SwePub - search: WFRF:(Brackmann Christian)

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1.	Afzelius, Mikael, et al. (author) Development of multipoint vibrational coherent anti-Stokes Raman spectroscopy for flame applications 2006 In: Applied Optics. - 2155-3165. ; 45:6, s. 1177-1186 Journal article (peer-reviewed)abstract A novel technique for coherent anti-Stokes Raman spectroscopy (CARS) measurements in multiple points is presented. In a multipass cavity the pump and Stokes laser beams are multiply reflected and refocused into a measurement volume with an adjustable number of separated points along a line. This optical arrangement was used in a vibrational CARS setup with planar BOXCARS phase-matching configuration. The CARS spectra from spatially separated points were recorded at different heights on a CCD camera. Measurements of temperature profiles were carried out in the burned gas zone of a premixed one-dimensional flame to demonstrate the applicability of this method for temperature measurements in high-temperature regions. The ability to measure in flames with strong density gradients was demonstrated by simultaneous measurements of Q-branch spectra of N-2 and CO in a Wolfhard-Parker burner flame. Interference phenomena found in multipoint spectra are discussed, and possible solutions are proposed. Merits and limitations of the technique are discussed. (c) 2006 Optical Society of America.
2.	Afzelius, Mikael, et al. (author) Pure rotational coherent anti-Stokes Raman spectroscopy in mixtures of CO and N-2 2004 In: Applied Optics. - 2155-3165. ; 43:36, s. 6664-6672 Journal article (peer-reviewed)abstract We present a model for quantitative measurements in binary mixtures of nitrogen and carbon monoxide by the use of dual-broadband rotational coherent anti-Stokes Raman spectroscopy. The model has been compared with experimental rotational coherent anti-Stokes Raman scattering spectra recorded within the temperature range of 294-702 K. Temperatures and concentrations were evaluated by spectral fits using libraries of theoretically calculated spectra. The relative error of the temperature measurements was 1-2%, and the absolute error of the CO concentration measurements was <0.5% for temperatures less than or equal to600 K. For higher temperatures, the gas composition was not chemically stable, and we observed a conversion of CO to CO2. The influence of important spectroscopic parameters such as the anisotropic polarizability and Raman line-broadening coefficients are discussed in terms of concentration measurements. In particular, it is shown that the CO concentration measurement was more accurate if N-2-CO and CO-N-2 line-broadening coefficients were included in the calculation. The applicability of the model for quantitative flame measurements is demonstrated by measuring CO concentrations in ethylene/air flames. (C) 2004 Optical Society of America.
3.	Alekseev, Vladimir A., et al. (author) Nitric oxide formation in flames of NH3/DME binary mixtures : Laser-induced fluorescence measurements and detailed kinetic analysis 2024 In: Fuel. - 0016-2361. ; 371 Journal article (peer-reviewed)abstract Binary mixture of ammonia (NH3) and dimethyl ether (DME) has been considered in literature as a potential fuel for practical use. Nitric oxide (NO) is a major product of combustion of NH3-containing fuels, and its formation routes have to be comprehensively studied. In this work, concentration profiles of NO were experimentally measured in laminar axisymmetric flames using planar laser-induced fluorescence. The molar percentage of NH3 in the NH3/DME fuel mixture varied from 10% to 60%. Emission levels of NO have reached as much as around 1% for mixtures with around 50% NH3. NO formation was analyzed with numerical simulations of 1D laminar flames and several detailed kinetic mechanisms. Modeling was performed in Chemkin with the steady-state burner-stabilized and free-propagating planar laminar flame reactor models. It was observed that the most recent versions of the contemporary NH3/DME models are able to reproduce the experiments, and their predictions agree with each other due to similarities in the NH3 submechanisms. Kinetic analysis has revealed some disagreement was observed in terms of how much direct chemical coupling between carbon- and nitrogen-containing species affects NO formation.
