Absolute ozone densities in a radio-frequency driven atmospheric pressure plasma using two-beam UV-LED absorption spectroscopy and numerical simulations

Abstract

The efficient generation of reactive oxygen species (ROS) in cold atmospheric pressure plasma jets (APPJs) is an increasingly important topic, e.g. for the treatment of temperature sensitive biological samples in the field of plasma medicine. A 13.56 MHz radio-frequency (rf) driven APPJ device operated with helium feed gas and small admixtures of oxygen (up to 1%), generating a homogeneous glow-mode plasma at low gas temperatures, was investigated. Absolute densities of ozone, one of the most prominent ROS, were measured across the 11 mm wide discharge channel by means of broadband absorption spectroscopy using the Hartley band centred at λ=255 nm. A two-beam setup with a reference beam in Mach-Zehnder configuration is employed for improved signal-to-noise ratio allowing high-sensitivity measurements in the investigated single-pass weakabsorbance regime. The results are correlated to gas temperature measurements, deduced from the rotational temperature of the N 2 (C 3 P + u  B 3 P + g , υ=0  2) optical emission from introduced air impurities. The observed opposing trends of both quantities as a function of rf power input and oxygen admixture are analysed and explained in terms of a zero-dimensional plasma-chemical kinetics simulation. It is found that the gas temperature as well as the densities of O and O 2 (b S + g 1 ) influence the absolute O 3 densities when the rf power is varied.

Chemical kinetics in an atmospheric pressure helium plasma containing humidity

“…Here, we assume T 0g = 295 K (room temperature) as the initial temperature of the gas before entering the plasma channel. Coupled powers are typically small in this work, therefore it is assumed that the reactor wall is not significantly heated and T s is set to 295 K. This is in good agreement with previous observations, 24 where the electrode temperature was measured using an infrared thermometer in a very similar plasma configuration under a variation of plasma power.…”

Chemical kinetics and density measurements of OH in an atmospheric pressure He + O2 + H2O radiofrequency plasma

“…The previous studies cited above present simulations, whereas this paper presents new experimental data, expanding on the limited experimental results that can be found in the literature [9,[44][45][46][47][48]. The OH densities were measured for various concentrations of O2 and H2O, up to 1 % in each case.…”

“…The OH ground state density is determined from the intensity of the integrated OH (X 2 Πi, '' = 0) → OH (A 2 Σ + , ' = 0) rotational envelope in the wavelength range 306-310 nm. As previously described in several papers [9,47,49], the absolute density of the hydroxyl radical, OH , is obtained from analysis of the measured plasma absorbance spectrum A(λ) in the spectral range from 306 to 311 nm, assuming the Beer-Lambert law (equations 1 and 2). Four signals are measured successively to correct for background and plasma emission.…”

“…Non-equilibrium atmospheric-pressure plasmas are known to be efficient sources of reactive species [1][2][3][4][5][6][7][8][9][10][11][12][13] and have been investigated for several different applications, in particular surface processing, environmental applications and plasma medicine [7,. One of the challenges is tailoring the plasma gas-phase chemistry to each specific application.…”

“…This light source has excellent temporal intensity stability, leading to an absorption baseline variability lower than 2.10 -5 over the range of the OH(X)→OH(A) transition (306-311 nm) [49]. This setup allows a detection limit one order of magnitude lower compared to the typical limit (about 10 -3 ) that can be achieved with the more commonly used UV-LEDs [9] . The RF-driven plasma source used here was described in detail in [5] and is illustrated in Figure 1.…”