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

Significantly Improved Detection of Molecular Oxygen by Two-Color Resonance-Enhanced Multiphoton Ionization

We report a new spectroscopic detection scheme for molecular oxygen that achieves roughly two orders of magnitude higher sensitivity for fully rotationally resolved spectra than the current state of the art. Two-color (2 + 1') resonance-enhanced multiphoton ionization (REMPI) via the 3d Rydberg complex yields state-selective spectra with signal comparable to the intense but diffuse C 3sσ 3Πg ← X 3Σg- (2 + 1) REMPI bands without significant saturation or broadening. The resulting increase in sensitivity permitted observation of the very weak 3dπ 1Δ2 ← X 3Σg- transitions and is independent of the intermediate state. This advance in ionization efficiency and quantum state-selective sensitivity for O2 promises to aid physical and chemical studies across a wide variety of fields.

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).

Measurement of nitric oxide concentrations in flames by using electronic-resonance-enhanced coherent anti-Stokes Raman scattering

We have measured nitric oxide (NO) concentrations in flames by using electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy (ERE-CARS). Visible pump and Stokes beams were tuned to a Q-branch vibrational Raman resonance of NO. A UV probe beam was tuned into resonance with specific rotational transitions in the (v"=1,v'=0) vibrational band in the A(2)Sigma(+)-X(2)Pi electronic transition, thus providing a substantial electronic-resonance enhancement of the resulting CARS signal. NO concentrations were measured at levels down to 50 parts in 10(6) in H(2)/air flames at atmospheric pressure. NO was also detected in heavily sooting C(2)H(2)/air flames at atmospheric pressure with minimal background interference.

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.

Disturbance of laser-induced-fluorescence measurements of NO in methane-air flames containing chlorinated species by photochemical effects induced by 225-nm-laser excitation

Laser-induced fluorescence measurements of NO in CH(4)-air flames seeded with CH(3)Cl and CH(2)Cl(2) are described. The measurements are perturbed by strong photochemical effects characterized by UV emissions. The contribution of these background emissions is taken into account on the basis of an on-line-off-line resonance procedure. First results indicate an important increase of NO in the presence of chlorinated species. Background emissions observed in the range 220-260 nm and at 278 nm are ascribed, respectively, to electronically excited HCl and CCl photofragments. It is shown that C(2)H(3)Cl and CHCl(2) species are responsible for the formation of HCl and CCl, respectively, and a photolytic mechanism for formation of the photofragments is proposed.

Temperature dependence of laser-induced fluorescence of nitric oxide in laminar premixed atmospheric-pressure flames

The temperature dependence of laser-induced NO A (2)?(+)-X (2)? fluorescence in the hot gases of natural gas-air flames, seeded with known quantities of NO, has been determined experimentally by means of a difference method. The flame temperature at three fixed equivalence ratios was changed when the mixture velocity was varied through a water-cooled, flat-flame burner and was measured by coherent anti-Stokes Raman spectroscopy. When the possible reburning of part of the seeded NO is allowed for, the results in the range 1700-2150 K are best described by the temperature dependence obtained from a model in which quenching corrections are neglected, as in the case of a saturated two-level system, when millijoule pulse energies are used. Measurements of the fluorescence intensity at constant seed concentration as a function of equivalence ratio between 0.75 and 1.3 also indicate that quenching corrections are unnecessary under these excitation conditions. Using the measured intensities of the seeded flame as a calibration factor, we determined the absolute NO concentrations as functions of the equivalence ratio at 1 cm above the burner. The results indicate that, with the calibration method presented here, a relative accuracy of 5% should be obtainable.