4.	Amdani, Rima Zuriah, et al. (author) Raman spectroscopy for characterizing porous carbon 2020 In: Laser Applications to Chemical, Security and Environmental Analysis, LACSEA 2020. - 9781557528209 Conference paper (peer-reviewed)abstract Raman spectroscopy is performed to disclose structural properties of porous carbon samples including lignocellulosic biomass. Analysis of spectral signatures of carbon, around 1350 cm-1 and 1580 cm-1, indicates higher porosity for the biomass sample.
5.	Bengtsson, Anton, 1979, et al. (author) Effects of thermal processing on the in vitro bioaccessibility and microstructure of beta-carotene in orange-fleshed sweet potato 2010 In: Journal of Agricultural and Food Chemistry. - : American Chemical Society (ACS). - 0021-8561 .- 1520-5118. ; 58:20, s. 11090-11096 Journal article (peer-reviewed)abstract The effects of different preparation methods on the bioaccessibility of beta-carotene in orange-fleshed sweet potato (OFSP), an important food crop in sub-Saharan Africa, have been evaluated using an in vitro digestion procedure. The preparation methods included, on fresh roots, boiling followed by pureeing and oil addition (BOL) and homogenization followed by boiling and oil addition (HOM); on milled flour from freeze-dried fresh roots, cooking of porridge followed by oil addition (POA) and oil addition to flour followed by cooking of porridge (POB). The retention of all-trans-beta-carotene ranged from 58% (POB) to 72% (BOL). The presence of oil during heating resulted in a significantly higher formation of 13-cis-beta-carotene for the POB-treated samples than for the other samples. The efficiency of micellarization of all-trans-beta-carotene after in vitro digestion was 50% (HOM), 48% (POB), 31% (POA), and 16% (BOL). Brightfield microscopy of the cell structure after processing and in vitro digestion showed a high degree of cell-wall rupture for the HOM-treated samples, whereas cells appeared intact for the BOL samples. Also, coherent anti-Stokes Raman scattering (CARS) microscopy showed smaller beta-carotene bodies residing in the HOM samples than in the BOL samples after digestion. These results suggest that the in vitro bioaccessibility of beta-carotene in an OFSP meal can be improved by processing methods that promote cell-wall rupture.
6.	Blomberg, Sara, et al. (author) Real-Time Gas-Phase Imaging over a Pd(110) Catalyst during CO Oxidation by Means of Planar Laser-Induced Fluorescence. 2015 In: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 5:4, s. 2028-2034 Journal article (peer-reviewed)abstract The gas composition surrounding a catalytic sample has direct impact on its surface structure, which is essential when in situ investigations of model catalysts are performed. Herein a study of the gas phase close to a Pd(110) surface during CO oxidation under semirealistic conditions is presented. Images of the gas phase, provided by planar laser-induced fluorescence, clearly visualize the formation of a boundary layer with a significantly lower CO partial pressure close to the catalytically active surface, in comparison to the overall concentration as detected by mass spectrometry. The CO partial pressure variation within the boundary layer will have a profound effect on the catalysts' surface structure and function and needs to be taken into consideration for in situ model catalysis studies.
7.	Bood, Joakim, et al. (author) Multipoint temperature and oxygen-concentration measurements using rotational coherent anti-Stokes Raman spectroscopy 2000 In: Optics Letters. - 0146-9592. ; 25:20, s. 1535-1537 Journal article (peer-reviewed)abstract A novel technique for coherent anti-Stokes Raman spectroscopy (CARS) measurements in multiple points is presented. With a system of cylindrical lenses, each laser beam is split into several focused beams, yielding separate planar boxcars configurations. Spectrally resolved CARS signals are detected at different heights on the CCD chip. With dual-broadband rotational CARS the setup is demonstrated for quantitative measurements of temperature- and oxygen-concentration profiles. The technique was demonstrated for three points only, but it can be extended to more points by use of special optics; this choice must be based on a sufficient signal-to-noise ratio in all points for the actual measurement condition.
