Photolytic Interference Research Articles

Quantitative oxygen atom measurements in lean, premixed, H2 tubular flames

Quantitative O-atom profiles are measured for the first time in tubular flames and used to assess the performance of chemical kinetic mechanisms in tubular flame simulations. Atomic oxygen is measured via femtosecond, Two-Photon Laser Induced Fluorescence (fs-TPLIF) to avoid photolytic interference. The fs-TPLIF signal is corrected for collisional quenching from major species concentrations measured by Raman scattering. Temperature-dependent quenching rate in the form of T−0.5 for H2O is applied to better represent the actual physics, and all simulations are found to agree with this method. Atomic oxygen is reported in H2/O2 flames diluted with N2 or CO2 at 200 and 400 s−1 stretch rates. The oxygen radical data is compared to predictions using three different, detailed chemical kinetic mechanisms. Predictions of profile shape vary slightly, but the peak O-atom number density is calculated within experimental uncertainty by eachmechanism.

Three-photon-excited laser-induced fluorescence detection of atomic hydrogen in flames.

In many recent studies, ultrashort femtosecond (fs) two-photon (2p) laser-induced fluorescence (LIF) of atomic hydrogen (H) has been demonstrated using a 205 nm excitation. However, 205-nm- deep ultraviolet (UV) pulses can be problematic in practical devices containing thick transmissive optics and can also be susceptible to photolytic production at high laser energies. In this Letter, we investigate the three-photon (3p) excitation scheme of H by using red-shifted 307.7 nm fs laser pulses. Efficient 3p excitation resulting from fs laser pulses enable the 3pLIF detection of H, which was previously unattainable in most flame conditions using ns or ps pulses. Measurements are reported in CH4/O2/N2 Bunsen jet flames and premixed CH4/air flames and compared to similar 2pLIF schemes with fs pulses. Saturation effects, photolytic interferences, and stimulated emissions effects are studied, as well as the benefits of 3pLIF in diagnostic hardware with thick optical windows.

Quantitative femtosecond, two-photon laser-induced fluorescence of atomic oxygen in high-pressure flames.

Quantitative femtosecond two-photon laser-induced fluorescence of atomic oxygen was demonstrated in an H2/air flame at pressures up to 10atm. Femtosecond excitation at 226.1nm was used to pump the 3pP3J'=0,1,2←←2pP3J''=0,1,2 electronic transition of atomic oxygen. Contributions from multiphoton de-excitation, production of atomic oxygen, and photolytic interferences were investigated and minimized by limiting the laser irradiance to ∼1011 W/cm2. Quantitative agreement was achieved with the theoretical equilibrium mole fraction of atomic oxygen over a wide range of fuel-air ratios and pressures in an H2/air laminar calibration burner.

Femtosecond, two-photon, laser-induced fluorescence (TP-LIF) measurement of CO in high-pressure flames.

Quantitative, kiloherz-rate measurement of carbon monoxide mole fractions by femtosecond two-photon, laser-induced fluorescence (TP-LIF) was demonstrated in high-pressure, luminous flames over a range of fuel-air ratios. Femtosecond excitation at 230.1nm was used to pump CO two-photon rovibrational X1Σ+→B1Σ+ transitions in the Hopfield-Birge system and avoid photolytic interferences with excitation irradiance ∼1.7×1010 W/cm2. The effects of excitation wavelength, detection scheme, and potential sources of de-excitation were also assessed to optimize the signal-to-background and signal-to-noise ratios and achieve excellent agreement with theoretically predicted CO mole fractions at low and high pressure.

Comparison of femtosecond- and nanosecond-two-photon-absorption laser-induced fluorescence (TALIF) of atomic oxygen in atmospheric-pressure plasmas

Absolute number densities of atomic species produced by nanosecond (ns)-duration, repetitively pulsed electric discharges are measured by two-photon-absorption laser-induced fluorescence (TALIF). Unique to this work is the development of femtosecond-laser-based TALIF (fs-TALIF) that offers a number of advantages over more conventional nanosecond (ns)-pulse-duration laser techniques, such as higher-fidelity quenching rate measurements over a wide pressure range, significantly reduced photolytic interference (including photo-dissociation and photo-ionization), ability to collect two-dimensional images of atomic-species number densities with high spatial resolution aided by higher signal level, and efficient and accurate measurements of atomic-species number densities due to the higher repetition rates of the laser. For full quantification of these advantages, atomic-oxygen TALIF signals are collected from an atmospheric-pressure plasma jet employing both ns- and fs-duration laser-excitation pulses and the results are compared and contrasted.

Femtosecond, two-photon, planar laser-induced fluorescence of carbon monoxide in flames.

