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.

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.

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.

Micro plasmas are operated around atmospheric pressure exhibiting pronounced non-equilibrium characteristics, i.e. they possess energetic electrons while ions and neutrals remain cold. They have gained significant interest due to their enormous application potential e.g. in the biomedical, surface modification and light source areas, just to name a few. Many different configurations are in use. Their understanding and quantification is mandatory for further progress in applications. We report on recent progress in the diagnostics and simulation of the entire micro plasma system from gas introduction, via the plasma discharge up to the samples at the example of a plasma jet operated in He/O2 in an ambient air environment.

The ascent of atomic oxygen densities created inside the micro-scaled atmospheric pressure plasma jet has been investigated spatially resolved under parameter variations such as applied power, gas mixture and gas velocity using two-photon absorption laser induced fluorescence spectroscopy.Along the discharge channel an increase of the atomic oxygen density within the plasma is observed. The density shows an exponentially asymptotic convergence into an equilibrium close to the effluent. In the post-discharge effluent an exponential spatial decrease can be found. Typical ascent distances of a few hundreds ofμm decrease with the applied power and increase with gas velocity and oxygen admixture. The maximum atomic oxygen density increases with applied power and admixed molecular oxygen up to more than1016 cm-3. An increase of the maximum atomic oxygen density with increasing gas velocity has been found. Optical emission spectroscopy measurements indicate a strong increase of the nitrogen emission at low gas flow rates along the channel.

The effluent of a microscale atmospheric pressure plasma jet (mu-APPJ) operated in helium with a small admixture of molecular oxygen (< 1.6%) has been analyzed by means of two independent diagnostics, quantitative molecular beam mass spectrometry (MBMS) and two-photon absorption laser-induced fluorescence spectroscopy (TALIF). The atomic oxygen density, the ozone density and the depletion of molecular oxygen have been measured by MBMS and the atomic oxygen density has been validated by TALIF. Absolute atomic oxygen densities in the effluent up to 4.7x10(15) cm(-3) could be measured with a very good agreement between both diagnostics. In addition, ozone densities in the effluent up to 1.4x10(15) cm(-3) and an O-2 depletion up to 10% could be measured by MBMS. The atomic oxygen density shows a maximum value at an O-2 admixture of 0.6%, whereas the ozone density continues to increase toward higher O-2 admixtures. With increasing distance from the jet, the atomic oxygen density decreases but is still detectable at a distance of 30 mm. The ozone density increases with distance, saturating at a distance of 40 mm. By applying higher powers to the mu-APPJ, the atomic oxygen density increases linearly whereas the ozone density exhibits a maximum.

Atomic oxygen formation in a radio-frequency driven micro-atmospheric pressure plasma jet is investigated using both advanced optical diagnostics and numerical simulations of the dynamic plasma chemistry. Laser spectroscopic measurements of absolute densities of ground state atomic oxygen reveal steep gradients at the interface between the plasma core and the effluent region. Spatial profiles resolving the interelectrode gap within the core plasma indicate that volume processes dominate over surface reactions. Details of the production and destruction processes are investigated in numerical simulations benchmarked by phase-resolved optical emission spectroscopy. The main production mechanisms are electron induced and hence most efficient in the vicinity of the plasma boundary sheath, where electrons are energized. The destruction is driven through chemical heavy particle reactions. The resulting spatial profile of atomic oxygen is relatively flat. The power dependence of the atomic oxygen density obtained by the numerical simulation is in very good agreement with the laser spectroscopic measurements.

Atmospheric pressure plasma jet sources are currently inthe focus of many researchers for their promising applications inmedical industry (e.g. treatment of living tissues), surfacemodification or material etching or synthesis. Here we report onthe study of excitation mechanisms of a coaxial microplasma jetwith a hollow capillary as an inner electrode and a ceramic tubewith metal ring as outer electrode. This microplasma jet isoperated in He and Ar gas and it is investigated by means ofelectrical measurements, optical emission spectroscopy and spaceand phase resolved wavelength integrated optical spectroscopy.Measurements of a microscale atmospheric pressure plasma jet withparallel metal electrodes operated in He are shown for comparisonas well. Four different modes are distinguished with He as plasmaforming gas. The α discharge in annular space between theelectrodes, observed at low applied voltages, is very similar tothe discharge in the jet with parallel electrodes. As the voltageincreases a γ discharge appears, first localized at the tipof the capillary. As the voltage increases further the γdischarge appears in the annular space as well. A hollow cathodeplasma is observed at any voltage used on the symmetry axis of thejet. Only one mode of plasma operation is observed in argon gaswith distinctively different behavior. We hypothesize that it iscomparable to a single microdischarge of a filamentary dielectricbarrier discharge.

We present clear evidence that ground state atomic oxygen shows two patterns near a surface in the helium plasma needle discharge. Two-photon absorption laser-induced fluorescence spectroscopy, combined with gas flow simulation, was employed to obtain spatially-resolved ground state atomic oxygen densities. When the feed gas flow rate is low, the radial density peaks along the axis of the needle. At high flow rate, a ring-shaped density distribution appears. The peak density is on the order of 1021 m−3 in both cases. The results are consistent with a previous report of the flow-dependent bacterial killing pattern observed under similar conditions.