Background corrections for laser-induced-fluorescence measurements of nitric oxide in lean, high-pressure, premixed methane flames

An experimental technique is presented that both minimizes and accounts for the interference background when laser-induced-fluorescence (LIF) measurements are made of NO in lean, high-pressure, premixed, CH(4)/O(2)/N(2) flames. Measurement interferences such as fluorescence and Raman scattering from secondary species become increasingly important for high-pressure LIF studies. O(2) fluorescence interferences are particularly troublesome in lean premixed flames. An excitation-detection scheme that minimizes the effects of these interferences is identified. A procedure that corrects the resulting LIF signal so as to account for any remaining interference signal is then developed. This correction is found to vary from less than 10% of the overall NO signal at atmospheric pressure to over 40% of the overall signal at 14.6 atm for LIF measurements of NO in a series of worst-case flames (phi = 0.6, dilution ratio 2.2). The correction technique is further demonstrated to be portable over a useful range of flame conditions at each pressure.

Experimental assessment of O(2) interferences on laser-induced fluorescence measurements of NO in high-pressure, lean premixed flames by use of narrow-band and broadband detection

We experimentally investigate the influence of O(2) interferences on laser-induced fluorescence measurements of NO in lean methane-fueled flames at a range of pressures for both narrow-band and broadband fluorescence detection. We identify NO excitation schemes that minimize O(2) interferences. From detection scans we obtain interference spectra for the different NO excitation schemes. We then identify optimum excitation-detection schemes for narrow-band detection measurements of NO. To simulate broadband detection experiments, we numerically apply five different filter combinations to the experimentally obtained detection scans. We develop filter-assessment parameters to judge the effectiveness of the different filtering schemes, and we establish a methodology for evaluating broadband excitation-detection strategies. From this research we identify optimum excitation-detection schemes for broadband detection measurements of NO.

Single-pulse, two-line temperature-measurement technique using KrF laser-induced O(2) fluorescence

A new single-pulse, two-line laser-induced O(2) fluorescence (LIF) temperature-measurement technique was demonstrated. The fluorescence spectrum obtained with multichannel detection following simultaneous excitation of two coincident transitions in the 0-6 and the 2-7 bands of the B(3)Σ(-)(u)-X(3)Σ(-)(g) Schumann-Runge system was used to determine the gas temperature. The rms error of 100-pulse average LIF temperature measurements, referenced to their corresponding thermocouple measurements, was 1.3% over a temperature range of 1300-1800 K in atmospheric air. Photon shot noise was found to be the primary source of uncertainty for these measurements in a quiescent environment. Single-pulse temperature-measurement uncertainties (1 σ) ranged from approximately 13% at 1300 K to 7% at 1800 K.

Laser-induced fluorescence spectroscopy of the (0, 0) Schumann-Runge band of O(2)

We report laser-induced fluorescence recording and identification of the R and P branches (J = 1-21) of the (0, 0) Schumann-Runge band of molecular oxygen, excited near 202 nm by a tunable pulsed laser beam (3-MW/cm(2) maximum intensity). The intensity of the excitation spectrum varies linearly with the density of the laser power. In the conditions of the experiment, the detection limit of ground-state O(2) molecules deduced from the R(5) line is ~10(15) cm(-3).

Avoidance of the photochemical production of oxygen atoms in one-dimensional, two-photon laser-induced fluorescence imaging

We demonstrate that one-dimensional, two-photon laser-induced fluorescence imaging of oxygen atoms in flames can be achieved with reasonable signal intensities and spatial resolution but without the interference caused by the photolysis of O(2), by the use of a beam telescope instead of a focusing lens. The increase in probed volume (and concomitantly the number of excitable atoms) caused by the larger beam diameter in the telescope experiments offsets, to a certain extent, the loss in signal that is due to the reduced laser fluence. Further, the results show that the "correct" dependence of the fluorescence intensity on laser fluence (in this case quadratic) does not guarantee the absence of photochemistry.

Photochemical effects in multiple species fluorescence imaging in hydrogen-nitrous oxide flames

We describe photochemical perturbations observed in flame studies using multiple species fluorescence imaging techniques. We model these perturbations using a sequential two-step mechanism that is specific to flames containing significant quantities of nitrous oxide: [equation], followed by O((1)D) + H(2)O ? 2OH.

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.