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.     

Flame turbulences recorded at 1 kHz using planar laser induced fluorescence upon hot band excitation of OH radicals

Laser-induced fluorescence (LIF) in flames is excited by a diode-pumped all-solid-state disk laser system which operates at a pulse repetition rate of 1 kHz and a tunable wavelength around 1030 nm. The laser fundamental is converted to 343 nm and used to excite into the hot band transition of OH radicals in H₂/O₂ diffusion flames of an industrial burner and in the premixed flame of a microburner. The OH radical emission around 308 nm is resolved spectrally and spatially using a light-sheet technique. Imaging of the planar LIF (PLIF) by a gated camera visualizes the turbulent flame behavior on the millisecond time scale without averaging. To our knowledge this is the first time that an all-solid-state laser providing at the same time a kHz repetition rate as well as pulse energies of up to 5.5 mJ is available for PLIF observation of OH radicals. PACS 33.50.-j; 42.55.Xi; 82.33.Vx     

Measurements and simulation of in-cylinder UV-absorption in spark ignition and Diesel engines

Spatially and spectrally resolved in-cylinder absorption measurements were performed in spark-ignited internal combustion engines and in Diesel engines. With UV-broadband illumination it was shown that the UV attenuation occurs throughout the burned gas area with roughly homogeneous absorption cross-sections. Model calculations based on the absorption properties of CO₂ at elevated temperatures show that this species gives the main contribution to in-cylinder UV absorption. A previously suggested technique of assessing UV absorption using O₂ laser-induced fluorescence (LIF) as probe light is successfully applied to in-cylinder measurements of the light absorption inside a fired heavy-duty Diesel engine. Even in this environment, the comparison with model calculations shows that CO₂ is the main contributor to UV light absorption. Since the O₂-LIF absorption technique is based on the identical geometry used for LIF concentration measurements, the results can directly be used for correcting LIF signal data such as that obtained from NO imaging. Received: 20 April 2001 / Published online: 18 July 2001     

Comparison of laser-induced and planar laser-induced fluorescence measurements of nitric oxide in a high-pressure, swirl-stabilized, spray flame

We report spatially resolved linear laser-induced fluorescence (LIF) and planar laser-induced fluorescence (PLIF) measurements of nitric oxide (NO) in a pre-heated, high-pressure (4.27 atm), lean direct-injection (LDI) spray flame. The feasibility of using PLIF in lieu of LIF is assessed with respect to measuring NO concentrations in high-pressure LDI spray flames. NO is excited via the resonant Q₂(26.5) transition of the γ(0,0) band while a non-resonant wavelength is employed to subtract background interferences. LIF detection is performed in a 2-nm region centered on the γ(0,1) band. PLIF detection is performed in a 68-nm window that captures fluorescence from several vibrational bands. An in situ NO doping scheme for fluorescence calibration is successfully employed to quantify the LIF signals. However, a similar calibration scheme for the reduction of PLIF images to quantitative field measurements is plagued by the laser-excited background. Excitation scans and calibration comparisons have been performed to assess the background contribution for PLIF detection. Quantitative radial NO profiles measured by LIF are presented and analyzed so as to correct the PLIF measurements to within the accuracy bars of the LIF measurements via a single-point scaling of the PLIF image. Received: 23 November 1999 / Revised version: 17 January 2000 / Published online: 27 April 2000     

Experimental evaluation of strategies for quantitative laser-induced-fluorescence imaging of nitric oxide in high-pressure flames (1-60 bar)

Nitric oxide laser-induced-fluorescence (NO-LIF) 2-D imaging measurements using a new multi-spectral detection strategy are reported for high-pressure flames (1-60 bar). This work builds on previous research that identified interference LIF from O₂ and CO₂ in high-pressure flames and optimized the choice of excitation strategies as a function of application conditions. In this study, design rules are presented to optimize the LIF detection wavelengths for quantitative 2-D NO-LIF measurements over a wide range of pressures (1-60 bar) and temperatures. Simultaneous detection of LIF in multiple wavelength regions enables correction of the NO signal for interference from O₂ and CO₂ and allows simultaneous imaging of all three species. New experiments of wavelength-resolved 1-D LIF in slightly lean (? = 0.9) and slightly rich (? = 1.1) methane/air flames are used to evaluate the design rules and estimate the NO detection limits for a wide range of flame conditions. The quantitative 2-D measurements of NO in the burnt gas are compared with model calculations (using GRI-Mech 3.0) versus pressure for slightly lean and slightly rich flames. The discussions and demonstrations reported in this study provide a practical guideline for application of instantaneous 1-D or 2-D NO-LIF imaging strategies in high-pressure combustion systems.     

