Two-photon Absorption Laser-induced Fluorescence Spectroscopy Research Articles

Contribution of vibrational excited molecular nitrogen to ammonia synthesis using an atmospheric-pressure plasma jet

The contribution of atomic nitrogen is fairly possible in plasma-assisted catalytic synthesis of ammonia since it has high adsorption probabilities on solid surfaces. On the other hand, recently, the contribution of vibrational excited molecular nitrogen to ammonia synthesis has been discussed. In this work, we compared the fluxes of atomic nitrogen and vibrational excited molecular nitrogen with the rate of plasma-assisted ammonia synthesis. We employed an atmospheric-pressure nitrogen plasma jet, and the spatial afterglow of the plasma jet and a hydrogen flow irradiated the surface of a ruthenium catalyst. The fluxes of atomic nitrogen and vibrational excited molecular nitrogen were measured by two-photon absorption laser-induced fluorescence spectroscopy and laser Raman scattering, respectively. The synthesis rate of ammonia had a positive correlation with the flux of vibrational excited molecular nitrogen, while the variation of the synthesis rate with the gas flow rate was opposite to the flux of atomic nitrogen. The experimental results indicate the contribution of vibrational excited molecular nitrogen to the synthesis of ammonia using the atmospheric-pressure plasma, where the flux of vibrational excited molecular nitrogen is more than four orders of magnitude higher than that of atomic nitrogen.

Enhancing selective production of atomic oxygen through nanosecond pulse-burst driven mode in a He/O2 dielectric barrier discharge

In this study, the temporal evolution of O atoms in a nanosecond burst-pulsed dielectric barrier discharge (DBD) is measured by two-photon absorption laser-induced fluorescence spectroscopy. The experiment is conducted at burst conditions of 50, 100, and 200 kHz pulse frequency, 10 Hz burst frequency, and 20–400 pulses in 0.1%–2% O2 + He mixtures. The accumulation effect of O atoms in the burst mode is observed and the density gradually saturates at around 100 pulses. Increasing the pulse frequency effectively enhances the O saturation density. The 0-dimensional kinetic model reveals that the saturation effect is primarily balanced by the formation and loss characteristics of O atoms. Similar saturation effect is also observed in the typical continuous periodic pulse mode (one pulse each cycle), but with a saturation density about one order of magnitude lower than that in the burst case, highlighting the burst excitation mode as an effective method for enhancing the instantaneous peak production of O atoms. Further investigations into the influence of O2 proportion on the selective production of O atoms are also performed. The results suggest that a low O2 proportion (<2%) and pulse-burst driven mode for the He/O2 DBD facilitates the selective production of O atoms while competing with O3 formation.

Single- and two-photon absorption laser-induced fluorescence spectroscopy in rare gases for gridded ion thruster diagnostics

Methods based on laser-induced fluorescence spectroscopy are widely used for spatially resolved non-intrusive diagnostics of atomic or molecular densities and velocity distributions in plasma applications. With regard to electric space propulsion, one focus is on the investigation of rare gases such as xenon or krypton, which are currently the favored propellants in gridded ion- and Hall-effect thrusters. For gridded ion engines, diagnostics of neutral atoms is of interest since charge-exchange processes between neutrals and ions are the main driver of accelerator grid erosion, which limits the lifetime of a gridded ion thruster. Extending the capabilities of the advanced electric propulsion diagnostics platform which has been developed by the IOM and partners, single- and two-photon absorption laser-induced fluorescence diagnostics have been set-up recently at our institute. Both experimental set-ups, and as a series of first applications, measurements of krypton neutrals in the plume of the radiofrequency ion thruster RIT-10 (ArianeGroup GmbH), and xenon neutrals within the discharge chamber of a gridded radiofrequency ion source developed at IOM, are presented.