8.	Brackmann, Christian, 1973, et al. (author) CARS microscopy of lipid stores in yeast: the impact of nutritional state and genetic background 2009 In: Journal of Raman Spectroscopy. - : Wiley. - 0377-0486 .- 1097-4555. ; 40:7, s. 748-756 Journal article (peer-reviewed)abstract We have developed a protocol for sub-micrometer resolved and chemically specific imaging of lipid storage in vivo employing coherent anti-Stokes Raman scattering (CARS) microscopy of one of the most important model organisms Saccharomyces cerevisiae - the yeast cell. By probing the carbon-hydrogen vibration using the nonlinear process of CARS, lipid droplets in the yeast cells clearly appear, as confirmed by comparative studies on relevant labeled organelles using two-photon fluorescence microscopy. From the images, unique quantitative data can be deduced with high three-dimensional resolution, such as the volume, shape, number, and intracellular location of the neutral lipid stores. We exemplify the strength and usability of the method for two cases: the impact on lipid storage of the nutritional condition (starvation and type of carbon source available) as well as of genetic modification of two fundamental metabolic regulation pathways involving carbohydrate and lipid storage (BCY1 and DGA1, LRO1, ARE1/2 deletions), respectively. While the impact of carbon source on the total cellular lipid volume was minimal, long-term starvation induces a significant accumulation of lipid droplets. We also confirm that the lipid-storage-deficient mutant is indeed unable to synthesize lipid droplets, and that the inability of the bcy1-mutant to store carbohydrates is compensated by a two-fold increase in stored neutral lipids. We note that there is a significant cell-to-cell variability in neutral lipid storage in general, i.e. that there is a correspondence to the noise found for gene expression also in lipidomics. Copyright (C) 2009 John Wiley & Sons, Ltd.
9.	Brackmann, Christian, et al. (author) Characterization of ammonia two-photon laser-induced fluorescence for gas-phase diagnostics 2014 In: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 115:1, s. 25-33 Journal article (peer-reviewed)abstract 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/cm(2). Stimulated emission, resulting in loss of excited molecules, is induced above 10 GW/cm(2), 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.
10.	Brackmann, Christian, 1973, et al. (author) Coherent anti-Stokes Raman scattering microscopy of human smooth muscle cells in bioengineered tissue scaffolds 2011 In: Journal of Biomedical Optics. - : SPIE - International Society for Optical Engineering. - 1083-3668 .- 1560-2281. ; 16:2 Journal article (peer-reviewed)abstract The integration of living, human smooth muscle cells in biosynthesized cellulose scaffolds was monitored by nonlinear microscopy toward contractile artificial blood vessels. Combined coherent anti-Stokes Raman scattering (CARS) and second harmonic generation (SHG) microscopy was applied for studies of the cell interaction with the biopolymer network. CARS microscopy probing CH(2)-groups at 2845 cm(-1) permitted three-dimensional imaging of the cells with high contrast for lipid-rich intracellular structures. SHG microscopy visualized the fibers of the cellulose scaffold, together with a small signal obtained from the cytoplasmic myosin of the muscle cells. From the overlay images we conclude a close interaction between cells and cellulose fibers. We followed the cell migration into the three-dimensional structure, illustrating that while the cells submerge into the scaffold they extrude filopodia on top of the surface. A comparison between compact and porous scaffolds reveals a migration depth of <10 μm for the former, whereas the porous type shows cells further submerged into the cellulose. Thus, the scaffold architecture determines the degree of cell integration. We conclude that the unique ability of nonlinear microscopy to visualize the three-dimensional composition of living, soft matter makes it an ideal instrument within tissue engineering.
