A comprehensive model for the quantification of linear and nonlinear regime laser-induced fluorescence of OH under A2Σ+←X2Π(1,0) excitation

An analytic model termed the ‘integrated quasi-steady-state’ (IQSS) model for the comprehensive quantification of both linear and nonlinear regime laser-induced fluorescence (LIF) is presented. The IQSS model is optimized for the hydroxyl radical (OH), subject to nanosecond A²Σ⁺←X²Π(1,0) excitation at pressures close to atmospheric. The IQSS model is particularly relevant to experimental conditions where the LIF signal is both spectrally and temporally integrated, such as in planar laser-induced fluorescence experiments. The IQSS model is based around a quasi-steady-state solution to a four-level rate-equation approximation of the OH molecule; this quasi-steady-state solution is then integrated with a triangular functional form for both the spatial and temporal variations to produce an analytic solution. In order to accurately predict LIF in the nonlinear regime, it is shown that both the temporal and the spatial variations of the laser pulse—or ‘wings’ of the laser pulse—must be adequately accounted for in the LIF model formulation. The IQSS model is successfully verified against detailed numerical simulations for variations in the laser irradiance, quenching environment and temperature. Experimentally, the IQSS model is successfully validated by comparing the predicted and measured OH LIF vs. irradiance dependence in the product gases of a methane–air laminar flame.     

Single-pulse collision-insensitive picosecond planar laser-induced fluorescence of OHA 2 Σ + (v′ = 2) in atmospheric-pressure flames

Single-pulse two-dimensional picosecond Laser-Induced Fluorescence (LIF) imaging of the OH density in a single quantum state was performed for the first time, using a premixed methane-oxygen flame at atmospheric pressure. A picosecond, excimer-Raman-laser system (268 nm, 470 ps FWHM) was used for excitation of OH. The fluorescence from the laser sheet was imaged onto a fast gated intensified camera with a 400 ps gate width. The short laser pulse minimizes the collisional redistribution of population in the ground state during excitation, while the short camera gate avoids significant quenching of the excited-state fluorescence. The fluorescence signal obtained in this way is a direct measure of the population in a selected quantum state. In contrast to common nanosecond LIF signals no corrections on variations of the collisional environment are necessary. This collision-insensitive approach to two-dimensional LIF yields an OH detection limit of 10 ppm in a cube of 330 µm per side with a single 1 mJ laser pulse. A rate-equation model is used to estimate the effects on the observed signal of fluctuations in pulse energy and duration, laser-camera timing jitter, and spatial variations in the collisional environment.     

Characterization of ammonia two-photon laser-induced fluorescence for gas-phase diagnostics

Two-photon laser-induced fluorescence (LIF) of ammonia (NH₃) with excitation of the C′-X transition at 304.8 nm and fluorescence detection in the 565 nm C′-A band has been investigated, targeting combustion diagnostics. The impact of laser irradiance, temperature, and pressure has been studied, and simulation of NH₃-spectra, fitted to experimental data, facilitated interpretation of the results. The LIF-signal showed quadratic dependence on laser irradiance up to 2 GW/cm². Stimulated emission, resulting in loss of excited molecules, is induced above 10 GW/cm², i.e., above irradiances attainable for LIF imaging. Maximum LIF-signal was obtained for excitation at the 304.8 nm bandhead; however, lower temperature sensitivity over the range 400–700 K can be obtained probing lines around 304.9 nm. A decrease in fluorescence signal was observed with pressure up to 5 bar absolute and attributed to collisional quenching. A detection limit of 800 ppm, at signal-to-noise ratio 1.5, was identified for single-shot LIF imaging over an area of centimeter scale, whereas for single-point measurements, the technique shows potential for sub-ppm detection. Moreover, high-quality NH₃-imaging has been achieved in laminar and turbulent premixed flames. Altogether, two-photon fluorescence provides a useful tool for imaging NH₃-detection in combustion diagnostics.     

