Laser-induced Fluorescence Signal Research Articles

Improvement of precision and accuracy of temperature imaging in sooting flames using two-line atomic fluorescence (TLAF)

Severe interference to the fluorescence signal from seeded indium has previously limited the application of current non-linear excitation regime two-line atomic fluorescence (NTLAF) to low sooting flames. The present paper reports new details of these interferences, from spatially- and temporally-resolved spectral measurements in a laminar, non-premixed sooting C2H4 flame burning in air. Three types of interference to the indium LIF signal have been identified: laser-induced fluorescence (LIF) from polycyclic aromatic hydrocarbons (PAH), elastic laser scattering (ELS) and laser-induced incandescence (LII) from soot particles. This knowledge was used to propose new methods to suppress interferences and select customized narrow-band filters for the signal collection, with a narrow bandwidth of ∼1.2 nm (full-width at half-maximum, FWHM) and a high peak transmission of ∼95%. The efficacy of the narrow-band filters for interference suppression in planar NTLAF measurements was assessed by comparing these measurements with those obtained with a set of conventional band-pass filters (FWHM = 10 nm), in both a soot-free flame and a sooting flame. The application of the narrow-band filters was found to increase both the accuracy and the precision of the temperature measurement, so that measurement accuracy that is due to the presence of interferences has been improved from 198 K to 10 K at a location where the soot volume fraction (fv) is 1.2 ppm. An average reduction of 40% in the standard deviation of measured temperature in single-shot measurements has also been achieved with the use of high transmission filters. With the use of narrow-band filters, NTLAF can be reliably applied in laboratory target sooting flames with peak soot volume fraction of several ppm.

Evidence for the Active Phase of Heterogeneous Catalysts through In Situ Reaction Product Imaging and Multiscale Modeling

We use multiscale modeling to analyze laser-induced fluorescence (LIF) measurements of the CO oxidation reaction over Pd(100) at near-ambient reaction conditions. Integrating density functional theory-based kinetic Monte Carlo simulations of the active catalyst into fluid-dynamical simulations of the mass transport inside the reactor chamber, we calculate the reaction product concentration directly above the catalyst surface. Comparing corresponding data calculated for different surface models against the measured LIF signals, we can discriminate the one that predominantly actuates the experimentally measured catalytic activity. For the probed CO oxidation reaction conditions, the experimental activity is due to pristine Pd(100) possibly coexisting with other (oxidic) domains on the surface.

Standoff Detection of Biomolecules by Ultraviolet Laser-Induced Fluorescence LIDAR

Standoff ultraviolet (UV) laser-induced fluorescence (LIF) Light Detection and Range (LIDAR) system has been developed and used for the detection of biological agents/simulants of Bacillus anthracis . The system employs a fiber coupled charge coupled devices spectrometer as sensor. Fourth harmonic (266 nm) of Nd-YAG laser has been used as excitation wavelength. Standoff fluorescence spectra were recorded for biomolecules in aerosol form, in liquid form, and also in solid form adsorbed on surfaces. The signature fluorescence peaks of biomolecules in all forms were dominated by tryptophan peak centered at $\sim 350$ nm. The UV-visible LIF broadband spans between 300 and 500 nm when excitation wavelength 266 nm was used. LIF spectra were recorded with high signal-to-noise ratio (>10) for concentrations down to 10 ppm and from standoff distances up to 100 m. The laser excitation energy dependence of standoff LIF signal was studied.

Analysis of Laser-Induced Fluorescence Spectra Obtained in Spray Flames of Diesel and Rapeseed Methyl Ester Using the Multiple-Excitation Wavelength Laser-Induced Incandescence Technique with IR, UV, and Visible Excitations

Laser induced fluorescence (LIF) measurements have been performed in spray flames of diesel and rapeseed methyl ester (RME) using laser excitations of 266 nm, 355 nm, and 532 nm. Ultraviolet (UV) measurements in cold sprays of both fuels have also been carried out and are reported in this article. The LIF spectra obtained when exciting the cold spray of diesel with 266-nm and 355-nm laser excitations have been correlated to the presence of petrogenic polycyclic aromatic hydrocarbons (PAHs), which are known to be present in petroleum-derived fuels. Low fluorescence signals have also been detected for such excitation wavelengths in the RME spray. These signals have been related to the presence of trace species derived from the vegetable oil used to obtain the studied biodiesel. LIF signals obtained in flame conditions have been originally derived using a spectrally resolved multiple-excitation wavelength technique coupling laser-induced incandescence and fluorescence (LII/LIF). Such a technique is well adapted to isolate fluorescence emissions with a high signal/noise ratio in flame regions where soot and soot precursors coexist. A significant reduction of the LIF intensities has been observed for the three considered excitation wavelengths when using RME instead of diesel. The spectra obtained as a function of the height above the burner (HAB) with UV excitations are red-shifted in the diesel flame, while they exhibit the same spectral width in the RME one. This could be explained by the presence of other species in the RME flame, such as carbonyl compounds, that can interfere with the fluorescence of PAHs. LIF spectra at 532 nm present the same shape in both flames regardless of the HAB. Similar LIF spectra have also been obtained at specific positions in both flames with UV and visible excitations, which can be related to a similar distribution of PAHs. Thus, such a finding suggests that the heavy aromatic compounds that are related to the formation of nascent soot are similar despite the absence of petrogenic PAHs in RME.

