Influence of trace oxygen on He(2 ³S) density in a He–O₂microwave discharge at atmospheric pressure: behaviour of the time afterglow

Abstract

The influence of weak concentrations of oxygen on the density of He(2 3S) metastable states was measured by laser atomic absorption spectroscopy in a microwave He–O2 plasma sustained in a microwave resonant cavity. The number density of He(2 3S) decreases by a factor of five when 500 ppm of O2 are introduced in helium. Since in pure helium, metastable states sustain the afterglow for hundreds of microseconds, the behaviour of the time afterglow of the plasma is next investigated. Photon counting is used to observe the evolution of four optical transitions from He(n = 3) to He(n = 2) levels in the afterglow. The last step of the time evolution of the intensity of the followed transitions is a first-order exponential decay. Introduction of about 300 ppm of oxygen shortens this duration to values that are close to the characteristic time needed to shut down the microwave power. These values are close to some data available for dielectric barrier discharges at room temperature. The influence of the plasma temperature on the main mechanisms induced by the addition of oxygen and involved in the behaviour of the afterglow is discussed.

Absorption spectroscopy measurements of resonant and metastable atom densities in atmospheric-pressure discharges using a low-pressure lamp as a spectral-line source and comparison with a collisional-radiative model

“…The numerically calculated values are in agreement with the experimental data, in particular both of them decreasing toward the wave launcher (increasing z). A similar trend has been observed by Cardoso et al [12] who showed, using a diode laser, that the density of the He I (2 3 S) metastable-state atoms in a cavity-sustained microwave plasma column decreases with increasing absorbed microwave power, which is equivalent to increasing z values in our case. Under the present experimental conditions (atmospheric-pressure plasmas), in brief our calculations show that the Ar(4s) level: i) is principally populated by dissociative recombination of Ar 2 + (Ar 2 + + e → Ar(4s)+ Ar) and depopulated by radiative de-excitation of its resonant energy levels (Ar(4s) → Ar+hν) and ii) is strongly coupled to the Ar(4p) level through the following two collisional reactions e + Ar(4s) → e + Ar(4p) and e + Ar(4p) → e + Ar(4s) and through the radiative reaction Ar(4p) → Ar(4s) + hν.…”

“…In expressions (11) and (12), L indicates the length of the absorbing layer. The value of k 0 L therein is obtained through the measured globalabsorption coefficient A L , which is defined in the coming section.…”

“…The density of metastable and resonant atoms can be determined in various ways: self-absorption [7][8][9][10][11] and absorption using the emission from a tunable diode laser [12][13][14][15][16][17][18] or the line from a spectral lamp [2,[19][20][21][22][23][24][25][26][27]. Utilizing lasers to determine the density of metastable and resonant atoms is advantageous, as it does not depend on the absorption-line spectral profile.…”

Dureza e resistência ao desgaste da camada de ZrN formada por nitretação a plasma sobre zircônia parcialmente estabilizada com ítria

“…1, onde se pode observar a posição da cavidade ressonante e da amostra de 3Y-PSZ, bem como a existência de uma fenda que possibilita a análise de espectroscopia óptica, utilizada para medir a temperatura das amostras durante o processo de nitretação. O reator de micro-ondas utilizado no presente trabalho está descrito com maiores detalhes nas referências [37,38].…”

Compact Z-pinch radiation source dedicated to broadband absorption measurements

“…If the medium to be investigated emits very weak light, emission spectroscopy is no longer suitable and absorption spectroscopy is necessary. The latter requires an auxiliary radiation source which may be a tunable laser whose wavelength can be scanned to obtain an absorption spectrum [2]. In this case, the medium to be probed must be in the steady state or repetitive with a good reproducibility.…”