Two Dimensional Rotational Temperature Measurement by Multiline Laser Induced Fluorescence of Nitric Oxide in Combustion Flame

Two-dimensional rotational temperature measurement was performed in a stable combustion flame of premixed butane and oxygen using multiline laser induced fluorescence (LIF) of nitric oxide molecules. Multiple rotational absorption lines of A²∑⁺Π;X²II(0,0) Q₁ and Q₂ lines were excited by laser light around 226 nm, and the LIF signal was observed by an image-intensified digital camera. Temperature was determined through least squares fitting correlation between LIF intensity and excitation rotational quantum number for the Boltzmann distribution function. The measured LIF intensity was approximated by the Boltzmann distribution with good accuracy, and the temperature obtained was between 500 K and 1800 K for the test flame. The measuring error of the temperature was evaluated and found to be 80 K, which corresponded to 8% of the measured fluorescence intensity. The two-line LIF scheme was evaluated by different pairs of excitation lines (Q₁(31.5)/Q₁(16.5) and Q₁(18.5)/Q₁(16.5)) for comparison with the multiline LIF approach. Temperature which was obtained by two-line LIF scheme corresponded well with multiline LIF results for Q₁(31.5)/Q₁(16.5) excitation. However, for Q₁(18.5)/Q₁(16.5) excitation, the obtained temperature did not agree with the multiline LIF result because the population of rotational states J=18.5 and J=16.5 is similar at high temperatures. We found that two-line LIF temperature measurement was reliable when excitation lines were suitably selected.     

Point and planar ultraviolet excitation/detection of hydroxyl-radical laser-induced fluorescence through long optical fibers

We demonstrate an all-fiber-coupled, UV, laser-induced-fluorescence (LIF) detection system of the hydroxyl radical (OH) in flames. The nanosecond-pulsed excitation of the (1,0) band of the OH A(2)∑(+)-X(2)Π system at ～283 nm is followed by fluorescence detection from the (0,0) and (1,1) bands around 310 nm. The excitation-laser beam is delivered through a 400 μm core UV-grade optical fiber of up to 10 m in length, and the fluorescence signal collected is transmitted through a 1.5 mm core 3 m long fiber onto the remote detector. Single-laser-shot planar LIF (PLIF) imaging of OH in flames is also demonstrated using fiber-based excitation. The effects of delivering intense UV beams through long optical fibers are investigated, and the system improvements for an all-fiber-coupled OH-PLIF imaging system are discussed. Development of such fiber-based diagnostics and imaging systems constitutes a major step in transitioning laser diagnostic tools from research laboratories to reacting flow facilities of practical interest.     

Fluorescence quantum yield of carbon dioxide for quantitative UV laser-induced fluorescence in high-pressure flames

The fluorescence quantum yield for ultraviolet laser-induced fluorescence of CO₂ is determined for selected excitation wavelengths in the range 215–250 nm. Wavelength-resolved laser-induced fluorescence (LIF) spectra of CO₂, NO, and O₂ are measured in the burned gases of a laminar CH₄/air flame (φ=0.9 and 1.1) at 20 bar with additional NO seeded into the flow. The fluorescence spectra are fit to determine the relative contribution of the three species to infer an estimate of fluorescence quantum yield for CO₂ that ranges from 2–8×10^?6 depending on temperature and excitation wavelength with an estimated uncertainty of ±0.5×10^?6. The CO₂ fluorescence signal increases linearly with gas pressure for flames with constant CO₂ mole fraction for the 10 to 60 bar range, indicating that collisional quenching is not an important contributor to the CO₂ fluorescence quantum yield. Spectral simulation calculations are used to choose two wavelengths for excitation of CO₂, 239.34 and 242.14 nm, which minimize interference from LIF of NO and O₂. Quantitative LIF images of CO₂ are demonstrated using these two excitation wavelengths and the measured fluorescence quantum yield.     

