Received 5 November 2015; accepted 14 December 2015; published 17 December 2015
1. Introduction
Afterglows of N_{2} flowing microwave discharges have been studied at medium gas pressures (1 - 20 Torr) for sterilization of medical instruments by N-atoms [1] [2] . The mentioned project of sterilization in N_{2} afterglow is based on N-atom etching of bacteria without oxidation by O-atoms. A part of the present study is to detect the O-atoms from air impurity to appreciate their influence on the sterilization process.
The main part concerns a study of Ar-N_{2} gas mixtures to enhance the sterilization process in the early afterglow. The interest of N_{2} dilution into Ar is to increase the electron energy in the plasma at constant values of transmitted power and of gas pressure. Superelastic collisions of electrons on the Ar metastable atoms produced in the plasma could enhance the electron energy. It is mentioned here that in the present measurements of flowing afterglow, the Ar metastable atoms have disappeared after collisions on the tube wall (destruction probability of about 1). As a consequence, the excitation transfers of Ar metastable atoms on N_{2} can be discarded at a distance of about 1 cm after the discharge end. Another interest of Argon dilution is to maintain the plasma at high gas pressure, up to the atmospheric gas pressure while keeping a plasma power as low than 100 Watt [3] .
The early flowing afterglows produced from Ar-N_{2} microwave plasmas are presently studied by emission spectroscopy with the same experimental methods as in N_{2}-H_{2} RF afterglow [4] [5] , in N_{2}, N_{2}-O_{2} [6] and in N_{2}-H_{2}, Ar-N_{2}-H_{2,} Ar-N_{2}-O_{2} microwave early afterglows [6] .
The present paper is focused on Ar-N_{2} early afterglow by directly introducing the discharge tube of 5 mm dia. inside the 5 litre reactor. By this way, it is expected to add the metastable N_{2}(A) and N_{2}(X, v > 13) molecules and ions to the N-atoms in the surface treatments as previously experimented [1] [2] . The studied active species are as in [6] the N and O-atoms, the N_{2}(A) and N_{2}(X, v > 13) metastable molecules and ions. The intensities emitted by the N_{2} first positive (1^{st} pos.) and N_{2} second positive (2^{nd} pos.) systems and by the NO_{β} bands are measured to obtain the mentioned active specie densities after NO titration to calibrate the N and O-atom densities [6] . The O-atoms are coming from air impurity in the discharge.
2. Experimental Setup and NO Titration
The experimental setup is changed in comparison to the one used in [6] . The dia. 5 mm discharge tube is now directly connected to the 5 litre reactor as shown in Figure 1. The Ar-N_{2} microwave plasmas is always produced by a surfatron cavity at 2450 MHz, 100 Watt, 1 slm, but lowering the gas pressure from 8 Torr in [6] to 4 Torr to allow a satisfactory diffusion of the afterglow inside the 5 litre reactor.
The plasma is located inside the dia.5 mm tube with a length after the surfatron gap varying from about 5 cm in pure N_{2} to 20 cm in the Ar-2%N_{2} gas mixture. With a discharge tube length of 30 cm after the surfatron gap, the residence time before the afterglow in the 5 litre reactor is 3 × 10^{−3} s.
The optical emission spectroscopy across the reactor is performed by means of an optical fiber connected to an Acton Spectra Pro 2500i spectrometer (grating 600 gr/mm) equipped with a Pixis 256E CCD detector (front illuminated 1024 × 256 pixels).
The N-atom density is obtained from the I_{580} measured intensity after calibration by NO titration as described in [6] .
Figure 1. Microwave discharge and post-discharge reactor of 5 liters.
3. The Ar-N_{2} Early Afterglow
3.1. N-Atom Density
As reported in [6] , the pure late afterglow emission is produced by reaction R1 in Table 1.
The N_{2} (580 nm) band head intensity () in arbitrary unit (a.u) was measured for constant parameters of the Acton spectrometer (grating 600 gr/mm, slit of 150 μm, integrating time 1 s).
is then deduced from reaction R1 with v’ = 11 and hυ = hc/λ (580 nm), as follows:
(1)
with k_{1} explicited in [4] - [6] .
The reaction R1 produced with an excess of Ar atoms results in a change of the N_{2}(B, v') distribution as compared to pure N_{2} at a given a_{N+N} value. The N + N recombination coefficient a_{N+N} has been calculated in [6] in conditions of pink and late afterglows for Ar-xN_{2} gas mixture with x from 2% to 100%.
