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Early afterglows of Ar-N
_{2} flowing microwave discharges are characterized by optical emission spectroscopy. The N and O atoms, the N
_{2}(A) and N
_{2}(X, v > 13) metastable molecules and N
_{2}
^{+} ion densities are determined by optical emission spectroscopy after calibration by NO titration for N and O-atoms and measurements of NO and N
_{2} band intensities. For an Ar-xN
_{2} gas mixture with × increasing from 2 to 100% at 4 Torr, 100 Watt and an afterglow time of 3 × 10
^{- 3} s at the 5 liter reactor inlet, it is found densities in the ranges of (2 - 6) × 10
^{14} cm
^{- 3} for N-atoms, one order of magnitude lower for N
_{2}(X, v > 13) and for 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 N
_{2}
^{+}.

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 [_{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 [

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 [_{2}, N_{2}-O_{2} [_{2}-H_{2}, Ar-N_{2}-H_{2,} Ar-N_{2}-O_{2} microwave early afterglows [

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 _{2}(A) and N_{2}(X, v > 13) metastable molecules and _{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 [

The experimental setup is changed in comparison to the one used in [_{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 [

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 [

As reported in [

The N_{2} (580 nm) band head intensity (

with k_{1} explicited in [

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 [_{2} gas mixture with x from 2% to 100%.

Equation (1) becomes:

By NO titration,it has been verified the same k_{1} value inside the error bars as for pure N_{2} [

k_{1} = 0.6 (+/− 0.3)10^{−26} cm^{6} counts/s with ^{−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 _{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% [

Reactions | |
---|---|

R1 | |

R2 | |

R3 | |

R4 | |

R5 | |

R6 | |

_{ } | R7_{ } _{ } |

R8 |

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 (

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}.

with k_{4} = k_{1}/k_{3}.

After several NO titration experiments, it was found in [_{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 _{2} percent into Ar. The results are reproduced in _{4} of 40% is 90% that is near the order of magnitude.

As shown in ^{13} cm^{−3} between 2% to 100%N_{2}.

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 [_{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 (

with k_{5} explicited in [

From Equations (3) and (5), it comes the following

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 _{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}.

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 [_{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 [

where k_{R4}, k_{R5} are the rate coefficients of reactions R4, R5. As

With the experimental values of a_{N+N} and of N and N_{2}(A) densities, it is found that ^{2}k_{1}.It results that Equation 8 can be simplified as:

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

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.

The emission of the

The

with_{391} is the spectral response of spectrometer, V is the detected afterglow volume, A_{391} the Einstein coefficient of the _{R7} the rate coefficient of reaction R7 with

By comparing the intensities of I^{m}_{316} from Equation (5) and I^{m}_{391} from equation (10), it is calculated:

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 ^{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 _{2}, the measurements have also be performed 5 cm above in the 5 litre reactor, keeping about the same results.

Compared to published data [_{2} appears to be in the same order of magnitude.

It is reported in _{2}, N_{2}(X, v > 13)/N_{2}, N_{2}(A)/N_{2} and _{2} into

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 _{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

The _{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 _{2} could be the result of the charge transfer R8 at the benefit of the

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 _{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

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

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 _{2} up to 10%N_{2}.

AndreRicard,HayatZerrouki,Jean-PhilippeSarrette, (2015) Determination of N and O-Atoms, of N_{2}(A) and N_{2}(X, v> 13) Metastable Molecules and N_{2}^{+} Ion Densities in the Afterglows of Ar-N_{2} Microwave Discharges. Journal of Analytical Sciences, Methods and Instrumentation,05,59-65. doi: 10.4236/jasmi.2015.54007