Temporally and spectrally resolved UV absorption and laser-induced fluorescence measurements during the pyrolysis of toluene behind reflected shock waves

Toluene is frequently used as fluorescence tracer in high-temperature combustion applications. A quantitative analysis of laser-induced fluorescence (LIF) signals requires the knowledge of photophysical properties and decomposition kinetics. Using spectrally and temporally resolved ultraviolet absorption and LIF measurements, we studied the spectral properties of toluene and its pyrolysis products behind shock waves between 810 and 1,755 K. Transient absorption spectra were acquired between 220 and 300 nm. The temporal behavior of the absorption at 266 nm was compared to simulations based on literature kinetics models of toluene pyrolysis and available high-temperature absorption cross-sections of toluene, benzyl radicals, and C7H6 as a product from benzyl decomposition. Experiment and simulation agree well at the beginning of the pyrolysis process, whereas for longer reaction times deviations occur presumably due to the build-up of high molecular weight species, which contribute to the observed absorption but have unknown spectral properties. Additionally, LIF emission spectra were recorded following 266-nm excitation at selected reaction times. From measurements up to 1,220 K, the relative fluorescence quantum yield of toluene was derived, extending existing data to higher temperatures. Products from toluene pyrolysis were found to be the major contributors to the LIF signal at higher temperatures.

Mechanism for laser-induced fluorescence signal generation in a nanoparticle-seeded flow for planar flame thermometry

The mechanism of atomic indium generation for laser-induced fluorescence (LIF) of indium from laser ablation seeding was investigated in a hydrogen/nitrogen non-premixed flame. The morphology and particle size distributions of the ablation products were examined with scanning electron microscopy and transmission electron microscopy. These investigations show that the ablation products comprise complex agglomerates of nano-sized primary particles of indium compounds and micron-sized spherical indium beads. Images of the atomic indium LIF, Mie scattering of ablation products and natural fluorescence emission of indium in the flame were recorded to investigate the mechanism of fluorescence signal generation. The relative contribution of natural fluorescence emission of indium towards the total indium fluorescence signal was assessed by comparing these images. These images also reveal the evolution of ablation products through the flame structure and the correlation between LIF signal and ablation products. It is found that the LIF signal generation is associated with the vapourisation of indium nanoparticles into the gas phase by thermal decomposition in the flame. A further mechanism for thermal decomposition of the nanoparticles was also identified, that of heating the ablation products by in situ laser ablation. This was assessed by means of a second laser, introduced prior to the excitation laser, to reveal that the LIF signal can be enhanced by in situ laser ablation, particularly in the upstream regions of the flame. These findings supersede the mechanism deduced previously by the authors that neutral atomic indium can survive a convection time of the order of tens of seconds and be directly seeded into reacting or non-reacting flows. The possible influences of laser ablation seeding on the nonlinear two-line atomic fluorescence thermometry technique were also assessed.

Ion mobility spectrometry coupled to laser-induced fluorescence for probing the electronic structure and conformation of gas-phase ions

We report on an improved design of a differential ion mobility analyzer (DMA) coupled to laser-induced fluorescence (LIF) for the simultaneous retrieval of two-dimensional information on the electric mobility and fluorescence spectroscopy of gas-phase ions. This enhanced design includes an ion funnel inter-face at the input orifice of the DMA and a nozzle beam stage at the output of the DMA. These improvements allow the detection of fluorescence not only from pure dyes and their clusters, as was demonstrated recently, but also from fluorophore-tagged biomolecules. Complex mixtures of fluorescent compounds can be separated by the DMA and studied by LIF. This unique combination of instruments also provides a powerful platform for probing fluorescent proteins in the gas phase. The green fluorescent protein (GFP) was tested on a new setup. In contrast to high vacuum, where no GFP fluorescence was detected, the presence of a LIF signal at the output of the DMA could explain some specific fluorescent properties of GFP in the gas phase. Given that both conformation and fluorescence are key properties of biological molecules in the gas phase, we expect that our enhanced design will answer the question whether gas-phase proteins retain their liquid-phase native structure or not.

