Electronic quenching of OH(A) by water in atmospheric pressure plasmas and its influence on the gas temperature determination by OH(A–X) emission

Abstract

In this paper it is shown that electronic quenching of OH(A) by water prevents thermalization of the rotational population distribution of OH(A). This means that the observed ro-vibrational OH(A-X) emission band is (at least partially) an image of the formation process and is determined not only by the gas temperature. The formation of negative ions and clusters for larger water concentrations can contribute to the non-equilibrium. The above is demonstrated in RF excited atmospheric pressure glow discharges in He-water mixtures in a parallel metal plate reactor by optical emission spectroscopy. For this particular case a significant overpopulation of high rotational states appears around 1000 ppm H 2 O in He. The smallest temperature parameter of a non-Boltzmann (two-temperature) distribution fitted to the experimental spectrum of OH(A-X) gives a good representation of the gas temperature. Only the rotational states with the smallest rotational numbers (J 7) are thermalized and representative for the gas temperature.

Power dissipation, gas temperatures and electron densities of cold atmospheric pressure helium and argon RF plasma jets

“…The rotational spectrum of molecules such as OH, N 2 , N + 2 is widely used for temperature determination. However, in this case the rotational states should follow a Boltzmann distribution, which is for atmospheric pressure plasmas in most conditions the case but not in general as work of Bruggeman et al [15] has shown with atmospheric pressure plasmas in and near liquids.…”

“…However, previous work by Bruggeman et al [15,22,26] has shown that also in some cases for atmospheric pressure plasmas the rotational states do not follow a Boltzmann population distribution because of different population mechanisms of the rotational states and quenching which reduces the lifetime of the excited states at atmospheric pressure significantly. Note that in the case of Verreycken et al [22], even when the rotational population distribution was a Boltzmann distribution it could lead to an overestimate of the gas temperature.…”

Hydrogen Peroxide Production in an Atmospheric Pressure RF Glow Discharge: Comparison of Models and Experiments

“…The gas is fed to the reactor section of the setup (8). The plasma is a capacitively coupled RF atmospheric pressure glow discharge operating at ambient pressure as investigated in [16,17]. In this configuration, the plasma is an APGD which can operate in He with small admixtures of molecular gases such as H 2 O.…”

Measurement of Electron Temperature and Number Density and Their Effects on Reactive Species Formation in a DC Underwater Capillary Discharge

“…Especially popular is the UV emission band of (A-X), which is around 309 nm [26]. However, it has recently been found that the rotational population distribution is not always in equilibrium with gas temperature and this method therefore sometimes leads to overestimation [27]. Fig.…”