High-speed planar laser-induced fluorescence of the CH radical using the C^{ 2} \varSigma ^{ + } {-}X^{ 2} \varPi \left( {0,0} \right) band

The potential for kHz-rate or high-speed planar laser-induced fluorescence (PLIF) of the Methylidyne (CH) radical using its \(C^{ 2} \varSigma ^{ + } {-}X^{ 2} \varPi\) (v′ = 0, v′′ = 0) band was investigated. We show that due to its strength and the excitation wavelength (~314 nm), which is conveniently generated by a dye laser operating with a red dye, the C–X(0,0) band is a good choice for CH LIF studies wherein suppression of background scattering is not required. While interference from polycyclic aromatic hydrocarbons is small, that caused by hydroxyl (OH) can be significant. Nonetheless, the OH lines can be avoided, and we observe good CH image fidelity. Most importantly, we show that due to the favorable properties of the CH C–X band (i.e., good signal strength and convenient wavelength), it can be used for PLIF at kHz acquisition rates using a continuously pulsing laser system. This is demonstrated in laminar and turbulent CH₄-air flames with a laser system operating at 10 kHz and delivering ~0.2 mJ/pulse at 314 nm.     

间-吡啶基紫外光化学中的氢原子解离通道

利用高里德堡态氢原子飞行时间探测技术, 在224～248 nm激发波长研究了间-吡啶基紫外光化学中的氢原子解离通道的动力学过程．氢原子光解碎片产率谱显示在234 nm附近有较宽的吸收．产物的平动能释放较小;在224～248 nm激发波长区间平均< f_T>是0.12～0.19．产物的平动能分布显示产物是H+HC≡C-CH=CH-C≡N,H 3,4-吡啶和H 2,3-吡啶,以H HC≡C-CH=CH-C≡N为主要的氢原子生成通道．氢原子碎片具有各向同性的角度分布．研究结果表明,在紫外电子态激发以后,间-吡啶基经过内转换到电子基态,再经由单分子解离到H HC≡C-CH=CH-C≡N,H 3,4-吡啶和H 2,3-吡啶产物．间-吡啶基的紫外光解机理和以前报道过的邻-吡啶基的紫外光解机理相似．     

Exhaust-gas imaging via planar laser- induced fluorescence of sulfur dioxide

The potential use of planar laser-induced fluorescence (PLIF) of sulfur dioxide (SO₂) for visualization of exhaust-gas distributions is outlined and demonstrated. Strong absorption features in the UV spectral range allow excitation of SO₂ with the fourth harmonic of a Nd:YAG laser at 266 nm. Fluorescence emissions are mostly red-shifted and can be easily detected in single-shot imaging arrangements with a good signal-to-noise ratio. This study uses a premixed methane/air flame that is doped with SO₂ to demonstrate the technique. The signal strength has a pronounced temperature dependence for excitation at 266 nm. Received: 14 January 2002 / Revised version: 30 January 2002 / Published online: 14 March 2002     

Detection of trace nitric oxide concentrations using 1-D laser-induced fluorescence imaging

Spectrally resolved laser-induced fluorescence (LIF) with one-dimensional spatial imaging was investigated as a technique for detection of trace concentrations of nitric oxide (NO) in high-pressure flames. Experiments were performed in the burnt gases of premixed methane/argon/oxygen flames with seeded NO (15 to 50 ppm), pressures of 10 to 60 bar, and an equivalence ratio of 0.9. LIF signals were dispersed with a spectrometer and recorded on a 2-D intensified CCD array yielding both spectral resolution and 1-D spatial resolution. This method allows isolation of NO-LIF from interference signals due to alternative species (mainly hot O₂ and CO₂) while providing spatial resolution along the line of the excitation laser. A fast data analysis strategy was developed to enable pulse-by-pulse NO concentration measurements from these images. Statistical analyses as a function of laser energy of these single-shot data were used to determine the detection limits for NO concentration as well as the measurement precision. Extrapolating these results to pulse energies of ～?16 mJ/pulse yielded a predicted detection limit of ～?10 ppm for pressures up to 60 bar. Quantitative 1-D LIF measurements were performed in CH₄/air flames to validate capability for detection of nascent NO in flames at 10–60 bar.     