Investigations of high-lying even-parity energy levels of atomic samarium using simultaneous observation of two-color laser-induced fluorescence and photoionization signals

Laser-induced photoionization and fluorescence signals were simultaneously observed in atomic samarium using two Nd:YAG-pumped dye lasers. Two-color, three-photon photoionization and two-color fluorescence signals were recorded simultaneously under similar experimental conditions and their dependence as a function of the second-step laser power was studied to understand the behavior of these signals. The laser-induced fluorescence and photoionization techniques have been used simultaneously for the first time to understand the behavior of these signals so that these techniques can be used efficiently in complementary manner to investigate the high-lying energy levels. The even-parity energy levels of atomic samarium in the energy region 36 510–36 875 cm-1 have been investigated based on these experiments. Total 21 high-lying even-parity energy levels have been identified and the two energy levels amongst them are reported as new energy levels after comparing our results with those reported in the literature.

Simultaneous two-phase flow measurement of spray mixing process by means of high-speed two-color PIV

In this article, a novel high-speed two-color PIV optical diagnostic technique has been developed and applied to simultaneously measure the velocity flow-fields of a multi-hole spark-ignition direct injection (SIDI) fuel injector spray and its ambient gas in a high-pressure constant volume chamber. To allow for the phase discrimination between the fuel droplets and ambient gas, a special tracer-filter system was designed. Fluorescent seeding particles with Sauter mean diameter (SMD) of 4.8 µm were used to trace the gas inside the chamber. With a single high-speed Nd:YLF laser sheet (527 nm) as the incident light source, the Mie-scattering signal marked the phase of the fuel spray, while the fluorescent signal generated from the seeding particles tracked the phase of ambient gas. A high-speed camera, with an image-doubler (mounted in front of the camera lens) that divided the camera pixels into two parts focusing on the same field of view, was used to collect the Mie-scattering signal and LIF (laser induced fluorescence) signal simultaneously with two carefully selected optical filters. To accommodate the large dynamic range of velocities in the two phases (1–2 orders of magnitude difference), two separation times (dt) were introduced. This technique was successfully applied to the liquid spray and ambient gas two-phase flow measurement. The measurement accuracy was compared with those from LDV (laser Doppler velocimetry) measurement and good agreement was obtained. Ambient gas motion surrounding the fuel spray was investigated and characterized into three zones. The momentum transfer process between the fuel spray and ambient gas in each zone was analyzed. The two-phase flow interaction under various superheated conditions was investigated. A strengthened momentum transfer from the liquid spray to the ambient was observed with increased superheat degree.

In-cylinder soot precursor growth in a low-temperature combustion diesel engine: Laser-induced fluorescence of polycyclic aromatic hydrocarbons

The growth of poly-cyclic aromatic hydrocarbon (PAH) soot precursors are observed using a two-laser technique combining laser-induced fluorescence (LIF) of PAH with laser-induced incandescence (LII) of soot in a diesel engine under low-temperature combustion (LTC) conditions. The broad mixture distributions and slowed chemical kinetics of LTC “stretch out” soot-formation processes in both space and time, thereby facilitating their study. Imaging PAH–LIF from pulsed-laser excitation at three discrete wavelengths (266, 532, and 633nm) reveals the temporal growth of PAH molecules, while soot-LII from a 1064-nm pulsed laser indicates inception to soot. The distribution of PAH–LIF also grows spatially within the combustion chamber before soot-LII is first detected. The PAH–LIF signals have broad spectra, much like LII, but typically with spectral profile that is inconsistent with laser-heated soot. Quantitative natural-emission spectroscopy also shows a broad emission spectrum, presumably from PAH chemiluminescence, temporally coinciding with of the PAH–LIF.

Measurements of excited-state-to-excited-state transition probabilities and photoionization cross-sections using laser-induced fluorescence and photoionization signals

Laser-induced photoionization and fluorescence signals were simultaneously observed in atomic samarium using Nd:YAG-pumped dye lasers. Two-color, three-photon photoionization and two-color fluorescence signals were recorded simultaneously as a function of the second-step laser power for two photoionization pathways. The density matrix formalism has been employed to analyze these signals. Two-color laser-induced fluorescence signal depends on the laser powers used for the first and second-step transitions as well as the first and second-step transition probability whereas two-color, three-photon photoionization signal depends on the third-step transition cross-section at the second-step laser wavelength along with the laser powers and transition probability for the first and second-step transitions. Two-color laser-induced fluorescence was used to measure the second-step transition probability. The second-step transition probability obtained was used to infer the photoionization cross-section. Thus, the methodology combining two-color, three-photon photoionization and two-color fluorescence signals in a single experiment has been established for the first time to measure the second-step transition probability as well as the photoionization cross-section.

