Two-photon Laser-induced Fluorescence Spectroscopy Research Articles

Neutral atom density measurements of xenon plasma inside a μ10 microwave ion thruster using two-photon laser-induced fluorescence spectroscopy

This paper reports measurements of the xenon ground state and excited state densities inside a μ10 microwave ion thruster. This thruster exhibits a 40% thrust enhancement upon changing from the waveguide to the discharge chamber propellant injection mode. In the present work, the associated mechanism was quantitatively evaluated using two-photon laser induced fluorescence (TALIF) spectroscopy to monitor the thruster waveguide. The 834.7 nm emission from excited state xenon was investigated with a 224.3 nm dye laser to excite the Xe I 5p61 S0 6pʹ [3/2]2 state, compared with the emission without the laser. The resulting data confirm that the neutral density exhibits a linear relationship with the propellant flow rate in the cold gas and ionized state, while the ion acceleration decreases the neutral density by the same order of magnitude as the propellant utilization efficiency is changed. As the propellant flow rate increases, the collisions of neutrals that generate excited states occur in the waveguide and, when this process plateaus, the ground state emission suddenly increases. Propellant injection from the discharge chamber is evidently effective at suppressing collisions with electrons in the waveguide that generate excited states and that potentially interfere with microwave propagation.

Application of Two-photon Laser-induced Fluorescence Spectroscopy to Microwave Cathode

Application of Two-photon Laser-induced Fluorescence Spectroscopy to Microwave Cathode

Neutral ground state particle density measurement of xenon plasma in microwave cathode by two-photon laser-induced fluorescence spectroscopy

This paper reports the first study to measure xenon neutral ground state particle density of microwave cathode by two-photon laser induced fluorescence spectroscopy (TALIF). Xenon is commonly used as a propellant in electric propulsion like Hall thrusters, ion thrusters, and their cathodes. For electric propulsion, information about neutral particles is important such as the ionization degree and the charged exchange collisions (CEX). The measurement target is XeI 5p61S06p[3/2]2, which absorbs at a wavelength of 224.29 nm and emits fluorescence of 834.7 nm. The measurement system was demonstrated for three cases: cold gas, without electron extraction, with electron extraction. From three cases, the measurement system can detect a neutral ground state particle density of 1019m−3 order without and with a plasma. In a cold gas, the neutral ground state particle density is (8.4±0.4)×1019m−3 at 0.098 mg/s. Without electron extraction, the neutral ground density decreases by ionization and excitation With electron extraction, the density varied from 0.6 to 2.3 times compared to without electron extraction depending on anode voltage.

Atomic oxygen dynamics in an air dielectric barrier discharge: a combined diagnostic and modeling approach

Cold atmospheric pressure plasmas are a promising alternative therapy for treatment of chronic wounds, as they have already shown in clinical trials. In this study an air dielectric barrier discharge (DBD) developed for therapeutic use in dermatology is characterized with respect to the plasma produced reactive oxygen species, namely atomic oxygen and ozone, which are known to be of great importance to wound healing. To understand the plasma chemistry of the applied DBD, xenon-calibrated two-photon laser-induced fluorescence spectroscopy and optical absorption spectroscopy are applied. The measured spatial distributions are shown and compared to each other. A model of the afterglow chemistry based on optical emission spectroscopy is developed to cross-check the measurement results and obtain insight into the dynamics of the considered reactive oxygen species. The atomic oxygen density is found to be located mostly between the electrodes with a maximum density of cm. Time resolved measurements reveal a constant atomic oxygen density between two high voltage pulses. The ozone is measured up to 3 mm outside the active plasma volume, reaching a maximum value of cm between the electrodes.