11.	Brackmann, Christian (author) Development and Application of Rotational Coherent Anti-Stokes Raman Spectroscopy and Laser-induced Fluorescence for Combustion Diagnostics 2004 Doctoral thesis (other academic/artistic)abstract The thesis concerns the development and application of two laser-based methods for combustion diagnostics: dual broadband rotational coherent anti-Stokes Raman spectroscopy (CARS) and laser-induced fluorescence (LIF). In efforts to increase the usefulness of the CARS technique, the applicability of a method involving a combination of the rotational CARS technique and the closely related vibrational CARS technique was investigated. In addition an extension of the rotational CARS technique such that not only single-point but also multiple-points measurements could be performed was presented. The rotational CARS technique was also used in an experimental investigation of a dimethyl ether (DME)-air diffusion flame for determining temperatures and relative oxygen concentrations. The major combustion application of the rotational CARS technique taken up in the thesis was its use for temperature measurements in an internal combustion engine operated under knocking combustion conditions. Temperature measurements were performed at different locations within the combustion chamber and at different points in time during the engine cycle. The insights these measurements provided, both concerning the experimental application of the rotational CARS technique in engines and in the evaluation of the measured data, were summarized in a review paper. The work with the LIF technique presented in the thesis involved primarily studies of formaldehyde, CH2O. Spectroscopic investigations of CH2O using excitation at a wavelength of 355 nm were carried out in laboratory flames. Laser-induced fluorescence by use of 355 nm excitation was also employed for measurements of formaldehyde in internal combustion engines. Measurements in a spark ignition engine involving two consecutive LIF measurements being performed within a given engine cycle provided data that could be used for determining the flame propagation within the combustion chamber. The characterization of the DME diffusion flame referred to above also included measurements of formaldehyde by use of the LIF technique, particular attention being directed at obtaining a qualitatively correct profile of the formaldehyde concentration in the flame by adjusting the measured signal to take account of quenching and temperature dependence. The laser-induced fluorescence technique was also used to investigate the devolatilisation of wood particles, a process representing the first step in the combustion of solid fuel of this type. The studies carried out included measurements of absorption and of both spectral and imaging LIF. The multi-species composition of the products emitted, hydrocarbon compounds in particular, resulted for both the absorption and the spectral LIF measurements in spectral signatures without distinct features, making identification of specific species difficult. Formaldehyde, which could be identified from the measured spectra was an exception to this. The imaging LIF measurements also provided some information on the flow of the emitted volatiles in relation to the internal wood fibre structure.
12.	Brackmann, Christian, et al. (author) Experimental and modeling study of nitric oxide formation in premixed methanol + air flames 2020 In: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 213, s. 322-330 Journal article (peer-reviewed)abstract Validation and further development of models for alcohol combustion requires accurate experimental data obtained under well-controlled conditions. To this end, measurements of nitric oxide, NO, mole fractions in premixed laminar methanol + air flames have been performed using saturated laser-induced fluorescence, LIF. The methanol flames have been stabilized at atmospheric pressure and initial gas temperature of 318 K at equivalence ratios ɸ = 0.7–1.5 using the heat flux method that allows for simultaneous determination of their laminar burning velocity. The LIF signal is converted into NO mole fraction via calibration measurements, which have been performed in flames of methane, methanol and syngas seeded with known amounts of NO. The experimental approach is verified by the measurements of NO mole fractions in the post flame zone of methane flames, investigated in previous studies at similar conditions. Data on the NO formation together with burning velocities for methanol and methane obtained under adiabatic flame conditions provide highly valuable input for model validation. They have been compared with predictions of six different chemical kinetic mechanisms. Summarizing the behavior of all models tested with respect to burning velocities and NO formation in flames of methane and methanol, the mechanism of Glarborg et al. (2018) and the San Diego mechanism (2019) demonstrate uniformly satisfactory performance.
13.	Brackmann, Christian, et al. (author) Experimental studies of nitromethane flames and evaluation of kinetic mechanisms 2018 In: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 190, s. 327-336 Journal article (peer-reviewed)abstract The present work reports new experimental data for premixed flames of nitromethane, CH3NO2, at atmospheric pressure, and an evaluation of two contemporary kinetic mechanisms based on these new flame studies as well as previously published experimental data on laminar burning velocity and ignition. Flames of nitromethane + air at lean (ϕ = 0.8) and rich (ϕ = 1.2) conditions were stabilized on a flat-flame burner, where profiles of CH2O, CO and NO were obtained using laser-induced fluorescence and temperature profiles using coherent anti-Stokes Raman spectroscopy. Laminar burning velocities for nitromethane + O2 + CO2 were measured using the heat flux method for ϕ = 0.8–1.3 at 348 K and ϕ = 0.8–1.6 at 358 K, and an oxidizer composition of 35% O2 and 65% CO2. In addition, the effect of the oxidizer composition was examined for a stoichiometric flame at 358 K by varying oxygen fraction from 30% to 40%. The mechanism by Mathieu et al. (Fuel 2016, 182, 597), previously not validated for flames, was able to reproduce experimental laminar burning velocities for nitromethane + air, but under predicted new results for CH3NO2 + O2 + CO2 mixtures. The mechanism by Brequigny et al. (Proc. Combust. Inst. 2014, 35, 703) under predicted experimental laminar burning velocities significantly at all investigated conditions. Previous studies have shown that none of the mechanisms can accurately predict ignition delay time over a wide range of conditions with respect to pressure, temperature, diluent and dilution ratio. The evaluation of the mechanisms reveals that the understanding of nitromethane combustion is at the present time not sufficient to produce a widely applicable mechanism.