Investigation of plasma mode transition and hysteresis in electron cyclotron resonance ion thrusters

Plasma mode-transition and hysteresis have been reported in several electron cyclotron resonance (ECR) plasma sources and have also been observed in ECR ion thrusters. From observation outside the thruster, the optical emission is significantly changed after plasma mode-transition, which indicates the electron heating, excitation and ionization processes are also changed. Therefore, to investigate these processes quantitatively, the ground-state neutral density and spontaneous emission by electron impact excitation are directly measured using two-photon absorption laser-induced fluorescence spectroscopy. This measurement system is applied to two types of thrusters. In one thruster, both mode-transition and hysteresis were observed, while in another, no hysteresis was observed due to the partial prevention of ECR heating. The experimental results indicate that the spontaneous emission sharply increases by the mode-transition and hysteresis. The tendency for this increase was relatively small with the partial prevention of ECR heating; however, the mode-transition was not deleted. Analysis of the excitation and ionization process revealed that the increase of indirect (stepwise) excitation and ionization from metastable particles can contributes to the sharp increase of the spontaneous emission. In addition, quantitative estimation of the collisional and ECR heating indicated that collisional heating cannot be neglected after the mode-transition, which indicates that collisional heating can contribute to the sharp increase due to the increase of electron temperature. The indirect ionization and collisional heating could thus be the main cause of the mode-transition and hysteresis.

Simultaneous quantification of atomic oxygen and ozone by full photo-fragmentation two-photon absorption laser-induced fluorescence spectroscopy

Atomic oxygen is one of the key reactive species in plasma chemistry and involved plasma treatments. Quantification of atomic O is essential and often accomplished by the method of two-photon absorption laser-induced fluorescence (TALIF) spectroscopy benefiting from its high resolution in time and space. However, photo-dissociation of ozone (O3), another active molecule formed commonly in O2-added plasmas, by the same UV laser often disturbs the TALIF measurement through in situ additional production of atomic O fragment. This interference of O3 fragmentation needs to be considered and separated from the plasma produced O atoms in the TALIF measurement. In this communication a novel conception benefiting from the photo-fragmentation effect of O3, is proposed for calibrating the TALIF signal of atomic oxygen in studied media. It is realized by TALIF detection of ground-state O(2p4 3P) fragment produced by fully photolyzing O3 by another synchronized 266 nm pulse laser. A robust 1:1 concentration ratio between the O(2p4 3P) fragment and photolyzed O3 is achieved, and therefore the known O3 density, e.g. from an ozonizer, can be utilized as a calibration reference for the TALIF signal of unknown-quantity O atoms in gaseous media of interested. This calibration method is straightforward to implement and simpler if same gas conditions are used in the calibration source (e.g. ozonizer) and diagnosed gaseous media, and no need for noble Xe gas. Furthermore, based on the proposed full photo-fragmentation TALIF principle, the O3 interference is able to be separated from atomic O originated from studied media, and the concentrations of O and O3 are able to be determined simultaneously if their populations are correlated with each other through kinetic chemical reactions, for instance in repetitive pulsed O2-mixed discharges. A successful exemplified diagnose by the proposed method is applied to a typical atmospheric-pressure line-to-plate pulsed-driven dielectric barrier discharge, where the time behaviors of O and O3 productions are quantified simultaneously in the post-discharge.

Modelling of turbulent reacting flow for a cold atmospheric pressure argon plasma jet

A two-dimensional, axisymmetric model of turbulent reacting flow for a cold atmospheric pressure argon plasma jet has been developed. The model is formulated within the framework of boundary-layer theory and allows to study the transport and chemical processes in the jet with low computational effort. The generation of primary reactive gas species is described using a local zero-dimensional reaction kinetics model. The proposed modelling approach is validated against available experimental data. The computations are performed for a turbulent cold plasma jet operated in argon with admixtures of oxygen, air and water. The effect of a shielding gas on the transport and chemical processes is discussed. The modelling results are compared with the results of quantitative schlieren diagnostics (for Ar), molecular-beam mass spectrometry (for Ar, N2, O2), laser-induced fluorescence (for NO), two-photon absorption laser-induced fluorescence (for O), ultraviolet absorption (for O3) and cavity ring-down (for HO2) spectroscopy measurements. It is shown that turbulent diffusion across the jet is an important factor influencing the behaviour of reactive species that are of interest for practical applications.