Equation (1) becomes:
(2)
By NO titration,it has been verified the same k_{1} value inside the error bars as for pure N_{2} [6] :
k_{1} = 0.6 (+/− 0.3)10^{−26} cm^{6} counts/s with in counts/s and [N] in cm^{−3}.
It is obtained a_{N+N} = 0.9 for pure N_{2} and a_{N+N} = 0.5 for the Ar-2% N_{2} mixture in the 5 litre reactor.
This result indicates that the early afterglow in N_{2} is dominated by the N+N recombination as expressed by R1.
The N-atom density is then obtained in the 5 litre reactor by taking into account the change of diameter from 2.1 cm in the tube to 15 cm in the reactor.
It is reported in Figure 2 the N-atom density variation with the %N_{2} into Ar
A slow increase of N-atom density is found in the range 2% - 10% N_{2} to reach a constant value of (5 - 6) × 10^{14} cm^{−3} between 10 and 100% N_{2}. The uncertainty on N-atom density is estimated to be 30% [6] .
Table 1. Kinetic reactions in Ar-N_{2} afterglow.
Figure 2. Active species density versus the %N_{2} into the Ar-N_{2} early afterglow in the 5 litre reactor at 4 Torr, 1 Slm, afterglow time of 3 × 10^{−3} s, plasma 100 Watt.
3.2. Density of O-Atoms in Impurity in the Ar-N_{2} Early Afterglow
The NO_{β} bands are presently observed as a result of the recombination of N and O atoms by reaction R2. In a similar way than for Equation (1), the NO (320 nm) measured band intensity () is deduced from reaction R2 as follows:
(3)
The coefficients in k_{3} are explicited in ref. 6 as for k_{1}.
The O atom density can be deduced from the N-atom density by considering the a_{N+N}. ratio of reactions 2 and 3, as follows:
(4)
with k_{4} = k_{1}/k_{3}.
After several NO titration experiments, it was found in [6] : k_{4} = 1(+/−0.4). From k_{4} obtained by NO titration, the O-atom density in the Ar-N_{2} early afterglow inside the reactor was determined by Equation (4) after measurements of and [N] versus the N_{2} percent into Ar. The results are reproduced in Figure 2. If the uncertainty on N-atom density is estimated to be 30% (see part 3.1), the experimental errors on O-atom density calculated from Equation (4), with the uncertainty on k_{4} of 40% is 90% that is near the order of magnitude.
As shown in Figure 2, there is a slow decrease of the O-atom density from 3 to 2 × 10^{13} cm^{−3} between 2% to 100%N_{2}.
3.3. Density of N_{2}(A) Metastable Molecules
It has been detected the N_{2}(C, 1® B, 0) emission at 316 nm near the NO_{β} emission at 320 nm which is used as in [4] - [6] to determine the density of the N_{2}(A) metastable molecule.
It is considered that the N_{2} 2^{nd} positive system in the early afterglow is produced by reaction R3.
The N_{2} (316 nm) measured intensity () is then given by:
(5)
with k_{5} explicited in [6] .
From Equations (3) and (5), it comes the following intensity ratio:
(6)
with k_{6} = k_{3}/k_{5}. The N_{2}(A) density is then obtained from equation (6) with the N and O atom densities previously determined. _{}
As shown in Figure 2, the N_{2}(A) density kept a constant value in the Ar-N_{2} gas mixture. It is estimated that it is obtained the order of magnitude of N_{2}(A) density in the range 10^{10} - 10^{11} cm^{−3}.
3.4. Density of N_{2}(X, v > 13) Molecules
The production of N_{2}(B, 11) by R1 in the early afterglow is less than 1 ( a_{N+N} < 1).
Other collisional processes in the pink afterglow [7] also excite the N_{2}(B) states, in addition to reaction R1.
For this other part (1 − a_{N+N}), it is considered the reactions R4 and R5 whose rate coefficients are reported in [4] - [6] . The contribution of reactions R4 and R5 on is then written as follows:
(7)
where k_{R4}, k_{R5} are the rate coefficients of reactions R4, R5. As, it is deduced:
(8)
With the experimental values of a_{N+N} and of N and N_{2}(A) densities, it is found that is about 2 orders of magnitude lower than [N]^{2}k_{1}.It results that Equation 8 can be simplified as:
(9)
From the obtained values of N-atom and N_{2}(A) density, it was deduced the values of [N_{2}(X, v > 13)] as reproduced in Figure 2.