14.	Brackmann, Christian, et al. (author) Formation of NO and NH in NH3-doped CH4 + N2 + O2 flame : Experiments and modelling 2018 In: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 194, s. 278-284 Journal article (peer-reviewed)abstract Co-combustion of 5200 ppm NH3 with a stoichiometric, atmospheric pressure, CH4 + N2 + O2 flame has been investigated with experiments and kinetic modelling. Profiles of the amidogen (NH) radical and nitric oxide (NO) have been measured using laser-induced fluorescence, the latter being quantitatively determined. Temperature profiles were measured using Rayleigh scattering and thermocouple, the nonintrusive measurements were considered more reliable and were used for evaluation of LIF data as well as input for flame modelling. Experimental results are compared with predictions of a chemical mechanism developed by Mendiara and Glarborg (2009), with simulations based on solution of energy equation as well as on experimental temperature profiles as input. Compared with a neat flame, the NH3-doped flame shows a shift in position ∼0.7 mm downstream, as established from the measurements of the NH profile. Modelling prediction of post-flame NO concentrations in the NH3-doped flame, around 1160 ppm, was within the evaluated uncertainty with experimental data (1460 ppm). Reaction path analysis indicated NH2 as a key species in the formation of NO and N2 from the nitrogen added to the flame by NH3. Altogether, the mechanism predicts concentration levels rather well but fails to predict the shift in flame position obtained with addition of NH3 to the rather slowly burning hydrocarbon flame.
15.	Brackmann, Christian, 1973, et al. (author) In situ Imaging of Collagen Synthesis by Osteoprogenitor Cells in Microporous Bacterial Cellulose Scaffolds. 2012 In: Tissue Engineering - Part C: Methods. - 1937-3384 .- 1937-3392. ; 18:3, s. 227-234 Journal article (peer-reviewed)abstract Microscopy techniques based on laser-induced nonlinear optical processes allow for chemically specific imaging of unmodified samples at high spatial resolution in three dimensions and provide powerful tools for characterization of tissue-engineering constructs. This is highlighted by the simultaneous imaging of scaffold material, cells, and produced extracellular matrix collagen in samples consisting of osteoprogenitor MC3T3-E1 cells seeded on microporous bacterial cellulose (BC), a potential scaffold material for synthesis of osseous tissue. BC and collagen have been visualized by second harmonic generation (SHG) microscopy, and verification of collagen identification on cellulose scaffolds has been carried out on sectioned samples by comparison with the conventional histological staining technique. Both methods showed similar collagen distributions and a clear increase in the amount of collagen when comparing measurements from two time points during growth. For investigations of intact cellulose scaffolds seeded with cells, SHG was combined with simultaneous coherent anti-Stokes Raman scattering (CARS) microscopy for visualization of cell arrangement in three dimensions and to be correlated with the SHG data. Results showed that the osteoprogenitor cells were able to produce collagen already during the first days of growth. Further on, developed collagen fiber networks could be imaged inside compact regions of cells located in the cellulose micropores. Collagen production, the initial step of tissue mineralization, demonstrates the potential of BC as a scaffold material for bone tissue engineering. Furthermore, the noninvasive in situ monitoring of collagen inside compact tissue clearly manifests the benefits of nonlinear microscopy techniques, such as SHG and CARS, for use in tissue engineering.