Spectroscopic studies of laser-produced plasmas formed in CO and CO2 using 193, 266, 355, 532 and 1064 nm laser radiation

The wavelength dependence of laser-produced breakdown in air, CO and CO₂ has been studied using the four Nd:YAG harmonics (266 nm, 355 nm, 532 nm and 1064 nm) and the ArF-excimer laser (193 nm). Breakdown thresholds at these wavelengths are reported for air, CO and CO₂. A significant reduction in the breakdown thresholds for both CO and CO₂ is apparent when comparing 193 nm with the four Nd:YAG harmonics. This reduction is attributed to the resonance-enhanced two-photon ionization of metastable carbon atoms generated in the laser focus at the ArF-laser wavelength. In addition to reporting breakdown thresholds, the laser-produced plasmas in CO and CO₂ are characterized in terms of plasma temperatures and electron densities which are measured by time-resolved emission spectroscopy. Electron densities range from 9 × 10¹⁷ cm^–3 to 1 × 10₁₇ cm^–3. Excitation temperatures range from 22 000 K at 0.2 µs to 11 000 K at 2 µs. Ionization temperatures range from 22 000 K at 0.1 µs to 16 000 K at 2 µs. Evidence is presented to indicate that, like ArF-laser-produced plasmas, Nd:YAG-laser-produced plasmas formed in CO and CO₂ are in or near a state of Local Thermodynamic Equilibrium (LTE) soon after their formation.     

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.     

Luminescence of undoped β-Ga2O3 single crystals excited by picosecond X-ray and sub-picosecond UV pulses

Undoped β-Ga₂O₃ single crystals were grown using the floating zone technique under a pressure of 2 atm oxygen. Luminescence spectra of the crystals were measured with steady-state X-ray (<15 keV) and UV (258 nm, 4.8 eV) sources. The X-ray excitation produced a spectrum with a peak at 390 nm (3.2 eV) whereas the UV excited spectrum had a peak at 430 nm (2.9 eV). The luminescence rise and decay were also examined by using picosecond X-ray and sub-picosecond UV pulses. It was found that the X-ray pulse excitation gave a slower rise and a faster decay of the luminescence compared with the UV pulse excitation. These results suggest that X-ray excitation generates high energy electrons, building up luminescent states until those electrons lose their kinetic energies, giving rise to the formation of local hot spots in the gallium oxide crystals.     

Examination of photoproducts in the ablation plume of doped PMMA

Optical emission and laser-induced fluorescence spectroscopies (OES and LIF) are employed to examine the plume ejected into vacuum upon UV pulsed laser ablation (248 nm, 20 ns, and 266 nm, 5 ns) of poly(methyl methacrylate) PMMA films doped with photostable compounds naphthalene (NapH) and phenanthrene (PhenH), and their photolabile iodide derivatives iodonaphthalene (NapI) and iodophenanthrene (PhenI). Spontaneous emissions observed in the irradiation of NapI and PhenI sensitised films can be assigned to excited products resulting from photodissociation of the dopants and to excited C₂, CH, and CN radicals. The presence in the plume of ground state products is revealed by LIF upon excitation at 266 nm. Measurements of the dependence of the spontaneous and LIF emissions on distance to the surface, ablation fluence, and flight velocities of species are discussed with reference to the mechanistic implications derived from the measurements. PACS 52.38.Mf; 61.82.Pv; 82.50     

Luminescence in trivalent rare earth activated Sr4Al2O7 phosphor

In this paper we report the combustion synthesis of trivalent rare-earth (RE³⁺ = Dy, Eu and Ce) activated Sr₄Al₂O₇ phosphor. The prepared phosphors were characterized by the X-ray powder diffraction (XRD) and photoluminescence (PL) techniques. Photoluminescence emission peaks of Sr₄Al₂O₇:Dy³⁺ phosphor at 474 nm and 578 nm in the blue and yellow region of the spectrum. The prepared Eu³⁺ doped phosphors were excited by 395 nm then we found that the characteristics emission of europium ions at 615 nm (⁵D₀?⁷F₂) and 592 nm (⁵D₀?⁷F₁). Photoluminescence (PL) peaks situated at wavelengths of 363 and 378 nm in the UV region under excitation at around 326 nm in the Sr₄Al₂O₇:Ce³⁺ phosphor.     

Coherence gating in optical heterodyne detection measurements of scattering and absorption in highly scattering media