It is observed about one order of magnitude lower N_{2}(X, v > 13) density as compared to N values.
Such values of [N_{2}(X, v > 13)] can be considered as an estimated value depending on the R5 rate coefficient.
3.5. Density of Ions
The emission of the band at 391 nm is observed in the present early afterglows. It is generally proposed [8] that the, 391 nm band is produced in the pink afterglow by reactions R6 and R7.
The intensity is then expressed as follows:
(10)
with, where c_{391} is the spectral response of spectrometer, V is the detected afterglow volume, A_{391} the Einstein coefficient of the (391 nm) transition, k_{R7} the rate coefficient of reaction R7 with [9] , and [10] .
By comparing the intensities of I^{m}_{316} from Equation (5) and I^{m}_{391} from equation (10), it is calculated:
(11)
with k_{11} increasing from 6.6 10^{−2} in pure N_{2} to 0.17 in Ar-2%N_{2}.
By assuming the equality [N_{2}, X, v>12] = [N_{2}, X, v > 13], it is found a density which decreases from about 10^{9} cm^{−3} in pure N_{2} to 2 × 10^{8} cm^{−3} in Ar-10%N_{2} and increases again to 10^{9} cm^{−3} in Ar-2%N_{2}. To verify that the ions are not coming from the end of a plasma jet at Ar-2%N_{2}, the measurements have also be performed 5 cm above in the 5 litre reactor, keeping about the same results.
Compared to published data [11] [12] , the value of density in pure N_{2} appears to be in the same order of magnitude.
4. Interest of Ar-N_{2} Gas Mixture for Surface Treatments
It is reported in Figure 3 the N/N_{2}, N_{2}(X, v > 13)/N_{2}, N_{2}(A)/N_{2} and density ratio versus the %N_{2} into
Figure 3. Density ratios of active species on N_{2} versus the %N_{2} in the Ar-N_{2} gas mixtures. In addition ratio.
Ar. Clearly, there is an interest of low %N_{2} to increase the active species density relative to N_{2} if it can be considered that the Ar atoms have no influence on the surface processes.
The density ratio is nearly constant from pure N_{2} to Ar-10%N_{2} with a new increase with Ar-2%N_{2}.
There is thus an interest of Ar-xN_{2} gas mixtures with x = 2% - 20% for surface treatments with high N, N_{2}(A, Xv > 13) and density values ( see Figure 2).
The density ratio decreased from pure N_{2} to Ar-10%N_{2} with an increase at Ar-2%N_{2} to find again the value in pure N_{2}.
This increase of density for Ar-2%N_{2} could be the result of the charge transfer R8 at the benefit of the ions [13] .
5. Conclusions
Densities of N and O atoms (the O-are coming from air impurity), N_{2}(A) and N_{2}(X, v > 13) metastable molecules and ions have been determined in Ar-N_{2} early afterglows of flowing microwave discharges at 1 slm, 4 Torr, afterglow time of 3 × 10^{−3} s and 100 W, after NO calibration.
The density of these active species are obtained by comparing the N_{2} (580 nm), NO_{β} (320 nm), N_{2} (316 nm) and (391 nm) band intensities and by writing the dominant kinetic equations.
It is found densities in the ranges of (2 - 6) × 10^{14} cm^{−3} for N-atoms, one order of magnitude lower for both N_{2}(X, v > 13) and O-atoms (coming from air impurity), of 10^{10} - 10^{11} cm^{−3} for N_{2}(A) and of 10^{8} - 10^{9} cm^{−3} for.
The densities obtained by these line-ratio measurements are with an uncertainty of 30% for N-atoms and the order of magnitude for O-atoms and N_{2}(A) metastable molecules. Estimated densities values are obtained for the N_{2}(X, v > 13) metastable and ions which are depending on the kinetics reaction rates.
It is found that the main interest of N_{2} dilution into Ar is to increase the N/N_{2} dissociation from 0.5% in N_{2} to about 10% in the Ar-2%N_{2} which could be of interest for surface reactions of N-atoms with less N_{2} molecules. The other N_{2}(A)/ N_{2}, N_{2}(X, v > 13)/N_{2} density ratios are also increasing at low %N_{2} into Ar. It is not the case for the and ratios which are constant or decreasing from pure N_{2} up to 10%N_{2}.