Measurement of radiative lifetime in atomic samarium using simultaneous detection of laser-induced fluorescence and photoionization signals

In this paper, we report the investigations of lifetime measurement of odd-parity energy level 19009.52 cm−1 of Sm I using simultaneous detection of laser-induced fluorescence and laser-induced photoionization signals employing pump–probe technique. To the best of our knowledge, this is for the first time that the results obtained using laser-induced fluorescence and photoionization techniques have been compared with each other. The obtained results match well with those reported in the literature.

OH density measurements in nanosecond pulsed discharges in atmospheric pressure N2–H2O mixtures

Time and spatially resolved OH densities are determined in a nanosecond pulsed filamentary discharge in N2–H2O for varying water concentration using laser-induced fluorescence (LIF). The OH densities are absolutely calibrated using Rayleigh scattering. A dip in the LIF signal is found at the centre of the filament. In the case of 2.6% H2O a remarkable increase in OH density is found in the far afterglow and is discussed in analogy with the pink splash or pink afterglow. It is believed that the large amount of energy released in the recombination of atomic nitrogen plays an important role in the production of OH in the far afterglow.

Validation and evaluation of a novel time-resolved laser-induced fluorescence technique

We present a novel technique to measure time-resolved laser-induced fluorescence signals in plasma sources that have a relatively constant Fourier spectrum of oscillations in steady-state operation, but are not periodically pulsed, e.g., Hall thrusters. The technique uses laser modulation of the order of MHz and recovers signal via a combination of band-pass filtering, phase-sensitive detection, and averaging over estimated transfer functions calculated for many different cycles of the oscillation. Periodic discharge current oscillations were imposed on a hollow cathode. Measurements were validated by comparison with independent measurements from a lock-in amplifier and by comparing the results of the transfer function average to an independent analysis technique triggering averaging over many oscillation cycles in the time domain. The performance of the new technique is analyzed and compared to prior techniques, and it is shown that this new technique has a niche in measurements where the analog photomultiplier signal has a nonwhite noise spectral density and cycles of oscillation are not sufficiently repeatable to allow for reliable triggering or a meaningful average waveform in the time domain.

Scalar gradient behaviour in MILD combustion

The results of three-dimensional Direct Numerical Simulation (DNS) of Moderate, Intense Low-oxygen Dilution (MILD) and conventional premixed turbulent combustion conducted using a skeletal mechanism including the effects of non-unity Lewis numbers and temperature dependent transport properties are analysed to investigate combustion characteristics using scalar gradient information. The DNS data is also used to synthesise laser induced fluorescence (LIF) signals of OH, CH2O, and CHO. These signals are analysed to verify if they can be used to study turbulent MILD combustion and it has been observed that at least two (OH and CH2O) LIF signals are required since the OH increase across the reaction zone is smaller in MILD combustion compared to premixed combustion. The scalar gradient PDFs conditioned on the reaction rate obtained from the DNS data and synthesised LIF signals suggests a strong gradient in the direction normal to the MILD reaction zone with moderate reaction rate implying flamelet combustion. However, the PDF of the normal gradient is as broad as for the tangential gradient when the reaction rate is high. This suggests a non-flamelet behaviour, which is due to interaction of reaction zones. The analysis of the conditional PDFs for the premixed case confirms the expected behaviour of scalar gradient in flamelet combustion. It has been shown that the LIF signals synthesised using 2D slices of DNS data also provide very similar insights. These results demonstrate that the so-called flameless combustion is not an idealised homogeneous reactive mixture but has common features of conventional combustion while containing distinctive characteristics.

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, Fi) 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.