Laser-Induced Fluorescence (LIF) from the S₁ state of acetone and 3-pentanone was studied as a function of temperature and pressure using excitation at 248 nm. Additionally, LIF of 3-pentanone was investigated using 277 and 312 nm excitation. Added gases were synthetic air, O₂, and N₂ respectively, in the range 0–50 bar. At 383 K and for excitation at 248 nm, all the chosen collision partners gave an initial enhancement in fluorescence intensity with added gas pressure. Thereafter, the signal intensity remained constant for N₂ but decreased markedly for O₂. For synthetic air, only a small decrease occurred beyond 25 bar. At longer excitation wavelengths (277 and 312 nm), the corresponding initial rise in signal with synthetic air pressure was less than that for 248 nm. The temperature dependence of the fluorescence intensity was determined in the range 383–640 K at a constant pressure of 1 bar synthetic air. For 248 nm excitation, a marked fall in the fluorescence signal was observed, whereas for 277 nm excitation the corresponding decrease was only half as strong. By contrast, exciting 3-pentanone at 312 nm, the signal intensity increased markedly in the same temperature range. These results are consistent with the observation of a red shift of the absorption spectra (9 nm) over this temperature range. Essentially, the same temperature dependence was obtained at 10 and 20 bar pressure of synthetic air. It is demonstrated that temperatures can be determined from the relative fluorescence intensities following excitation of 3-pentanone at 248 and 312 nm, respectively. This new approach could be of interest as a non-intrusive thermometry method, e.g., for the compression phase in combustion engines.     

The laser-diode-excited 5d-4f luminescence of Ce3+ and Pr3+ ions embedded into a BaR2F8 matrix

We show the possibility of obtaining UV luminescence from 5d-4f transitions of rare-earth ions in the BaY₂F₈: (Yb³⁺, Pr³⁺, Ce³⁺) crystal under upconversion excitation by standard laser diodes with lasing wavelengths of 960, 808, and 840 nm. Various upconversion mechanisms of pumping for populating the higher-lying energy levels of the active ions, as well as methods of adaptation of the active medium BaY₂F₈: (Yb³⁺, Pr³⁺, Ce³⁺) to these mechanisms, are considered.     

Luminescence of oxygen molecular ion in neodymium-doped yttria

Pulsed cathodoluminescence spectra of Nd³⁺:Y₂O₃ compacts registered after their annealing in air at a temperature above 950°C exhibit a structured band in the range 610–660 nm with four well-resolved components located at wavelengths of 620.6, 630.6, 645.3, and 655.6 nm. At the same time, the lattice parameter of the cubic yttria increases, and the color of samples changes from light blue to yellowish. In addition, the nearly complete absorption of the intrinsic luminescence band of yttria is observed in the range of 380–600 nm with a center at λ = 485 nm. We assume that these effects are caused by oxygen molecular ions O₂⁻, which are formed in cubic yttria due to the penetration of oxygen into natural anion vacancies. The frequencies of vibrations of the ground and excited states of the oxygen molecular ion has been determined.     

Luminescence properties of Eu3+ and Sm3+ coactivated Gd(III) tungstate phosphor for light-emitting diodes

Rare-earth ions coactivated red phosphors Gd_0.2RE_1.8(WO₄)₃ (RE=Eu³⁺ and Sm³⁺) were synthesized by conventional solid-state reaction using boric acid as a flux agent. The samples were characterized by X-ray diffractometer (XRD), energy-dispersive X-ray spectrometer (EDS) and luminescence spectrometer (LS). The results showed that the Eu–Sm system exhibits higher emission intensity than those of the Eu single-doped system and Sm separate-doped system under ultraviolet (UV) radiation. Samarium(III) ions are effective in broadening and strengthened absorptions around 400 nm. Furthermore, it exhibits enhanced luminescence emission. when the mole ratio of boric acid is about 0.16, the luminescence capability is optimum. Two strongest lines at ultraviolet (394 nm) and blue (465 nm) in excitation spectra of these phosphors match well with the output wavelengths of UV and blue GaN-based light-emitting diodes (LEDs) chips.     

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.     

Attenuation corrections for in-cylinder NO LIF measurements in a heavy-duty Diesel engine

Quantification of the nitric oxide (NO) concentration inside the cylinder of a Diesel engine by means of laser-induced fluorescence (LIF) measurements requires, amongst others, knowledge of the attenuation of the ultraviolet radiation involved. We present a number of laser diagnostic techniques to assess this attenuation, enabling a correction for laser intensity and detection efficiency of the raw NO LIF data. Methods discussed include overall laser beam transmission, bidirectional laser scattering (bidirectional LIF), spectrally resolved fluorescence imaging, and Raman scattering by N₂. A combination of techniques is necessary to obtain the complete attenuation of laser beam and NO fluorescence. The overall laser beam transmission measurements and bidirectional LIF measurements (the latter yielding spatially resolved transmission) provide evidence of a non-uniform attenuation distribution, with predominant attenuation within or near the piston bowl. Fluorescence imaging of multiple vibrational bands through a spectrograph is shown to be a powerful method for obtaining spatially resolved data on the transmission losses of fluorescence. Special attention is paid to the role of CO₂ and O₂ as UV light absorbers, and the consequences to different excitation-detection schemes for NO. PACS 82.33.Vx; 42.62.Fi; 33.20.t