H and O atom detection for combustion applications: study of quenching and laser photolysis effects

Progresses on the Use of Two-Photon Absorption Laser Induced Fluorescence (TALIF) Diagnostics for Measuring Absolute Atomic Densities in Plasmas and Flames

Recent developments in plasma science and technology have opened new areas of research both for fundamental purposes (e.g., description of key physical phenomena involved in laboratory plasmas) and novel applications (material synthesis, microelectronics, thin film deposition, biomedicine, environment, flow control, to name a few). With the increasing availability of advanced optical diagnostics (fast framing imaging, gas flow visualization, emission/absorption spectroscopy, etc.), a better understanding of the physicochemical processes taking place in different electrical discharges has been achieved. In this direction, the implementation of fast (ns) and ultrafast (ps and fs) lasers has been essential for the precise determination of the electron density and temperature, the axial and radial gradients of electric fields, the gas temperature, and the absolute density of ground-state reactive atoms and molecules in non-equilibrium plasmas. For those species, the use of laser-based spectroscopy has led to their in situ quantification with high temporal and spatial resolution, with excellent sensitivity. The present review is dedicated to the advances of two-photon absorption laser induced fluorescence (TALIF) techniques for the measurement of reactive species densities (particularly atoms such as N, H and O) in a wide range of pressures in plasmas and flames. The requirements for the appropriate implementation of TALIF techniques as well as their fundamental principles are presented based on representative published works. The limitations on the density determination imposed by different factors are also discussed. These may refer to the increasing pressure of the probed medium (leading to a significant collisional quenching of excited states), and other issues originating in the high instantaneous power density of the lasers used (such as photodissociation, amplified stimulated emission, and photoionization, resulting to the saturation of the optical transition of interest).

Advances in imaging of chemically reacting flows

Many important chemically reacting systems are inherently multi-dimensional with spatial and temporal variations in the thermochemical state, which can be strongly coupled to interactions with transport processes. Fundamental insights into these systems require multi-dimensional measurements of the thermochemical state as well as fluid dynamics quantities. Laser-based imaging diagnostics provide spatially and temporally resolved measurements that help address this need. The state of the art in imaging diagnostics is continually progressing with the goal of attaining simultaneous multi-parameter measurements that capture transient processes, particularly those that lead to stochastic events, such as localized extinction in turbulent combustion. Development efforts in imaging diagnostics benefit from advances in laser and detector technology. This article provides a perspective on the progression of increasing dimensionality of laser-based imaging diagnostics and highlights the evolution from single-point measurements to 1D and 2D multi-parameter imaging and 3D high-speed imaging. This evolution is demonstrated using highlights of laser-based imaging techniques in combustion science research as an exemplar of a complex multi-dimensional chemically reacting system with chemistry-transport coupling. Imaging diagnostics impact basic research in other chemically reacting systems as well, such as measurements of near-surface gases in heterogeneous catalysis. The expanding dimensionality of imaging diagnostics leads to larger and more complex datasets that require increasingly demanding approaches to data analysis and provide opportunities for increased collaboration between experimental and computational researchers in tackling these challenges.

Effect of discharge polarity on the propagation of atmospheric-pressure helium plasma jets and the densities of OH, NO, and O radicals

The atmospheric-pressure helium plasma jet is an emerging technology for plasma biomedical applications. In this paper, the authors focus on the effect of discharge polarity on propagation of the discharge and the densities of OH, NO, and O radicals. The plasma jet is applied to a glass surface placed on a grounded metal plate. Positive or negative voltage pulses with 25 μs duration, 8 kV amplitude, and 10 kpps repetition rate are used for the plasma jet. The plasma propagation is measured using a short-gated ICCD camera. The light emission intensity of the discharge generated at the rising phase of the voltage pulse is approximately equivalent for both polarities, while that generated during the falling phase is much higher for the negative discharge than the positive one. The shape of the discharge changes with the discharge polarity. The OH, NO, and O densities in the plasma jet are also measured for both polarities. It is found that the OH density is almost the same regardless the discharge polarity. Conversely, the negative discharge produces more O atoms and the positive discharge produces more NO molecules. These results indicate that the polarity of the discharge affects the densities of some reactive species produced in the plasma jet.