Measurements of Absolute Atomic Nitrogen Density by Two-Photon Absorption Laser-Induced Fluorescence Spectroscopy in Hot Air Plasma Generated by Microwave Resonant Cavity

For the first time, absolute densities of atomic nitrogen in its ground state (N4S) have been measured in hot dry and humid air plasma columns under post-discharge regime. The determination of space-resolved absolute densities leads to obtain the dissociation degrees of molecular nitrogen in the plasma. The hot plasmas are generated inside an upstream gas-conditioning cell at 600 mbar when the air gas flow is directly injected at 10 slm in a microwave resonant cavity (2.45 GHz, 1 kW) placed in the downstream side. Density measurements based on laser induced fluorescence spectroscopy with two-photon excitation (TALIF), are more particularly performed along the radial and axial positions of the plasma column. Calibration of TALIF signals is performed in situ (i.e. in the same gas-conditioning cell but without plasma) using an air gas mixture containing krypton. Optical emission spectroscopy is considered to estimate the rotational gas temperature by adding a small amount of H2 in dry air to better detect OH (A-X) spectra. The rotational temperatures in humid air plasma column (50% of humidity) are larger than those of dry air plasma column by practically 30% near the nozzle of resonant cavity on the axis of the plasma column. This is partly due to attachment heating processes initiated by water vapor. A maximum of the measured absolute nitrogen density is also observed near the nozzle which is also larger for humid air plasma column. The obtained dissociation degrees of molecular nitrogen in both dry and humid air plasma along the air plasma column are lower than the cases where only thermodynamic equilibrium is assumed. This is characteristic of the absence of chemical and energetic equilibria not yet reached in the air plasma column dominated by recombination processes.

Time-Selective TALIF Spectroscopy of Atomic Oxygen Applied to an Atmospheric Pressure Argon Plasma Jet

Two-photon laser excitation diagnostics is applied to characterize the atomic oxygen production in an atmospheric pressure Ar+O2% plasma jet. The plasma jet device is driven by a 13.56-MHz radio-frequency power supply. It is found that in addition to the fast decaying two-photon absorption laser-induced fluorescence (TALIF), unexpected emissions from excited atomic argon and oxygen species with a long decay time are existent in TALIF processes. This phenomenon is believed to be common in TALIF diagnostics of atomic oxygen, especially in high-pressure plasmas because of intense collisional energy transfer processes. In order to avoid the influence of extra emissions on TALIF performance, time-selective detection is utilized to capture the pure atomic oxygen fluorescence through nanosecond imaging for characterizing the spatial distribution of O atoms in the plasma effluent. An approximate method for the absolute density calibration based on the atomic oxygen fluorescence in the open air, due to the laser-induced photodissociation of O2 molecules, is applied to the TALIF signal from a plasma jet. The 2-D spatial distributions of the ground state density of atomic oxygen are built in the plasma effluent under different O2 contents. With the increase in O2 admixtures, the generation of atomic oxygen is constricted to the plasma core area close to the gas outlet, and a maximum O density is obtained with 0.3% O2 in the core area.

Mechanisms behind surface modification of polypropylene film using an atmospheric-pressure plasma jet

