Absolute O3 and OH densities measurement by two-beam UV-LED absorption spectroscopy in atmospheric pressure plasmas

Low temperature atmospheric pressure plasmas (APPs) create rich environment of reactive
particle species and chemical-physical interactions at close-to-room temperature and ambient
pressure which calls for a wide range of fundamental and application studies. APPs for
biomedical applications is one of the emerging interdisciplinary researches. Its fundamental
mechanisms have been studied using different numerical models and various diagnostic
techniques. With hundreds of particle species and complex reactions, each species requires
unique measurement techniques. In a typical APP, ozone (O3), one of the key species in
living-cells inactivation, is produced from the complex reaction chain of short-lived oxygen
atoms and excited molecules. Measurement and theoretical predictions of O3 densities can
have high uncertainties. The measurements of O3 densities inside the small plasma volume
are challenging due to the sensitivity to non-plasma parameters. In this work, two-beam
UV-LED absorption spectroscopy has been developed by using a Mach-Zehnder configuration
for O3 density measurements on the core of a homogeneous, He-O2 capacitively
coupled, 13.56 MHz RF-driven APP. The improved technique allows for high-sensitivity
measurement in the order of 10−3 absorption signal with 10−4 of uncertainty. The anticorrelation
between O3 density and gas temperature was observed and described based on
the plasma chemistry models. For controlling-parameter effect, the duty-cycle in frequency
modulations showed a significant influence on the spatial profile of O3 density in the plasma
channel. From an application perspective, the developed technique was able to provide 2D
O3 density distribution in the effluent region of a co-axial DBD kHz-driven APPJ when applied
to biological samples. The correlation between radial O3 density profiles and bacterial
inactivation areas was investigated. In the relatively realistic condition with higher H2O
vapour admixture, hydroxyl (OH) density, which is one of important radical species, can be
measured using the UV absorption technique. Thus, the setup has been adjusted in order
to measure both species. Furthermore, O3 density in the CO2-CO conversion 40.68 MHz
RF-driven APP, an important process in chemical research, was observed. The O3 density
as a function of plasma power and CO2 concentration provided a significant contribution to
the main production and destruction channels of the conversion processes.