Two-photon Absorption Laser-induced Fluorescence Spectroscopy Research Articles

Modeling and experimental study of molecular nitrogen dissociation in an Ar–N2 ICP discharge

The dissociation of nitrogen molecules in an Ar–N2 inductively coupled plasma (ICP) discharge is studied both experimentally and theoretically. To measure the absolute N atom density and emission intensity of Ar and N2 excited levels, two-photon absorption laser-induced fluorescence (TALIF) spectroscopy and optical emission spectroscopy are used. We observe an increase in N atom density with increasing pressure whereas the N atom density decreases for pressures higher than 100 mTorr in a pure nitrogen discharge. On adding argon to the mixture, we observe that the dissociation rate is enhanced when going from a pure nitrogen discharge to an argon mixed discharge. To calculate the plasma parameters, a global (volume-averaged) model is developed. The variation of the electron temperature and the particle densities are calculated by solving the particle and energy balance equations. The model calculations are compared with the measurement results and the production and loss rates of each species are described under each discharge condition. From the model calculation, the dissociation of N2 molecules in the Ar–N2 mixed discharge occurs mainly by electron impact dissociation at low pressures, while at high pressures the dissociative recombination is enhanced by charge transfer between Ar+ and N2(X) as well as metastable–metastable pooling dissociation due to the high density. In addition, the surface sticking coefficient of nitrogen atoms in a planar ICP discharge (including glass and stainless steel walls) is deduced from TALIF measurements and is estimated to be 0.02 under our set-up conditions.

Gas flow dependence of ground state atomic oxygen in plasma needle discharge at atmospheric pressure

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.

Diagnostic-based modeling on a micro-scale atmospheric-pressure plasma jet

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.

Characterization of the effluent of a He/O2 microscale atmospheric pressure plasma jet by quantitative molecular beam mass spectrometry

The effluent of a microscale atmospheric pressure plasma jet (μ-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 4.7×1015 cm-3 could be measured with a very good agreement between both diagnostics. In addition, ozone densities in the effluent up to 1.4×1015 cm-3 and an O2 depletion up to 10% could be measured by MBMS. The atomic oxygen density shows a maximum value at an O2 admixture of 0.6%, whereas the ozone density continues to increase toward higher O2 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 μ-APPJ, the atomic oxygen density increases linearly whereas the ozone density exhibits a maximum.

Characterization of an N2 flowing microwave post-discharge by OES spectroscopy and determination of absolute ground-state nitrogen atom densities by TALIF

A flowing microwave post-discharge source sustained at 2.45 GHz in pure nitrogen has been investigated by optical emission spectroscopy (OES) and two-photon absorption laser-induced fluorescence (TALIF) spectroscopy. Variations of the optical emission along the post-discharge (near, pink and late afterglow) have been studied and the gas temperature has been determined. TALIF spectroscopy has been used in the late afterglow to determine the absolute ground-state nitrogen atomic densities using krypton as a reference gas. Measurements show that the microwave flowing post-discharge is an efficient source of N (4S) atoms in late afterglow. In our experimental conditions, the maximum N (4S) density is about 2.2 × 1015 cm−3 for a pressure of 22 Torr, at 300 K. The decay of N (4S) density as a function of the time spent in the quartz tube has been modelled and a wall recombination probability γ of (2.1 ± 0.3) × 10−4 is obtained.

Absolute atomic oxygen density profiles in the discharge core of a microscale atmospheric pressure plasma jet

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.

Absolute atomic oxygen density distributions in the effluent of a microscale atmospheric pressure plasma jet

The coplanar microscale atmospheric pressure plasma jet (μ-APPJ) is a capacitively coupled radio frequency discharge (13.56 MHz, ∼15 W 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 (∼0.5%). Ground state atomic oxygen densities in the effluent up to 2 × 1014 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 15 W (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.

Spatially resolved diagnostics on a microscale atmospheric pressure plasma jet

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 (μ-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.

Direct measurements of neutral density depletion by two-photon absorption laser-induced fluorescence spectroscopy

The ground state density of xenon atoms has been measured by spatially resolved laser-induced fluorescence spectroscopy with two-photon excitation in the diffusion chamber of a magnetized Helicon plasma. This technique allows the authors to directly measure the relative variations of the xenon atom density without any assumptions. A significant neutral gas density depletion was measured in the core of the magnetized plasma, in agreement with previous theoretical and experimental works. It was also found that the neutral gas density was depleted near the radial walls.

Determination of the degree of dissociation in an inductively coupled hydrogen plasma using optical emission spectroscopy and laser diagnostics

The knowledge of absolute concentrations of radicals in molecular discharges, such as the density of atomic hydrogen in H2 plasmas, is crucial for understanding processes at the surface and is, therefore, of great interest in a variety of plasma applications. Reliable spectroscopic methods are indispensable for the determination of the degree of dissociation in processing plasmas. In this paper, we describe a novel optical emission spectroscopy method called two-gas actinometry (TGA). It is based on using a second actinometer gas to take into account dissociative excitation, which is of particular importance at comparatively low degrees of dissociation. TGA is employed for measuring the characteristic dependence of the degree of dissociation, in a wide range of plasma parameters, in an inductively coupled radio-frequency hydrogen plasma. Absolute densities with high accuracy are obtained using two-photon absorption laser-induced fluorescence spectroscopy.

Absolute atomic oxygen density measurements by two-photon absorption laser-induced fluorescence spectroscopy in an RF-excited atmospheric pressure plasma jet

The atmospheric pressure plasma jet is a capacitively coupled radio frequency discharge (13.56 MHz) running with a high helium flux (2 m3 h−1) between concentric electrodes. Small amounts (0.5%) of admixed molecular oxygen do not disturb the homogeneous plasma discharge. The jet effluent leaving the discharge through the ring-shaped nozzle contains high concentrations of radicals at a low gas temperature—the key property for a variety of applications aiming at treatment of thermally sensitive surfaces. We report on absolute atomic oxygen density measurements by two-photon absorption laser-induced fluorescence (TALIF) spectroscopy in the jet effluent. Calibration is performed with the aid of a comparative TALIF measurement with xenon. An excitation scheme (different from the one earlier published) providing spectral matching of both the two-photon resonances and the fluorescence transitions is applied.

Detection of H Atoms in RF-Discharge SiH4, CH4 and H2 Plasmas by Two-Photon Absorption Laser-Induced Fluorescence Spectroscopy

Hydrogen atoms were detected quantitatively by a two-photon absorption laser-induced fluorescence technique in capacitively coupled and inductively coupled RF-discharge plasmas. In a parallel-plate RF-discharge reactor the density of H atoms measured in a SiH4 plasma diluted with Ar was higher than that diluted with H2. A comparison of the densities in SiH4, CH4 and H2 plasmas measured by changing dilution ratio with Ar also showed that H2 molecules were not easily decomposed and H atoms were mostly produced from decomposition of source SiH4 and CH4 molecules at low RF-power level. Spatial distribution of H atoms effused from an inductively coupled RF-discharge radical source was also measured. Its axial and radial profiles showed the expansion of H atoms towards the chamber wall by diffusion and gas flow. The absolute calibration of H atom density was carried out by a comparison with absorption of Lyα line at 121.6 nm.