On the rotational–translational equilibrium in non-thermal argon plasmas at atmospheric pressure

Abstract

This work examines the rotational–translational equilibrium in non-thermal, argon-based plasmas at atmospheric pressure. In particular, rotational temperatures (T rot) and neutral gas temperatures (T g) are compared along the axis of plasma columns sustained by either radiofrequency (RF) or microwave (MW) electromagnetic fields. Water vapours or N2 admixtures are added to the high-purity argon plasmas to record the rotational temperatures from the emission spectra of either the OH(A2Σ + − X2Π i ) or the N2 +(B2Σ u + − X2Σ g + ) rovibrational systems. T g values are also deduced from the line broadening of selected Ar emission lines using an hyperfine spectrometer. In the MW Ar/H2O plasma, T g decreases from ∼2100 K close to the wave launcher to ∼1600 K near the end of the plasma column, while T rot is mostly constant in the 1500 K range. In presence of N2 admixtures instead of water vapours, T g is higher by about 300 K (from ∼2400 K to ∼1900 K), while T rot decreases from ∼3200 K to ∼2750 K along the plasma column. A discrepancy between T g and T rot is also observed in the much colder RF plasmas with T g ∼ 400 K and T rot ∼ 515 K. Such departure from the rotational–translational equilibrium in both plasmas is ascribed to the influence of electrons competing with neutrals to impose their own temperature on the distribution of rotational levels of both ground and excited states.

Different Radial Modification Profiles Observed on APPJ-Treated Polypropylene Surfaces according to the Distance between Plasma Outlet and Target

“…it can be seen that the values for the wire configuration are higher than those obtained with the mesh. In any case, such values are in agreement with ones reported in the literature for Ar plasma jets impinging on a dielectric surface [ 41 , 47 , 48 ]. Despite the difference observed in the values of for the different configurations, both curves present similar trends as a function of d .…”

“…The main reason for that is because, when Ar is the working gas, the population and excitation of rotational levels of the (C) vibrational band are not dominated by electron impact collisions, but by collisions with Ar atoms and molecules in metastable states, which can produce (C) excited states with specific rotational distributions. It is also important to mention that the values obtained using OES with Ar as the working gas are usually higher than the ones, even when is measured using emissions from OH radicals, like what was reported recently in [ 47 ].…”

“…Among the thermal parameters of the APPJs produced using the configurations under study, we evaluated only the rotational and vibrational temperatures ( and , respectively). These two thermal parameters can be considered the most important ones for APPJs, since has a close relationship with the gas temperature ( ), with in most cases, and is related to the rate of chemical reactions [ 38 , 45 , 46 , 47 ]. It is important to mention that, when Ar is the working gas, the values measured using band emissions from ( ) are not the same as , as was extensively discussed in a review paper by Bruggeman et al [ 38 ].…”

Optical response of plasmonic silver nanoparticles after treatment by a warm microwave plasma jet

“…This system was used to measure the line broadening of the Ar 2p 2 -1s 2 and 2p 3 -1s 2 (Paschen notation) emission lines at 826 and 841 nm, respectively. These two lines are strongly affected by resonance broadening in atmospheric pressure plasmas, and the Lorentzian portion of their broadening is completely dominated by mechanisms related to the neutral gas temperature (T g ) (resonance and van der Waals broadening) [41,49]. A typical measurement along with the corresponding Voigt fit is presented in figure 2.…”

Axial uniformity diagnosis of coaxial surface wave linear plasma by optical emission spectroscopy

“…Due to the gas pressure in this work, the low-pressure limits collisions to occur during the lifetime of ground state whereby the electron excitation is the dominant excitation mechanism. In that case, the rotational distribution of the excite states is a mapping of the ground state, because the electron collision from the ground state is without change of rotational quantum numbers [37,38]. Therefore, using rotational spectra of diatomic molecules (mostly diatomic) under excited states as a probe for gas temperature has made great success for many years, such as OH, NO [39] and N 2 [40].…”