Comparison of nanosecond and picosecond excitation for two-photon laser-induced fluorescence imaging of atomic oxygen in flames

Two-photon laser-induced fluorescence (LIF) imaging of atomic oxygen is investigated in premixed hydrogen and methane flames with nanosecond and picosecond pulsed lasers at 226 nm. In the hydrogen flame, the interference from photolysis is negligible compared with the LIF signal from native atomic oxygen, and the major limitations on quantitative measurements are stimulated emission and photoionization. Excitation with a nanosecond laser is advantageous in the hydrogen flames, because it reduces the effects of stimulated emission and photoionization. In the methane flames, however, photolytic interference is the major complication for quantitative O-atom measurements. A comparison of methane and hydrogen flames indicates that vibrationally excited CO2 is the dominant precursor for laser-generated atomic oxygen. In the methane flames, picosecond excitation offers a significant advantage by dramatically reducing the photolytic interference. The prospects for improved O-atom imaging in hydrogen and hydrocarbon flames are presented.

Photochemical effect in two-photon laser-induced fluorescence detection of carbon monoxide in hydrocarbon flames

The CO formation as a result of the CO(2) photodissociation at 230.08 nm was observed by using the two-photon laser-induced fluorescence (LIF) method. The measurements were performed in a propane-air combustion product flow and in mixtures of CO(2) and O(2). The temperature dependence of the fluorescence signal caused by CO molecules, produced in the photodissociation of CO(2) molecules under the action of laser radiation at a wavelength of 230.08 nm, was measured at temperatures ranging from 1300 to 2000 K. It is shown that consideration of CO(2) photodissociation under the action of the probing radiation is necessary when one applies the two-photon LIF method for the measurement of small CO concentrations in high-temperature gas mixtures containing CO(2). As an example, a correction is given of the CO concentration profiles measured by the LIF method in the combustion product flow around a cooled metallic plate.

Application of two-photon laser-induced fluorescence for visualization of water vapor in combustion environments

<p>Investigations concerning the potential for the visualization of water vapor in combustion processes have been made. The water molecules were excited through a two-photon excitation process at 248 nm with a tunable excimer laser; this was followed by fluorescence detection between ~ 400 and 500 nm. In the experimental work special care was taken to map the possible spectral interferences from hot O(2), which also absorbs in the same spectral region and which produces fluorescence emission that interferes with the water fluorescence. Experimental investigations of high-pressure applications are also presented.</p><p>Finally, two-dimensional (2-D) measurements made at room temperature, taken in an atmosphericpressure flame, and taken in an engine simulator at elevated pressure are presented. These results indicate that the detection limit for 2-D single-shot registrations under optimized experimental conditions was estimated to 0.2% at atmospheric pressure and at room temperature. Extrapolations to flame conditions are also presented.</p>

Detection of nitrogen atoms in flames using two-photon laser-induced fluorescence and investigations of photochemical effects

Nitrogen atoms, in a flame or produced by photodissociation of nitrogen-containing molecules, were excited through a two-photon absorption process at 211 nm, and the near IR fluorescence was detected. Both in flame and in photodissociation experiments of cold gases, fluorescence, which was resonantly enhanced at the atomic nitrogen wavelength but which originated from excited oxygen atoms, was also identified. This phenomenon, the photochemical effects and those of molecular absorption, stimulated emission, and the mixing of gases are discussed in connection with the detection of atomic nitrogen in flames.

Two-photon excited LIF determination of H-atom concentrations near a heated filament in a low pressure H(2)environment

With respect to the investigation of low pressure filament-assisted chemical vapor deposition processes for diamond formation, absolute concentrations of atomic hydrogen were determined by two-photon laserinduced fluorescence in the vicinity of a heated filament in an environment containing H(2) or mixtures of H(2)and CH(4). Radial H concentration profiles were obtained for different pressures and filament temperatures, diameters, and materials. The influence of the addition of various amounts of methane on the H atom concentrations was examined.

Simultaneous multiple species detection in a flame using laser-induced fluorescence: Errata

An approach for simultaneous detection of NO, OH and O in flames using laser-induced fluorescence is presented. The technique is based on spectral coincidences using a Nd:YAG-based laser system producing a frequency-doubled and frequency-mixed laser beam at 287 and 226 nm, respectively. The possibility of making spatially resolved measurements using a diode-array detector was also investigated. The use of this technique for studying potential laser-induced disturbances was also demonstrated.