16.	Brackmann, Christian, et al. (author) Laser-induced fluorescence of formaldehyde in combustion using third harmonic Nd : YAG laser excitation 2003 In: Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy. - 1386-1425. ; 59:14, s. 3347-3356 Journal article (peer-reviewed)abstract Formaldehyde (CH2O) is an important intermediate species in combustion processes and it can through laser-induced fluorescence measurements be used for instantaneous flame front detection. The present study has focussed on the use of the third harmonic of a Nd:YAG laser at 355 nm as excitation wavelength for formaldehyde, and different dimethyl ether (C2H6O) flames were used as sources of formaldehyde in the experiments. The investigations included studies of the overlap between the laser profile and the absorption lines of formaldehyde, saturation effects and the potential occurrence of laser-induced photochemistry. The technique was applied for detection of formaldehyde in an internal combustion engine operated both as a spark ignition engine and as a homogenous charge compression ignition engine. (C) 2003 Elsevier B.V. All rights reserved.
17.	Brackmann, Christian, 1973, et al. (author) Non-linear microscopy of smooth muscle cells in artificial extracellular matrices made of cellulose 2012 In: Journal of Biophotonics. - : Wiley. - 1864-063X. ; 5:5-6, s. 404-414 Journal article (peer-reviewed)abstract Non-linear microscopy has been used to characterize bovine smooth muscle cells and their proliferation, migration, and differentiation in hydrogel cellulose scaffolds, toward the development of fully functional blood vessel implants. The extracellular matrix (ECM) composed of cellulose and endogenous collagen fibers was imaged using Second Harmonic Generation (SHG) microscopy and the cell morphology by Coherent Anti-Stokes Raman Scattering (CARS) microscopy. Images prove that cells adhere on the cellulose scaffold without additional surface modification and that both contractile and proliferating phenotypes are developed. This work shows that non-linear microscopy contributes with unique insights in cell interactions with (artificial) ECM components and has the potential to become an established characterization method in tissue engineering.
18.	Brackmann, Christian, 1973, et al. (author) Nonlinear microscopy of lipid storage and fibrosis in muscle and liver tissues of mice fed high-fat diets 2010 In: Journal of Biomedical Optics. - : SPIE-Intl Soc Optical Eng. - 1083-3668 .- 1560-2281. ; 15:6 Journal article (peer-reviewed)abstract Hallmarks of high-fat Western diet intake, such as excessive lipid accumulation in skeletal muscle and liver as well as liver fibrosis, are investigated in tissues from mice using nonlinear microscopy, second harmonic generation (SHG), and coherent anti-Stokes Raman scattering (CARS), supported by conventional analysis methods. Two aspects are presented; intake of standard chow versus Western diet, and a comparison between two high-fat Western diets of different polyunsaturated lipid content. CARS microscopy images of intramyocellular lipid droplets in muscle tissue show an increased amount for Western diet compared to standard diet samples. Even stronger diet impact is found for liver samples, where combined CARS and SHG microscopy visualize clear differences in lipid content and collagen fiber development, the latter indicating nonalcoholic fatty liver disease (NAFLD) and steatohepatitis induced at a relatively early stage for Western diet. Characteristic for NAFLD, the fibrous tissue-containing lipids accumulate in larger structures. This is also observed in CARS images of liver samples from two Western-type diets of different polyunsaturated lipid contents. In summary, nonlinear microscopy has strong potential (further promoted by technical advances toward clinical use) for detection and characterization of steatohepatitis already in its early stages.
