Determination of chemically active species in a novel microwave plasma source by laser-induced fluorescence

Abstract

The downstream zone of a new type of plasma source was investigated. Microwave power is fed to the plasma by an annular waveguide ring with slotted line radiators on the inner side. In a pure oxygen plasma the conditions for the generation of atomic oxygen were studied by laser-induced fluorescence. Excitation from the ground state is achieved by two-photon absorption (3p 3P1,2,0←2p 3P2,1,0 transitions) of 226 nm laser radiation. Fluorescence is observed at 845 nm. From the Doppler-broadened absorption line profile a temperature of 350 K is determined. A rate equation model combined with a measurement of laser properties and a calibration of the detection system by Raman scattering in hydrogen gas enabled an absolute calculation of the atomic oxygen ground state density. In the downstream zone values in the range of 1013–1014 cm−3 are obtained. The scaling of the atomic oxygen density with external parameters was investigated.

Investigation of a pulsed oxygen microwave plasma by time-resolved two-photon allowed laser-induced fluorescence

“…For a pure oxygen plasma (p = 50 Pa) in cw operation at 1 kW microwave power, an absolute atomic oxygen density of 2.0×10 14 cm −3 or a degree of dissociation of 1.6% is obtained. This value is in good agreement with the results obtained by the mentioned Raman calibration [6].…”

“…It turns out that below 100 µJ a quadratic dependency of the fluorescence signal from the laser intensity is achieved. This is well below critical laser intensity values predicted in [6,12]. The possible influence of photodissociation on the TALIF measurement will be discussed later.…”

“…The resulting laser beam is focused into the vacuum vessel by a quartz lens (f = 500 mm). Effects like multiphoton ionization [6], photodissociation [11] and amplified spontaneous emission [12] lead to a non-quadratic dependency of the TALIF signal from the laser intensity.…”

“…Having these facts in mind, there are only three possibilities left. One way is the calibration of the detection scheme by Raman scattering of hydrogen as described in [6]. Unfortunately, this method does not take into account the photon statistics of the twophoton excitation used in this experiment.…”

Collisional Energy Transfer of NO D 2Σ+ (v‘ = 0) and A 2Σ+ (v‘ = 4) by O2, N2, Ar, and NO

“…A numerical kinetic model for the D v ‘ = 0 two-photon excitation was implemented to assess these photoionization effects; , that is, the potential influence of the ions or atoms. The two-photon absorption cross section for D−X(0,0) is not known, and we use that for A−X(0,0) by Burris et al, resulting in an approximate two-photon cross section for the P 11 + O 21 bandhead of 10 -48 cm 4 s. A Gaussian laser pulse and a laser fluence of 3 J/cm 2 are used.…”