Influence of target surfaces on the atomic oxygen distribution in the effluent of a micro-scaled atmospheric pressure plasma jet

Micro-scaled plasma jets with reactive process gases, e.g. oxygen, are applied for localized surface treatment. Here, investigations of the development and the spatial distribution of atomic oxygen are reported in the post-discharge effluent of a micro-scaled atmospheric pressure plasma jet. These measurements are supplemented by installation of planar targets of various materials in the effluent. The reactive species are detected by means of two-photon laser-induced fluorescence spectroscopy from the discharge, through the free effluent, up to distances of about 200 µm in front of a planar surface. Ozone density profiles are measured by UV absorption spectroscopy. The effect of the effluent on gold and plastic substrates and vice versa is investigated. In the free effluent, the atomic oxygen density falls off exponentially to about 2 × 1015 cm−3 at a distance of 6 mm from the jet's nozzle. The implementation of a plastic target does not disturb the O distribution, resulting in a strictly localized etching of the target. In contrast, mounting of a gold target increases the oxygen density and spreads its distribution close to the target. For correlation, surface modifications by plasma treatment of plastic and gold substrates are analysed by UV laser microscopy and x-ray photoelectron spectroscopy.

Absolute nitrogen atom density measurements by two-photon laser-induced fluorescence spectroscopy in atmospheric pressure dielectric barrier discharges of pure nitrogen

In this paper, two-photon absorption laser induced fluorescence spectroscopy is used to follow the nitrogen atom density in flowing dielectric barrier discharges fed with pure nitrogen and operating at atmospheric pressure. Two different dielectric barrier discharge regimes are investigated: the Townsend regime, which is homogeneous although operating at atmospheric pressure, and the more common filamentary regime. In both regimes, densities as high as 3×1014∕cm3 are detected. However, the N atoms kinetic formation depends on the discharge regime. The saturation level is reached more rapidly with a filamentary discharge. For a given discharge regime, the N atom density depends strongly on the energy dissipated in the plasma between the gas inlet and the measurement position, whether the energy is varied by varying the position of the measurements, the gas flow, or the dissipated power. Experiments performed in the postdischarge show that the N atom decay cannot be simply attributed to three-body recombination of atomic nitrogen with nitrogen molecules, meaning that other mechanisms such as surface recombination or gas impurities play a role.

Diagnostics on an atmospheric pressure plasma jet

The atmospheric pressure plasma jet (APPJ) is a homogeneous non-equilibrium discharge at ambient pressure. It operates with a noble base gas and a percentage-volume admixture of a molecular gas. Applications of the discharge are mainly based on reactive species in the effluent. The effluent region of a discharge operated in helium with an oxygen admixture has been investigated. The optical emission from atomic oxygen decreases with distance from the discharge but can still be observed several centimetres in the effluent. Ground state atomic oxygen, measured using absolutely calibrated two-photon laser induced fluorescence spectroscopy, shows a similar behaviour. Detailed understanding of energy transport mechanisms requires investigations of the discharge volume and the effluent region. An atmospheric pressure plasma jet has been designed providing excellent diagnostics access and a simple geometry ideally suited for modelling and simulation. Laser spectroscopy and optical emission spectroscopy can be applied in the discharge volume and the effluent region.

Two-photon laser induced fluorescence spectroscopy performed on free nitrogen plasma jets

The properties of free nitrogen plasma jets are examined by studying the ground-state nitrogen atom flow characteristics. The plasma is created by a cascaded arc and subsequently it expands freely into a low pressure vessel. In such a way supersonic flows with high Mach number are achieved. N(4S) atoms are locally probed by means of two-photon absorption laser induced fluorescence spectroscopy. Axial and radial N(4S) atom temperature and velocity profiles present the shape predicted by the neutral gas supersonic expansion theory. The adiabatic exponent γ is equal to 1.45 in the supersonic domain. A Mach number M of 4.4 is measured ahead of the normal shock wave. In contrast, density profiles reveal a departure from the classical expansion picture. Too small density jumps over the shock region indicate a non-conservation of the N(4S) atom forward flux. Moreover the partial N(4S) atom static pressure decreases in the subsonic domain. Loss of nitrogen atoms during the plasma expansion is a direct consequence of plasma–wall interactions. However, losses are limited because of the relatively high N atom mass and because of a low surface recombination probability of N atoms. The dissociation degree at the arc exit is around 40%. Under such circumstances N2(A) molecules cannot survive in the jet. The local electron density is estimated from a measure of the radiative lifetime of the nitrogen atom excited state.