19.	Brackmann, Christian, et al. (author) Optical and mass spectrometric study of the pyrolysis gas of wood particles 2003 In: Applied Spectroscopy. - : SAGE Publications. - 1943-3530 .- 0003-7028. ; 57:2, s. 216-222 Journal article (peer-reviewed)abstract A detailed experimental investigation has been made of the pyrolysis-the first step in biomass combustion-of single birchwood particles. In addition to mass spectrometric and gravimetric analysis, the pyrolysis volatiles were characterized by different optical techniques. Absorption measurements showed a nearly featureless absorption in the ultraviolet spectral region with a continuously stronger absorption for shorter wavelengths. Using different excitation wavelengths, laser-induced fluorescence measurements revealed generally broad spectra in the spectral region from 300-500 nm, which are characteristic spectral signatures for larger hydrocarbons. The optical data were monitored at different times in the pyrolysis process of the particles and compared with the results from the mass spectrometric and gravimetric analysis. The sensitivity of the optical techniques for differentiation between specific molecules was rather low, although formaldehyde could be observed both in absorption and fluorescence spectra. Laser-induced fluorescence measurements were also made for two-dimensional visualization of the pyrolysis volatiles; emitted from heated birchwood particles, indicating much higher flows along the fiber direction than across.
20.	Brackmann, Christian, et al. (author) Picosecond excitation for reduction of photolytic effects in two-photon laser-induced fluorescence of CO 2013 In: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 34, s. 3541-3548 Journal article (peer-reviewed)abstract Two-photon laser-induced fluorescence for detection of carbon monoxide (CO) frequently shows interferences by emission from photolytically produced C-2 radicals encountered under fuel-rich combustion conditions. Reduced C-2 interference for excitation with laser pulses in the picosecond regime is here demonstrated by comparison with excitation using nanosecond pulses for measurements in laminar premixed ethene-air flames. Compared with nanosecond pulses of 8 ns duration and 4 mJ pulse energy, picosecond pulses of 80 ps duration and around 0.5 mJ pulse energy gave similar to 10 times higher peak power, which allowed for efficient CO excitation and resulted in stronger signal with lower C-2 interference. CO fluorescence with picosecond excitation showed a linear to quadratic power dependence, indicating photoionization, whereas a more quadratic dependence was found for the C-2 interference. A sub-nanosecond effective lifetime of CO resulted in a rapid fluorescence decay compared with C-2 and allowed for efficient reduction in C-2 interference by minimizing the detection gate. In addition, interference compensation using time-resolved detection could be demonstrated. Altogether, picosecond pulses provide efficient two-photon excitation of CO in terms of signal strength as well as reduced C-2 interference. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
21.	Brackmann, Christian, et al. (author) Quantitative measurements of species and temperature in a DME-air counterflow diffusion flame using laser diagnostic methods 2006 In: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 178:6, s. 1165-1184 Journal article (peer-reviewed)abstract A diffusion flame of dimethyl ether (DME) and air in a counterflow burner has been investigated experimentally by means of different laser-based methods. Quantitative measurements of temperature, O-2 concentration, and OH concentration have been carried out using dual broadband rotational coherent anti-Stokes Raman Spectroscopy (CARS), Rayleigh scattering and laser-induced fluorescence (LIF). Furthermore a qualitative formaldehyde profile has been measured using LIF. The measured profile has been compensated for fluorescence signal quenching and the Boltzmann distribution to obtain a representative qualitative formaldehyde profile for the flame.
22.	Brackmann, Christian, et al. (author) Quantitative picosecond laser-induced fluorescence measurements of nitric oxide in flames 2017 In: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 36:3, s. 4533-4540 Journal article (peer-reviewed)abstract Quantitative concentrations measurements using time-resolved laser-induced fluorescence have been demonstrated for nitric oxide (NO) in flame. Fluorescence lifetimes measured using a picosecond Nd:YAG laser and optical parametric amplifier system have been used to directly compensate the measured signal for collisional quenching and evaluate NO concentration. The full evaluation also includes the spectral overlap between the ∼15cm-1 broad laser pulse and multiple NO absorption lines as well as the populations of the probed energy levels. Effective fluorescence lifetimes of 1.2 and 1.5ns were measured in prepared NO/N2/O2 mixtures at ambient pressure and temperature and in a premixed NH3-seeded CH4/N2/O2 flame, respectively. Concentrations evaluated from measurements in NO/N2/O2 mixtures with NO concentrations of 100-600ppm were in agreement with set values within 3% at higher concentrations. An accuracy of 13% was estimated by analysis of experimental uncertainties. An NO profile measured in the flame showed concentrations of ∼1000ppm in the post-flame region and is in good agreement with NO concentrations predicted by a chemical mechanism for NH3 combustion. An accuracy of 16% was estimated for the flame measurements. The direct concentration evaluation from time-resolved fluorescence allows for quantitative measurements in flames where the composition of major species and their collisional quenching on the probed species is unknown. In particular, this is valid for non-stationary turbulent combustion and implementation of the presented approach for measurements under such conditions is discussed.