Plasma treatments are common for increasing the surface energy of plastics, such as polypropylene (PP), to create improved adhesive properties. Despite the significant differences in plasma sources and plasma properties used, similar effects on the plastic film can be achieved, suggesting a common dominant plasma constituent and underpinning mechanism. However, many details of this process are still unknown. Here we present a study into the mechanisms underpinning surface energy increase of PP using atmospheric-pressure plasmas. For this we use the effluent of an atmospheric-pressure plasma jet (APPJ) since, unlike most plasma sources used for these treatments, there is no direct contact between the plasma and the PP surface; the APPJ provides a neutral, radical-rich environment without charged particles and electric fields impinging on the PP surface. The APPJ is a RF-driven plasma operating in helium gas with small admixtures of O2 (0–1%), where the effluent propagates through open air towards the PP surface. Despite the lack of charged particles and electric fields on the PP surface, measurements of contact angle show a decrease from 93.9° to 70.1° in 1.4 s and to 35° in 120 s, corresponding to a rapid increase in surface energy from 36.4 mN m−1 to 66.5 mN m−1 in the short time of 1.4 s. These treatment effects are very similar to what is found in other devices, highlighting the importance of neutral radicals produced by the plasma. Furthermore, we find an optimum percentage of oxygen of 0.5% within the helium input gas, and a decrease of the treatment effect with distance between the APPJ and the PP surface. These observed effects are linked to two-photon absorption laser-induced fluorescence spectroscopy (TALIF) measurements of atomic oxygen density within the APPJ effluent which show similar trends, implying the importance of this radical in the surface treatment of PP. Analysis of the surface reveals a two stage mechanism for the production of polar bonds on the surface of the polymer: a fast reaction producing carboxylic acid, or a similar ketone, followed by a slower reaction that includes nitrogen from the atmosphere on the surface, producing amides from the ketones.

Absolute OH and O radical densities in effluent of a He/H2O micro-scaled atmospheric pressure plasma jet

The effluent of a micro-scaled atmospheric pressure plasma jet (μ-APPJ) operated in helium with admixtures of water vapor ( ppm) has been analyzed by means of cavity ring-down laser absorption spectroscopy and molecular beam mass spectrometry to measure hydroxyl (OH) radical densities, and by two-photon absorption laser-induced fluorescence spectroscopy to measure atomic oxygen (O) densities. Additionally, the performance of the bubbler as a source of water vapor in the helium feed gas has been carefully characterized and calibrated. The largest OH and O densities in the effluent of and , respectively, have been measured at around 6000 ppm. The highest selectivity is reached around 1500 ppm, where the OH density is at ∼63% of its maximum value and is 14 times larger than the O density. The measured density profiles and distance variations are compared to the results of a 2D axially symmetric fluid model of species transport and reaction kinetics in the plasma effluent. It is shown that the main loss of OH radicals in the effluent is their mutual reaction. In the case of O, reactions with other species than OH also have to be considered to explain the density decay in the effluent. The results presented here provide additional information for understanding the plasma-chemical processes in non-equilibrium atmospheric pressure plasmas. They also open the way to applying μ-APPJ with He/H2O as a selective source of OH radicals.

Absolute atomic oxygen density measurements for nanosecond-pulsed atmospheric-pressure plasma jets using two-photon absorption laser-induced fluorescence spectroscopy

Nanosecond-pulsed plasma jets that are generated under ambient air conditions and free from confinement of electrodes have become of great interest in recent years due to their promising applications in medicine and dentistry. Reactive oxygen species that are generated by nanosecond-pulsed, room-temperature non-equilibrium He–O2 plasma jets among others are believed to play an important role during the bactericidal or sterilization processes. We report here absolute measurements of atomic oxygen density in a 1 mm-diameter He/(1%)O2 plasma jet at atmospheric pressure using two-photon absorption laser-induced fluorescence spectroscopy. Oxygen number density on the order of 1013 cm−3 was obtained in a 150 ns, 6 kV single-pulsed plasma jet for an axial distance up to 5 mm above the device nozzle. Temporally resolved O density measurements showed that there are two maxima, separated in time by 60–70 µs, and a total pulse duration of 260−300 µs. Electrostatic modeling indicated that there are high-electric-field regions near the nozzle exit that may be responsible for the observed temporal behavior of the O production. Both the field-distribution-based estimation of the time interval for the O number density profile and a pulse-energy-dependence study confirmed that electric-field-dependent, direct and indirect electron-induced processes play important roles for O production.