Two-photon, planar laser-induced fluorescence (TP-PLIF) of carbon monoxide was performed in steady and driven flames using femtosecond (fs) laser pulses at 1kHz. Excitation radiation at 230.1nm (full-width at half-maximum bandwidth of 270 cm-1) was used to pump many rovibrational two-photon transitions in the B1∑+←X1∑+ system. Visible fluorescence in the range 362-516nm was captured using an image intensifier and high-speed camera. The signal dependence on excitation energy and wavelength is presented. Photolytic interferences from the ultraviolet laser were explored in a sooting diffusion flame. Using an excitation laser intensity of 1010 W/cm2, negligible photolytic interferences were observed, and PLIF imaging of dynamic flame events was performed at 1 kHz.

Diel phosphorus variation and the stoichiometry of ecosystem metabolism in a large spring‐fed river

Elemental cycles are coupled directly and indirectly to ecosystem metabolism at multiple time scales. Understanding coupling in lotic ecosystems has recently advanced through simultaneous high‐frequency measurements of multiple solutes. Using hourly in situ measurements of soluble reactive phosphorus (SRP), specific conductance (SpC), and dissolved oxygen (DO), we estimated phosphorus (P) retention pathways and dynamics in a large (discharge, Q ≈ 7.5 m3/s) spring‐fed river (Ichetucknee River, Florida, USA). Across eight multi‐day deployments, highly regular diel SRP variation of 3–9 μg P/L (mean ∼50 μg P/L) was strongly correlated with DO variation, suggesting photosynthetic control directly via assimilation, and/or indirectly via geochemical reactions. Consistent afternoon SRP maxima and midnight minima suggest peak removal lags gross primary production (GPP) by ∼8 hours. Two overlapping processes were evident, one dominant with maximum removal near midnight, the other smaller with maximum removal near midday. Hourly [Ca] measurements during three 24‐hour deployments showed consistent afternoon minima, suggesting that calcite precipitates as GPP increases pH and mineral saturation state. Resulting P co‐precipitation was modeled using SpC as a [Ca] proxy, yielding a diel P signal adjusted for the primary geochemical retention pathway. P assimilation, the dominant diel signal, was computed by interpolating between daily concentration maxima, both with and without geochemical adjustment, and converting concentration deficits to fluxes using discharge and benthic area. Adjusting for calcite co‐precipitation yielded assimilation rates (13.7 ± 5.8 mg P·m−2·d−1) strongly correlated with GPP, accounting for 72% ± 9% of gross removal, and ecosystem C:P stoichiometry (466 [± 12]:1) consistent with dominant vascular autotrophs (478 [± 24]:1). Without adjusting for co‐precipitation, covariance with GPP was weaker, and C:P implausibly high. Consistent downstream P accumulation, likely from P‐rich porewater seepage, varied across deployments (3–22% of total flux) and co‐varied with discharge and respiration. Asynchronous N and P assimilation may arise from differential timing of protein and ribosome production, with the latter requiring maximal carbohydrate stores and minimal photolytic interference. This study demonstrates direct and indirect coupling of biological, hydrological, and geochemical processes and the utility of high‐resolution time series of multiple solutes for understanding these linkages.

Interference-free two-photon LIF imaging of atomic hydrogen in flames using picosecond excitation

Interference-free, two-photon-excited planar laser-induced fluorescence (PLIF) imaging of atomic hydrogen is demonstrated in steady premixed methane flames. PLIF measurements of atomic hydrogen present a challenge because of the relatively weak two-photon absorption cross-sections and the nonlinear dependence of the laser-induced fluorescence (LIF) signal on laser intensity. Previous two-photon LIF measurements of atomic hydrogen in hydrocarbon flames using nanosecond laser excitation have identified complications from photolytic production of hydrogen atoms by the excitation laser. Recent results from line-imaging studies in our laboratory indicate a significant advantage to using picosecond excitation for imaging atomic hydrogen with negligible photolytic interference. In the current study, we extend our capabilities for interference-free PLIF imaging of atomic hydrogen in steady premixed CH4/O2/N2 flames. Peak single-shot signal-to-noise ratios of approximately 6–8 are achieved, and the estimated single-shot detection limit is on the order of 1016cm-3. Avoidance of interference and stimulated emission and the effects of fluorescence quenching are discussed. Averaged composite PLIF images are generated by combining images from multiple axial locations in the flame. The images show enhanced number densities of atomic hydrogen near the flame tip, in accordance with numerical predictions of diffusional focusing of H-atoms resulting from the sharp curvature of the flame front.