Inside a miniaturized capacitively coupled cold atmospheric pressure plasma jet operated at a helium base gas flow with a minor molecular oxygen admixture atomic oxygen is created. The build up of atomic oxygen along the discharge channel and its further decay in the effluent is investigated by means of xenon calibrated two photon laser induced fluorescence spectroscopy (TALIF). The longitudinal and the transversal atomic oxygen distribution is measured from the discharge core through the transition area into the effluent. A particular emphasis is set on the influence of collisional quenching at elevated pressures.

Diagnostic-based modeling (DBM) actively combines complementary advantages of numerical plasma simulations and relatively simple optical emission spectroscopy (OES). DBM is applied to determine spatial absolute atomic oxygen ground-state density profiles in a micro atmospheric-pressure plasma jet operated in He–O2. A 1D fluid model with semi-kinetic treatment of the electrons yields detailed information on the electron dynamics and the corresponding spatio-temporal electron energy distribution function. Benchmarking this time- and space-resolved simulation with phase-resolved OES (PROES) allows subsequent derivation of effective excitation rates as the basis for DBM. The population dynamics of the upper O(3p3P) oxygen state (λ = 844 nm) is governed by direct electron impact excitation, dissociative excitation, radiation losses, and collisional induced quenching. Absolute values for atomic oxygen densities are obtained through tracer comparison with the upper Ar(2p1) state (λ = 750.4 nm). The resulting spatial profile for the absolute atomic oxygen density shows an excellent quantitative agreement to a density profile obtained by two-photon absorption laser-induced fluorescence spectroscopy.

Diagnostic based modelling (DBM) actively combines complementary advantages of numerical plasma simulations and relatively simple optical emission spectroscopy (OES). DBM is employed to determine absolute atomic oxygen ground state densities in a helium-oxygen radio-frequency driven atmospheric pressure plasma jet. A comparatively simple one-dimensional simulation yields detailed information on electron properties governing the population dynamics of excited states. Important characteristics of the electron dynamics are found to be largely insensitive to details of the chemical composition and to be in very good agreement with space and phase-resolved OES. Benchmarking the time and space resolved simulation allows us to subsequently derive effective excitation rates as the basis for DBM with simple space and time integrated OES. The population dynamics of the upper O 3p P-3 (lambda = 844 nm) atomic oxygen state is governed by direct electron impact excitation, dissociative excitation, radiation losses and collisional induced quenching. Absolute values for atomic oxygen densities are obtained through tracer comparison with the upper Ar 2p(1) (lambda = 750.4 nm) state. The presented results for the atomic oxygen density show excellent quantitative agreement with independent two-photon laser-induced fluorescence measurements.

The micro atmospheric pressure plasma jet is an rf driven (13.56 MHz, ∼20 W) capacitively coupled discharge producing a homogeneous plasma at ambient pressure when fed with a gas flow of helium (1.4 slm) containing small admixtures of oxygen (∼0.5%). The design provides excellent optical access to the plasma core. Ground state atomic oxygen densities up to 3×1016 cm−3 are measured spatially resolved in the discharge core by absolutely calibrated two-photon absorption laser-induced fluorescence spectroscopy. The atomic oxygen density builds up over the first 8 mm of the discharge channel before saturating at a maximum level. The absolute value increases linearly with applied power.

Despite enormous potential for technological applications, fundamentals of stable non-equilibrium micro-plasmas at ambient pressure are still only partly understood. Micro-plasma jets are one sub-group of these plasma sources. For an understanding it is particularly important to analyse transport phenomena of energy and particles within and between the core and effluent of the discharge. The complexity of the problem requires the combination and correlation of various highly sophisticated diagnostics yielding different information with an extremely high temporal and spatial resolution. A specially designed rf microscale atmospheric pressure plasma jet (mu APPJ) provides excellent access for optical diagnostics to the discharge volume and the effluent region. This allows detailed investigations of the discharge dynamics and energy transport mechanisms from the discharge to the effluent. Here we present examples for diagnostics applicable to different regions and combine the results. The diagnostics applied are optical emission spectroscopy (OES) in the visible and ultraviolet and two-photon absorption laser-induced fluorescence spectroscopy. By the latter spatially resolved absolutely calibrated density maps of atomic oxygen have been determined for the effluent. OES yields an insight into energy transport mechanisms from the core into the effluent. The first results of spatially and phase-resolved OES measurements of the discharge dynamics of the core are presented.

The coplanar microscale atmospheric pressure plasma jet (mu-APPJ) is a capacitively coupled radio frequency discharge (13.56 MHz, similar to 15W rf power) designed for optimized optical diagnostic access. It is operated in a homogeneous glow mode with a noble gas flow (1.4 slm He) containing a small admixture of molecular oxygen (similar to 0.5%). Ground state atomic oxygen densities in the effluent up to 2 x 10(14) cm(-3) are measured by two-photon absorption laser-induced fluorescence spectroscopy (TALIF) providing space resolved density maps. The quantitative calibration of the TALIF setup is performed by comparative measurements with xenon. A maximum of the atomic oxygen density is observed for 0.6% molecular oxygen admixture. Furthermore, an increase in the rf power up to about 15W (depending on gas flow and mixture) leads to an increase in the effluent's atomic oxygen density, then reaching a constant level for higher powers.