23.	Brackmann, Christian, et al. (author) Simultaneous vibrational and pure rotational coherent anti-Stokes Raman spectroscopy for temperature and multispecies concentration measurements demonstrated in sooting flames 2002 In: Applied Optics. - 2155-3165. ; 41:3, s. 564-572 Journal article (peer-reviewed)abstract The potential of measuring temperature and multiple species concentrations (N-2, O-2, CO) by use of combined vibrational coherent anti-Stokes Raman spectroscopy (CARS) and pure rotational CARS has been investigated. This was achieved with only one Nd:YAG laser and one dye laser together with a single spectrograph and CCD camera. From measurements in premixed sooting C2H4-air flames it was possible to evaluate temperatures from both vibrational CARS and rotational CARS, spectra, O-2 concentration from the rotational CARS spectra, and CO concentration from the vibrational CARS spectra. Quantitative results from premixed sooting C2H4-air flames are presented, and the uncertainties in the results as well as the possibility of extending the combined CARS technique fur probing of additional species are discussed. (C) 2002 Optical Society of America.
24.	Brackmann, Christian, et al. (author) Strategies for formaldehyde detection in flames and engines using a single-mode Nd:YAG/OPO laser system 2005 In: Applied Spectroscopy. - : SAGE Publications. - 0003-7028 .- 1943-3530. ; 59:6, s. 763-768 Journal article (peer-reviewed)abstract This paper presents technical developments for the detection of formaldehyde (CH2O) using laser-induced fluorescence. The easily accessible third harmonic of the Nd:YAG laser at 355 nm was used for excitation of formaldehyde. In order to investigate potential background fluorescence, e.g., from large molecules such as polyaromatic hydrocarbons, special attention was paid to investigating the possibility of scanning the wavelength of a single-mode Nd: YAG laser under the gain profile, ∼3 cm-1, on and off resonance. Furthermore, a technique for simultaneous detection of formaldehyde and OH using one laser system is presented. The single-mode Nd: YAG laser at 355 nm in combination with an optical parametric oscillator (OPO) laser tuned to 283 nm was used for simultaneous two-dimensional imaging of both species using one charge-coupled device (CCD) detector equipped with a dual filter image separator. The techniques are demonstrated with measurements in laboratory flames and the combined measurements are also demonstrated in an engine.
25.	Brackmann, Christian, et al. (author) Strategies for formaldehyde engines using a single-mode detection in flames and Nd : YAG/OPO laser system 2005 In: Applied Spectroscopy. - 1943-3530. ; 59:6, s. 763-768 Journal article (peer-reviewed)abstract This paper presents technical developments for the detection of formaldehyde (CH2O) using laser-induced fluorescence. The easily accessible third harmonic of the Nd:YAG laser at 355 nm was used for excitation of formaldehyde. In order to investigate potential background fluorescence, e.g., from large molecules such as polyaromatic hydrocarbons, special attention was paid to investigating the possibility of scanning the wavelength of a single-mode Nd:YAG laser under the gain profile, similar to 3 cm(-1), on and off resonance. Furthermore, a technique for simultaneous detection of formaldehyde and 011 using one laser system is presented. The single-mode Nd:YAG laser at 355 nm in combination with an optical parametric oscillator (OPO) laser tuned to 283 nm was used for simultaneous two-dimensional imaging of both species using one charge-coupled device (CCD) detector equipped with a dual filter image separator. The techniques are demonstrated with measurements in laboratory flames and the combined measurements are also demonstrated in an engine.

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