Influence of trace oxygen on He(23S) density in a He-O2 microwave discharge at atmospheric pressure:: behaviour of the time afterglow

The influence of weak concentrations of oxygen on the density of He(2(3)S) metastable states was measured by laser atomic absorption spectroscopy in a microwave He-O-2 plasma sustained in a microwave resonant cavity. The number density of He(2(3)S) decreases by a factor of five when 500 ppm of O-2 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.