Comparison of nanosecond and picosecond excitation for interference-free two-photon laser-induced fluorescence detection of atomic hydrogen in flames

Two-photon laser-induced fluorescence (TP-LIF) line imaging of atomic hydrogen was investigated in a series of premixed CH4/O2/N2, H2/O2, and H2/O2/N2 flames using excitation with either picosecond or nanosecond pulsed lasers operating at 205 nm. Radial TP-LIF profiles were measured for a range of pulse fluences to determine the maximum interference-free signal levels and the corresponding picosecond and nanosecond laser fluences in each of 12 flames. For an interference-free measurement, the shape of the TP-LIF profile is independent of laser fluence. For larger fluences, distortions in the profile are attributed to photodissociation of H2O, CH3, and/or other combustion intermediates, and stimulated emission. In comparison with the nanosecond laser, excitation with the picosecond laser can effectively reduce the photolytic interference and produces approximately an order of magnitude larger interference-free signal in CH4/O2/N2 flames with equivalence ratios in the range of 0.5< or =Phi< or =1.4, and in H2/O2 flames with 0.3< or =Phi< or =1.2. Although photolytic interference limits the nanosecond laser fluence in all flames, stimulated emission, occurring between the laser-excited level, H(n=3), and H(n=2), is the limiting factor for picosecond excitation in the flames with the highest H atom concentration. Nanosecond excitation is advantageous in the richest (Phi=1.64) CH4/O2/N2 flame and in H2/O2/N2 flames. The optimal excitation pulse width for interference-free H atom detection depends on the relative concentrations of hydrogen atoms and photolytic precursors, the flame temperature, and the laser path length within the flame.

Two-photon LIF imaging of atomic oxygen in flames with picosecond excitation

Interference-free planar laser-induced fluorescence measurements of atomic oxygen are demonstrated in steady and unsteady laminar premixed methane flames. Two-dimensional LIF measurements of atomic oxygen present a challenge because of the relatively weak two-photon absorption cross-sections and the nonlinear dependence of the LIF signal on laser intensity. Previous two-photon LIF measurements of atomic oxygen in hydrocarbon flames using nanosecond laser excitation were complicated by significant interference from photolytically produced oxygen atoms. Recent results from line-imaging studies in our laboratory indicate that CO 2 is the dominant photolytic precursor. Here, we use picosecond excitation to image atomic oxygen with negligible photolytic interference. We demonstrate this capability in premixed CH 4/O 2/N 2 ( ϕ = 1.08 , X N 2 = 0.63 ) flames with an adiabatic flame temperature of 2480 K and an estimated peak O-atom mole fraction of 0.5%. Peak single-shot signal-to-noise ratios of 9 are achieved, and the estimated detection limit is approximately 80 ppm. Quantitative O-atom LIF measurements also require knowledge of the temperature-dependent quenching cross-sections, which are not currently available. The effects of collisional quenching on the O-atom LIF are predicted using two models for the temperature dependence of quenching cross-sections, and the results indicate that O-atom LIF signals can provide accurate measurements of O-atom profiles in flames. Measurements in an acoustically forced Bunsen flame illustrate the feasibility of using O-atom LIF to investigate flow–flame interactions.

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.

Photolytic interference affecting two-photon laser-induced fluorescence detection of atomic oxygen in hydrocarbon flames

This study reports on photochemical interferences affecting atomic oxygen detection using two-photon laser-induced fluorescence at 226 nm. In contrast to previous studies in which molecular oxygen was proven to be the relevant photochemical precursor molecule in a hydrogen-fueled flame, the present investigations were carried out in a laminar diffusion flame of methane and air. The most significant interferences were found at the fuel side of the flame in the absence of molecular oxygen, and vibrationally excited carbon dioxide was identified as the most probable precursor molecule for the photochemical production of oxygen atoms.

LIF Spectroscopy of NO and O2in High-Pressure Flames

We report new models of absorption, excitation, and fluorescence spectra for joint excitations of the A ← X(0,0) band of NO and the B← X bands of 02. Supporting measurements obtained from the burned regions of flames operated at pressures from 1 to 10 atm are also presented. As developed through these models, a strategy appropriate for imaging planar laser-induced flourescence is proposed that, for lean combustion environments, should afford sensitive measurements of NO concentration through the use of a single excitation wavelength despite the photolytic interference from 02 that intensifies with increasing pressure and temperature.

Third‐generation FAGE instrument for tropospheric hydroxyl radical measurement

We have constructed a single‐stage, frequency‐doubled, copper vapor laser‐pumped dye laser to be used in the measurement of atmospheric hydroxyl radical concentrations. This laser can be tuned to either the 1←0 or 0←0 vibrational transition in the electronic transition 2Σ←2Π. A new photon counting instrument is used for HO fluorescence detection. Theoretical and experimental studies of instrument performance show better sensitivities and reduced photolytic interferences than have been possible with previous systems based upon Nd:YAG pumping.

Laser-induced fluorescence of O(3p^3P), 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.

Pressure dependence of fluorescent and photolytic interferences in HO detection by laser-excited fluorescence

In the measurement of HO concentrations by laser-excited fluorescence, expansion of the sampled air offers a way to reduce fluorescent and photolytic interference by other species. The decrease in [HO] upon expansion is balanced by an increase in HO fluorescence yield over a wide range of pressures. Background air fluorescence is reduced if the responsible species have fluorescence yields higher than those of HO. Preliminary experiments indicate that most of the fluorescence observed in laboratory air is due to such species. Upon expansion, the suppression of fluorescent interference can be no greater than the reduction in pressure, whereas the suppression of photolytic interference can be no less.