Detection of hydrogen atoms in SiH4–H2 radio-frequency plasmas using two-photon laser-induced fluorescence

Two-photon laser-induced fluorescence spectroscopy is used to detect ground-state atomic hydrogen in highly diluted SiH4–H2 radio-frequency discharges currently used for microcrystalline silicon thin film deposition. Along with the fluorescence coming from the hydrogen atoms created in the discharge, additional atoms arising from photodissociation of silicon containing species at 205 nm are observed. This parasitic effect is identified as resulting mainly from the disilane molecules formed in the plasma. Hydrogen atoms created in the plasma and originating from the photodissociation are discriminated by a spectral analysis of the laser-induced fluorescence line profile and relative densities are deduced. Some results on the hydrogen atom and disilane concentrations as functions of the silane dilution, radio-frequency power, gas pressure, and gas temperature are given to illustrate the potentiality of such a diagnostic in SiH4–H2 discharges containing up to 6% of silane.

Nonlinear optical techniques for plasma diagnostics

The application of various nonlinear optical laser spectroscopic techniques to plasma diagnostics are reported. The techniques discussed in this paper are two-photon laser induced fluorescence spectroscopy, double-resonant four-wave mixing, coherent anti-Stokes Raman-scattering (CARS), and a combination of emission spectroscopy and CARS. They are applied to measurements of atomic hydrogen densities, molecular temperature, chemical composition, electric field distributions, and vibrational population distribution. The basic principles are described and important aspects of the methods are discussed in context with application to various kinds of discharges at low and elevated pressures.

Dynamic Variation of the Sticking Coefficient of Oxygen Atoms in Helicon-Wave Excited High-Density Oxygen Plasmas

The sticking coefficient of O atoms on a chamber wall was determined by lifetime measurements in afterglow oxygen plasmas using two-photon laser-induced fluorescence spectroscopy. The sticking coefficient was strongly dependent on the discharge conditions. When the discharge duration was short or the oxygen gas pressure was low, the sticking coefficient was close to unity and the lifetime of O atoms was equal to the diffusion time determined by the chamber geometry. However, for a long discharge duration and a high oxygen pressure, the sticking coefficient was considerably lower and saturated at ∼0.1. The decrease in the sticking coefficient can be explained by the increase in the surface coverage due to adsorption of O atoms during the discharge. The number of O atoms which was sufficient to decrease the sticking coefficient to ∼0.1 was determined experimentally to be on the order of 1015 cm-2.

Analysis of ion-bombarded and laser-irradiated surfaces of ZnS and Zn via two-photon high-resolution laser-induced fluorescence spectroscopy

Doppler-shifted two-photon laser-induced fluorescence spectroscopy (LFS) has been used to measure the velocity distributions of Zn atoms ejected by Ar+ ion bombardment from both a Zn surface and a ZnS single crystal. Doppler-shifted spectroscopy in combination with time of flight(TOF), has also been employed to investigate Zn atoms ejected from a ZnS single crystal under irradiation by 308-nm photons (XeCl excimer laser). The absolute Zn sputtering yields from ion-bombarded and laser-irradiated ZnS samples were determined by comparison with LFS measurements of Zn atoms sputtered from pure Zn targets by Ar+ ions since the absolute sputtering yield is known. For the Zn target the velocity distributions of the ion-induced sputtered Zn atoms were in good agreement with the Sigmund–Thompson distribution based on the known binding energy of 1.35 eV. On the other hand, the velocity distribution of Zn atoms ion sputtered from ZnS showed some deviation from the Sigmund–Thompson distribution and had a lower binding energy, ∼0.9 eV. The velocity distributions of the laser-ablated Zn atoms from ZnS were Maxwell–Boltzmann in nature with characteristic temperatures increasing from 2000 to 9000 K at fluences of 20 and 80 mJ/cm2, respectively. The absolute sputtering yield (Y) of Zn sputtered from ZnS was found to be Y≂8 atoms/ion for ion sputtering and Y≂10−6 atoms/photon at a laser fluence of 30 mJ/cm2 for laser ablation. High-resolution Doppler-shift techniques have been combined with TOF techniques. This promising combination permits unambiguous detection of prompt and delayed emission of ablated atoms, as well as the observation of possible molecular or cluster fragmentation. The results obtained indicate that no delayed emission is occurring and suggest the possibility of molecule or cluster emission from ZnS.