Photochemical effects of KrF excimer excitation in laser-induced fluorescence measurements of OH in combustion environments

This paper presents experimental evidence that using the KrF excimer laser for quantitative laser-induced fluorescence (LIF) studies of the OH A-X (3,0) system is highly problematic if the effects of both photobleaching and photochemistry are not included for laser spectral irradiances greater than 20 MW/cm² cm^-1. Pump-probe and time-resolved measurements of the OH LIF signal in an atmospheric pressure, premixed CH₄-air flame at low- and high-laser-spectral-irradiance conditions show that a significant amount of OH is produced from photofragments resulting from the simultaneous 2-photon predissociation of H₂O molecules in the C-X system. A 5+2-level rate-equation model that includes the effects of both photobleaching and photochemical OH production is shown to satisfactorily predict the data using a single adjustable parameter given by the effective, spectrally integrated 2-photon cross-section of H₂O near 248 nm. The time-integrated OH LIF signal was found to depend on both the laser spectral irradiance and the local concentration of H₂O. Additionally, use of the KrF excimer laser for 2-line rotational thermometry can produce temperature errors as great as +550 K at high laser-pulse energies. Received: 21 August 2000 / Revised version: 30 October 2000 / Published online: 21 February 2001     

Effect of laser intensity and of lower-state rotational energy transfer upon temperature measurements made with laser-induced fluorescence

4 –air flame, with OH at 2000 K. We calculate the ratio of LIF intensities that would be induced by doubled dye-laser light near 283 nm, by means of the A←X, 1←0, P₁(7), and Q₂(11) transitions in OH. Here we show that the ratio of LIF signals from those two transitions, and thus the deduced temperature, is sensitive to laser intensity. That is caused mainly by the competition between laser-pumping of molecules out of the lower rotational state and of rotational energy transfer (RET) collisions into that state. A-state collisional effects are normally important, but are minimized here by assuming that they are the same for both transitions. The laser spectral intensity dependence of the fluorescence ratio depends heavily upon the value of the RET coefficients within the X-state. While RET reduces the sensitivity of the observed signal to the laser spectral intensity, the conversion of a measured fluorescence ratio to temperature is particularly difficult. That is because RET rates, and quenching rates, can be a function of local conditions and of the rotational state being populated. Two different models are used to demonstrate these effects, and both predict large effects upon temperature. Received: 19 February 1998/Revised version: 16 June 1998     

氨基氰分子在193nm的光离解动力学

本文研究了氨基氰分子(H₂NCN)的光离解动力学。利用激光诱导荧光技术(LIF)检测了这一分子在193nm下光解所生成的CN(X²Σ⁺)基,在激光能量为0.35mJ/脉冲的条件下,CN(X)基的LIF信号十分弱,其0—0带的转动温度为1590±90K;当激光能量较高时,LIF信号明显增加,并观察到=2能级上的布居。在50℃的温度下,本实验测量了氨基氰分子的紫外吸收光谱,在193nm其吸收截面小于2.4×10^-19cm²。二次光解机理可用来解释本实验的结果。 关键词：     

1+1′ resonant multiphoton ionisation of OH radicals via the A2Σ+ state: insights from direct comparison with A-X laser-induced fluorescence detection

A 1+1′ resonance-enhanced multiphoton ionisation (REMPI) scheme for OH X²Π radicals is characterised for a broad range of intermediate A²Σ⁺ (v = 1, J, F_i) levels. The intensities of OH A-X (1,0) transitions detected by subsequent fixed-frequency VUV ionisation are compared with those obtained by near simultaneous laser-induced fluorescence (LIF) measurements. The ratios of the 1+1′ REMPI to LIF signals are used to derive enhancement factors which reflect the VUV absorption to the OH A³Π, 3d, v = 0 Rydberg state and/or the fast autoionisation process that yields OH⁺ ions. The determination of the enhancement factors permits 1+1′ REMPI to be utilised as a quantitative state-specific probe of OH X²Π radicals.     