Spatiotemporal behaviors of absolute density of atomic oxygen in a planar type of Ar/O2 non-equilibrium atmospheric-pressure plasma jet

A method combining of two-photon absorption laser-induced fluorescence and vacuum ultra-violet absorption spectroscopy is constructed and used to get the fine spatial distribution of O generated by a planar type of Ar/O2 non-equilibrium atmospheric-pressure plasma jet. The O density in the quasi-uniform region, which occupies about 80% of the total plasma width, is as high as 1015 cm−3. The lifetime of O is estimated to be about 360 ± 60 µs. The loss of O atoms is due to the three-body reaction of O + O2 + Ar → O3 + Ar confirmed by a simple calculation with a few rate equations. The results are very useful for the simulation of Ar/O2 plasmas, the design of large-scale planar plasma jets and the development of their potential applications.

Measurements of Absolute Atomic Oxygen Density by Two-photon Absorption Laser-induced Fluorescence Spectroscopy in Hot Air Plasma Generated by Microwave Resonant Cavity

Measurements of Absolute Atomic Oxygen Density by Two-photon Absorption Laser-induced Fluorescence Spectroscopy in Hot Air Plasma Generated by Microwave Resonant Cavity

Measurements of Absolute Atomic Oxygen Density by Two-photon Absorption Laser-induced Fluorescence Spectroscopy in Hot Air Plasma Generated by Microwave Resonant Cavity

Measurements of Absolute Atomic Oxygen Density by Two-photon Absorption Laser-induced Fluorescence Spectroscopy in Hot Air Plasma Generated by Microwave Resonant Cavity

Enhanced oxygen dissociation in a propagating constricted discharge formed in a self-pulsing atmospheric pressure microplasma jet

We report on the propagation of a constricted discharge feature in a repetitively self-pulsing microplasma jet operated in helium with a 0.075 vol% molecular oxygen admixture in ambient air environment. The constricted discharge is about 1 mm in width and repetitively ignites at the point of smallest electrode distance in a wedge-shaped electrode configuration, propagates through the discharge channel towards the nozzle, extinguishes, and re-ignites at the inlet at frequencies in the kHz range. It co-exists with a homogeneous, volume-dominated low temperature (T ⋍ 300 K) α-mode glow. Time-resolved measurements of nitrogen molecule C-state and nitrogen molecule ion B-state emission bands reveal an increase of the rotational temperature within the constricted discharge to about 600 K within 50 µs. Its propagation velocity was determined by phase-resolved diagnostics to be similar to the gas velocity, in the order of 40 m s−1. Two-photon absorption laser-induced fluorescence spectroscopy synchronized to the self-pulsing reveals spatial regions of increased oxygen atom densities co-propagating with the constricted discharge feature. The generated oxygen pulse density is about ten times higher than in the co-existing homogeneous α-mode. Densities reach about 1.5 × 1016 cm−3 at average temperatures of 450 K at the nozzle. This enhanced dissociation of about 80% is attributed to the continuous interaction of the constricted discharge to the co-propagating gas volume.

Investigation of atomic oxygen density in a capacitively coupled O2/SF6 discharge using two-photon absorption laser-induced fluorescence spectroscopy and a Langmuir probe

Two-photon absorption laser-induced fluorescence (TALIF) spectroscopy was used for detection of absolute atomic oxygen density in a low-pressure capacitively coupled plasma source. We investigated the variation of atomic oxygen density for various mixtures of O2/SF6 and report a significant five-fold increase of [O] when oxygen plasma was diluted with SF6 by only 5%. We attribute this increase in [O] to a combination of a change in surface conditions caused by constituents of SF6 plasma reacting with the reactor walls and also due to an increase in the electron temperature. Atomic oxygen production rates were determined using electron-energy distribution functions obtained with a cylindrical Langmuir probe. It was found that the effective electron temperature dramatically increased from approximately 1–8 eV as the SF6 content varied from 0% to 60% which consequently resulted in a three-fold increase in the atomic oxygen production rate. TALIF was also used to investigate the variation of [O] due to fluorination of the reactor walls and also after etching resist-coated wafers. It was found that [O] increased by over a factor of three after fluorinating the walls with SF6 plasma; on the other hand a coating formed on the reactor walls after a resist etch process resulted in a reduction of [O] by only 20%.