Laser-induced fluorescence measurement of oxygen atoms above a catalytic combustor surface

Two-photon laser-induced fluorescence has been used to detect oxygen atoms in the gas phase above a heated, catalytically stabilized combustor. Excitation was at 226 nm and detection at 777 nm. Special care using the collection optics was necessary to avoid detector saturation from intense thermal emission background from the heated plate. The experimental configuration together with considerations of quenching collisions and photochemical production of oxygen atoms are described.

Laser-induced fluorescence of O(3p(3)P), O(2), and NO near 226 nm: photolytic interferences and simultaneous excitation in flames

Spectral overlaps between O atom two-photon transitions and one-photon O(2) and NO transitions near 226 nm have been studied. Excitation of photodissociating O(2) levels can cause anomalously high concentration population measurements of O atoms in flames. We find that excitation of O (3)P(1) will minimize the photochemical interference, and the data presented here are necessary to quantify the extent of such effects. Alternatively, these overlaps can be exploited to obtain simultaneous laser-induced fluorescence detection of O, O(2), and NO with a single laser.

Simultaneous multiple species detection in a flame using laser-induced fluorescence

An approach for simultaneous detection of NO, OH and O in flames using laser-induced fluorescence is presented. The technique is based on spectral coincidences using a Nd:YAG-based laser system producing a frequency-doubled and frequency-mixed laser beam at 287 and 226 nm, respectively. The possibility of making spatially resolved measurements using a diode-array detector was also investigated. The use of this technique for studying potential laser-induced disturbances was also demonstrated.

Doppler-free laser-induced fluorescence of oxygen atoms in an atmospheric-pressure flame

Doppler-free laser-induced fluorescence of oxygen atoms has been measured in the burnt gases of an atmospheric-pressure flame, where the Doppler and collisional widths are similar. The 3p(3)P level was excited using counterpropagating beams at 226 nm. Upper-state fine-structure splitting could be partially resolved, and a collisionally broadened Lorentzian width of 0.42 cm(-1) was measured. This corresponds to a broadening cross section of 10(-13) cm(2), which is larger than that expected for collisional quenching alone.

Photochemical effects in two-photon-excited fluorescence detection of atomic oxygen in flames

This paper describes photochemical effects observed using 226-nm two-photon-excited fluorescence detection to measure the atomic oxygen concentration in hydrogen-oxygen flames. In a study of a lean atmospheric-pressure flame, we observed artificially high atomic-oxygen concentration levels in the postflame gases using all but the most gentle excitation conditions (intensities greater than ~0.1 GW/cm(2)). A similar study of a lean low-pressure (72-Torr) flame showed little evidence of photochemical production of atomic oxygen. Using a second laser system in a pump-probe configuration, with the probe laser monitoring the atomic oxygen concentration in a very lean flame while the pump laser was scanned across molecular-oxygen Schumann-Runge bands at 221 nm, we demonstrated that excess atomic oxygen concentrations can be produced by single-photon excitation of these bands in vibrationally excited oxygen molecules present in the flame. This production mechanism explains at least part of the artificially high concentration levels observed in the atmospheric-pressure flame.

Absolute concentration measurements of atomic hydrogen in subatmospheric premixed H(2)/O(2)/N(2) flat flames with photoionization controlled-loss spectroscopy

An experimental protocol, using photoionization controlled-loss spectroscopy (PICLS), has been developed for obtaining absolute number densities of atomic hydrogen from laser-induced fluorescence measurements in flames. Two laser beams are employed, the first to excite hydrogen atoms from the ground state to the second excited state via two-photon absorption and the second to strongly photoionize the excited atoms. The resulting fluorescence measurements are independent of quenching. A model is presented that assures the viability of PICLS as long as the photoionization rate is greater than or equal to the quenching rate. The model is verified in fuel-lean, stoichiometric, and fuel-rich flat premixed H(2)/O(2)/N(2) flames at pressures of 20 and 72 Torr. Over this range in pressure, the ratio of number densities obtained from PICLS to those calculated from partial equilibrium is constant to within 20%. Most of the error arises from the sensitivity of the partial equilibrium calculat ions to small uncertainties in both the fuel-oxidizer ratio and the measured OH concentration. Because of the quenching-independent nature of PICLS, quantitative fluorescence measurements can be made by calibrating at a single favorable flame condition.