Laser-induced fluorescence detection of singlet CH2 in low-pressure methane/oxygen flames

Methylene, CH₂, is a chemically important intermediate in hydrocarbon combustion but has previously eluded optical detection in a combustion environment. The CH₂ signal as a function of height above the burner surface in a premixed, laminar, methane/oxygen flame (5.6 Torr and fuel equivalence ratio 1.05) is measured by laser-induced fluorescence (LIF) in the B ₁ – ã¹ A ₁ electronic system. The ã state which lies 3165 cm^–1 above the ground state is populated at the high temperatures of the flame (800–1800 K). Although less than one photon for each laser pulse is detected, we can unambiguously attribute the LIF features in the region 450 to 650 nm to CH₂ by both scanning the excitation laser and dispersing fluorescence. LIF temperatures and CH and OH LIF concentration profiles are also obtained for the flame. The CH₂ radical concentration maximum occurs closer to the burner than that of either OH or CH, as expected from models of methane combustion chemistry.     

燃烧产物组成激光诱导荧光光谱的测量

对激光诱导荧光（LIF）光谱技术在燃烧过程中的应用进行研究，介绍测量燃烧过程中常见自由基OH和NO的LIF光谱的实验方案，以及采用激光诱导荧光光谱技术测量小分子荧光光谱的方法，利用YAG激光器、染料激光器、CO2激光器、光谱仪、ICCD等设备对燃烧产物中常见小分子自由基OH和NO进行了测量，从实验中得到了自由基OH和NO的荧光光谱。实验结果表明，荧光光谱与激发波长无关，但是激发波长改变后，荧光强度因离开最佳波长而有所下降，这符合分子荧光光谱的特征。与其他光谱技术相比，激光诱导荧光光谱技术具有极高的选择性和灵敏度。     

Single-shot imaging of OH radicals and simultaneous OH radical/acetone imaging with a tunable Nd : YAG laser

Single shot imaging capability for OH radical distributions in various atmospheric pressure methane flames upon excitation with a tunable frequency-quadrupled Nd:YAG laser is demonstrated. The laser wavelength can be tuned with an intra-cavity etalon to produce laser-induced fluorescence (LIF) signals from OH via absorption in the OH A–X (2,0) P ₁(10) line. Simultaneous single-shot imaging of the burnt and unburnt zones in laminar nonpremixed, premixed and turbulent flames is presented. The unburnt areas are visualized with LIF of acetone that is seeded to the methane fuel. Acetone levels are set to match signal intensities to that of the OH signals to allow imaging on a single intensified CCD camera. PACS 42.62.Fi; 33.50.Dq; 82.33.Vx     

Oxygen quenching of toluene fluorescence at elevated temperatures

Gas-phase oxygen quenching of toluene laser-induced fluorescence (LIF) is studied between 300 and 650 K in a nitrogen/oxygen bath gas of 1-bar total pressure with oxygen partial pressures up to 400 mbar. With increasing vibrational excitation of the laser-excited toluene, intramolecular decay becomes faster, resulting in a decreasing relative strength of collisional quenching by oxygen. Additionally, Stern–Volmer plots are found to be non-linear for temperatures above 500 K in the case of 266-nm excitation and at all temperatures for 248-nm excitation. This is attributed to the onset of internal conversion from specific vibrational levels. A photophysical model is developed that describes the experimental data and predicts toluene LIF signal strengths for higher oxygen partial pressures. One important result for practical application is that oxygen quenching is not the dominant de-excitation process for engine-related temperature and pressure conditions, and thus application of the popular fuel–air ratio LIF (FARLIF) concept leads to erroneous signal interpretation.This revised version was published online in August 2005 with a corrected cover date.     

