
its concentration within the liquid are strongly dependent on the reaction pathways
occurring in the effluent region, such that the trends in OH density at the end of
the plasma region differ from those in the liquid. While the concentration of OH in
the liquid is always much lower than that of H2O2, the ratio of the two species can
be controlled over orders of magnitude by varying water admixture and power. The
highest selectivity to OH is at low water admixtures, low powers and short treatment
times, while the highest selectivity to H2O2 is at high water admixtures, high powers
and long treatment times.