Investigation of the chemical stability of the laser-induced fluorescence tracers acetone, diethylketone, and toluene under IC engine conditions using Raman spectroscopy

This paper reports on an investigation of the chemical stability of the common laser-induced fluorescence (LIF) tracers acetone, diethylketone, and toluene. Stability is analyzed using linear Raman spectroscopy inside a heated pressure cell with optical access, which is used for the LIF calibration of these tracers. The measurements examine the influence of temperature, pressure, and residence time on tracer oxidation, which occurs without a rise in temperature or pressure inside the cell, highlighting the need for optical detection. A comparison between the three different tracers shows large differences, with diethylketone having the lowest and toluene by far the highest stability. An analysis of the sensitivity of the measurement shows that the detection limit of the oxidized tracer is well below 3% molar fraction, which is typical for LIF applications in combustion devices such as internal combustion (IC) engines. Furthermore, the effect on the LIF signal intensity is examined in an isothermal turbulent mixing study.

Soot, PAH and OH measurements in vaporized liquid fuel flames

Qualitative measurements of both OH and polycyclic aromatic hydrocarbons (PAH) as well as quantitative data of soot volume fraction fv have been obtained in vaporized liquid fuels, with the main goal of providing experimental data for combustion models and numerical mechanism validation. Measurements were carried out in a laminar coflow burner which was designed, built and integrated with an evaporation system, enabling the combustion of vaporized liquid fuels at pressures of up to 3.0MPa to be studied. The fuels n-heptane and n-decane, referred to in the literature as being important surrogate fuels, were selected for measurements at atmospheric pressure that were carried out in the experimental setup described, making use of a combination of the Laser Induced Fluorescence (LIF) and Laser Induced Incandescence (LII) techniques. Partially premixed flames of n-heptane and n-decane showed similar combustion characteristics in the range of 1.9<ϕ<3.7. For both of the fuels the threshold for soot formation was found to be at about ϕ∼3.3. Comparison of the LIF and LII measurements for the n-heptane flames within the range of 3.7⩽ϕ⩽8.5 indicated the maximum PAH LIF signal to be a good predictor of fvmax obtained from LII. Excitation at 266nm using delayed detection was found to result in the signals obtained showing a close linear correlation with the soot volume fraction obtained from LII when prompt detection at 1064nm excitation was used. Neither the effects of any additional photo-chemical processes making use of high laser power at 266nm nor the effects of particle size on the delayed gate times appeared to be of any significance for the flame conditions studied here.

A study on the temporally and spatially resolved OH radical distribution of a room-temperature atmospheric-pressure plasma jet by laser-induced fluorescence imaging

In this paper, the time and spatially resolved OH distribution of a room-temperature atmospheric-pressure plasma jet is investigated using a laser-induced fluorescence (LIF) method. The plasma jet is generated in room air by applying a nanosecond pulsed high voltage onto a ceramic tube with helium gas flow. It is found that, before the plasma ‘bullet’ propagates through the region detected by the laser, there are low OH LIF signals, which are from the OH left from previous discharge pulses. After the propagation of the primary plasma ‘bullet’ (generated by the primary discharge at the rising edge of the voltage pulse) through the detected region, the OH LIF signals are more than doubled. Furthermore, after the propagation of the secondary plasma ‘bullet’ (generated by the secondary discharge at the falling edge of the voltage pulse) through the detected region, the OH LIF signals are doubled again. Then, after the voltage pulse, the OH LIF signals decay slowly until about 120 µs. Starting from about 120 µs after the voltage pulse, the OH LIF signals have a third increase; its peak value is more than doubled. Detailed investigations find that this is due to the gas flow, which blows the OH generated inside the discharge tube to the detected region. In addition, spatially resolved OH LIF signals show that the signal intensity is stronger on both edges, which gives rise to the donut shape of the OH distribution. Further studies reveal that this might be due to the interaction of the plasma plume with the surrounding water vapor in the air.

Laser velocimetry with fluorescent dye-doped polystyrene microspheres

Simultaneous Mie scattering and laser-induced fluorescence (LIF) signals are obtained from individual polystyrene latex microspheres dispersed in an air flow. Microspheres less than 1 μm mean diameter were doped with two organic fluorescent dyes, Rhodamine B (RhB) and dichlorofluorescein (DCF), intended either to provide improved particle-based flow velocimetry in the vicinity of surfaces or to provide scalar flow information (e.g., marking one of two fluid streams). Both dyes exhibit measureable fluorescence signals that are on the order of 10(-3) to 10(-4) times weaker than the simultaneously measured Mie signals. It is determined that at the conditions measured, 95.5% of RhB LIF signals and 32.2% of DCF signals provide valid laser-Doppler velocimetry measurements compared with the Mie scattering validation rate with 6.5 W of 532 nm excitation, while RhB excited with 1.0 W incident laser power still exhibits 95.4% valid velocimetry signals from the LIF channel. The results suggest that the method is applicable to wind tunnel measurements near walls where laser flare can be a limiting factor and monodisperse particles are essential.