Single-shot laser-induced fluorescence imaging of formaldehyde with XeF excimer excitation

Single-shot formaldehyde laser-induced fluorescence (LIF) imaging measurements in a technical scale turbulent flame have been obtained using XeF excimer laser excitation in the ?¹A₂-˜X¹A₁ transition at 353.2 nm. Measurements have been carried out in a 150 kW natural gas swirl burner where formaldehyde distribution fields have the potential, in combination with OH concentration fields, to visualize the heat release distribution and therefore give an optimal visualization of flame-front positions. The extended areas where formaldehyde was detected in the swirl flame indicates the presence of low temperature chemistry in preheated gas pockets before ignition. Received: 31 January 2000 / Revised version: 2 March 2000 / Published online: 5 April 2000     

Experimental study of signal trapping of OH laser induced fluorescence and chemiluminescence in flames

The absorption of OH^∗ chemiluminescence and laser-induced fluorescence (LIF) in the exhaust gas of confined premixed laminar CH₄/air flames at atmospheric pressure was investigated. One flame was used as source and a second as absorber. OH LIF was excited in the ν″=0→ν′=1 band of the A–X electronic system around ≈283 nm and spectrally resolved detected in the (0,0) and (1,1) vibrational bands around 305–320 nm. For OH^∗ chemiluminescence, spectrally resolved detection was performed in the wavelength range 280–340 nm. For an absorption path of 54 mm and at T≈2000 K, signal trapping on the order of 10–40% was observed. Signal trapping was most pronounced in the (0,0) band, as expected from the thermal population distribution of OH in the electronic ground state. The spectral distribution of the signals and the wavelength dependence of the signal trapping are addressed in this paper. Implications from the results with respect to detection strategies and chemiluminescence-based equivalence ratio measurements are discussed.     

Correction of LIF temperature measurements for laser absorption and fluorescence trapping in a flame

A computational method is described in order to correct OH LIF temperature measurements for absorption of laser energy and trapping of fluorescence. Calculations are performed in a large range of flame conditions and can be used as a correction data base both in case of (0-0) and (1?0) excitations. Comparison of corrected temperatures profiles obtained in a 40 Torr methanol/air flame, for both kinds of Laser-Induced Fluorescence (LIF) excitations shows a very good agreement. This method is applied to measure the temperature profile of a methanol flame perturbed by a sampling probe. The LIF collection volume is located at the actual probe sampled volume using an experimental procedure already described. Spatial resolution and sensitivity of temperature measurements are sufficiently efficient to highlight, for the first time by LIF, an indubitable cooling effect due to the probe presence that induces important OH profile change. According to flame chemical modelling, it is shown that both effects are strongly correlated.     

Multiphoton excitation and dissociation of CH3OH molecules by intense broadband YSGG:Cr:Er laser radiation in the range 2.7 µm

We have observed efficient multiphoton excitation of CH₃OH molecules by broadband pulsed YSGG:Cr:Er-laser radiation (λ=2.7 µm,τ _p =60 ns, and Δν_las～1.5 cm^?1) at fluences of 0.3-600 J/cm² under collisional and collisionless conditions. Multiphoton dissociation of CH₃OH has been realized. We discuss the role of intermode interaction, the intensity of the laser radiation, and the laser bandwidth in the excitation process.     

Soot-visualization strategies using laser techniques

Strategies for spatially resolved soot volume-fraction measurements have been investigated in sooting laboratory flames with known soot characteristics. Two techniques were compared: Laser-Induced Fluorescence in C₂ from Laser-Vaporized Soot (LIF(C₂)LVS), and Laser-Induced Incandescence of soot (LII). The LII signal is the increased temperature radiation from soot particles which have been heated to temperatures of several thousand degrees as a consequence of absorption of laser radiation. The LIF(C₂)LVS technique is based on the production of C₂ radicals from laser-vaporized soot which occurs for laser intensities ≥10⁷ W/cm². A laser wavelength is chosen such that besides vaporizizng the soot, it also excites the C₂ radicals, and the subsequent C₂ fluorescence signal is detected. The signals from both techniques showed good correlation with soot volume fractions in the studied flame. The dependence of the signals on experimental parameters was studied, and the influence of interfering radiation, such as background flame luminosity and fluorescence from polyaromatic hydrocarbons, on studied signals was established. The potential of the two techniques for imaging of soot volume fractions in laboratory flames was demonstrated. Advantages and disadvantages of the studied techniques are discussed.