Effects of N2O and O2 addition to nitrogen Townsend dielectric barrier discharges at atmospheric pressure on the absolute ground-state atomic nitrogen density

Absolute ground-state density of nitrogen atoms N (2p3 4S3/2) in non-equilibrium Townsend dielectric barrier discharges (TDBDs) at atmospheric pressure sustained in N2/N2O and N2/O2 gas mixtures has been measured using Two-photon absorption laser-induced fluorescence (TALIF) spectroscopy. The quantitative measurements have been obtained by TALIF calibration using krypton as a reference gas. We previously reported that the maximum of N (2p3 4S3/2) atom density is around 3 × 1014 cm−3 in pure nitrogen TDBD, and that this maximum depends strongly on the mean energy dissipated in the gas. In the two gas mixtures studied here, results show that the absolute N (2p3 4S3/2) density is strongly affected by the N2O and O2 addition. Indeed, the density still increases exponentially with the energy dissipated in the gas but an increase in N2O and O2 amounts (a few hundreds of ppm) leads to a decrease in nitrogen atom density. No discrepancy in the order of magnitude of N (2p3 4S3/2) density is observed when comparing results obtained in N2/N2O and N2/O2 mixtures. Compared with pure nitrogen, for an energy of ∼90 mJ cm−3, the maximum of N (2p3 4S3/2) density drops by a factor of 3 when 100 ppm of N2O and O2 are added and it reduces by a factor of 5 for 200 ppm, to reach values close to our TALIF detection sensitivity for 400 ppm (1 × 1013 cm−3 at atmospheric pressure).

Investigation of Surface Etching of Poly(Ether Ether Ketone) by Atmospheric-Pressure Plasmas

An atmospheric-pressure argon plasma jet with varying admixtures of molecular oxygen was used to study the etching mechanism of poly(ether ether ketone) (PEEK). Furthermore, a correlation between plasma-based etching processes on PEEK with the generation of chemically reactive plasma species is proposed. The surface analysis was performed by X-ray photoelectron spectroscopy, atomic force microscopy, and surface profilometry which showed a dramatic increase in the content of oxygen functionalities and surface roughness after long-time Ar/O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -plasma treatment. For the plasma diagnostics, two-photon absorption laser-induced fluorescence spectroscopy was applied. The obtained etching mass as well as the surface roughness for different molecular oxygen admixtures revealed a strong dependence on the atomic-oxygen density. Furthermore, the radial surface profile, affected by plasma etching, might be attributed to the distribution of plasma-generated oxygen species in the plasma jet effluent.

Atomic oxygen in a cold argon plasma jet: TALIF spectroscopy in ambient air with modelling and measurements of ambient species diffusion

By investigating the atomic oxygen density in its effluent, two-photon absorption laser-induced fluorescence (TALIF) spectroscopy measurements are for the first time performed in a cold argon/oxygen atmospheric pressure plasma jet. The measurements are carried out in ambient air and quenching by inflowing air species is considered. We propose a novel absorption technique in the VUV spectral range, where emission originating from within the discharge is used as light source to determine the inflow of atmospheric oxygen into the effluent. Furthermore, we propose a modelling solution for the on-axis density of inflowing ambient air based on the stationary convection–diffusion equation.

Spectroscopic characterization of an atmospheric pressure μ-jet plasma source

A new method for determination of plasma parameters under atmospheric pressure conditions is formulated and applied for characterization of a radio-frequency μ-jet plasma source using He/O2 mixture. By applying absolutely calibrated optical emission spectroscopy and numerical simulation, the gas temperature in the active plasma region and plasma parameters (electron density and electron distribution function) are determined. The steady-state concentrations of different species such as oxygen atom and ozone in the plasma channel and in the effluent of the plasma source are calculated using measured plasma parameters and gas temperature. On the other hand, spatial distribution of steady-state densities of these species are measured using emission and absorption spectroscopy. A comparison of the results thus obtained and the validation of the new method against two-photon absorption laser-induced fluorescence spectroscopy measurements are discussed. In addition, the influence of the surface processes and gas flow regime on the loss of the active species in the plasma source are discussed.