Papers by Johannes Kiefer
Optics letters, Jan 15, 2016
The determination of the absorption and emission spectra of strongly absorbing molecules is chall... more The determination of the absorption and emission spectra of strongly absorbing molecules is challenging, and the data can be biased by self-absorption of the fluorescence signal. To overcome this problem, a total internal reflection approach is proposed. The strongly absorbing sample is placed in an evanescent field of the radiation of a supercontinuum source. The collimated reflected light encodes the absorption spectrum, and the isotropic fluorescence emission is collected in a direction perpendicular to the surface at the same time. This ensures that the emitted light has a minimum possibility of self-absorption inside the sample.
Applied spectroscopy, 2017
Gaining information of species, temperature, and velocity distributions in turbulent combustion a... more Gaining information of species, temperature, and velocity distributions in turbulent combustion and high-speed reactive flows is challenging, particularly for conducting measurements without influencing the experimental object itself. The use of optical and spectroscopic techniques, and in particular laser-based diagnostics, has shown outstanding abilities for performing non-intrusive in situ diagnostics. The development of instrumentation, such as robust lasers with high pulse energy, ultra-short pulse duration, and high repetition rate along with digitized cameras exhibiting high sensitivity, large dynamic range, and frame rates on the order of MHz, has opened up for temporally and spatially resolved volumetric measurements of extreme dynamics and complexities. The aim of this article is to present selected important laser-based techniques for gas-phase diagnostics focusing on their applications in combustion and aerospace engineering. Applicable laser-based techniques for investi...
The journal of physical chemistry. B, Jan 2, 2017
Dielectric relaxation of the ionic liquid, 1-ethyl-3-methylimidazolium ethyl sulfate (EMI(+)ETS(-... more Dielectric relaxation of the ionic liquid, 1-ethyl-3-methylimidazolium ethyl sulfate (EMI(+)ETS(-)), is studied using molecular dynamics (MD) simulations. The collective dynamics of polarization arising from cations and anions are examined. Characteristics of the rovibrational and translational components of polarization dynamics are analyzed to understand their respective roles in the microwave and terahertz regions of dielectric relaxation. The MD results are compared with the experimental low-frequency spectrum of EMI(+)ETS(-), obtained via ultrafast optical Kerr effect (OKE) measurements.
Journal of Molecular Structure, 2017
Proceedings of the Combustion Institute, 2016
Applied Spectroscopy, Sep 1, 2013
A spectral filtering approach for improving the sensitivity of two-color laser-induced incandesce... more A spectral filtering approach for improving the sensitivity of two-color laser-induced incandescence measurements is proposed. The commonly used bandpass filters providing wavelength selection, and hence temperature sensitivity, are replaced by shortpass and longpass filters, respectively, allowing significantly higher signal intensities to be detected. This modification is of particular interest when nanoparticles with low emissivity, for instance, metal and metal oxide particles, are investigated. An example case in which the conventional optical components are compared with the new approach reveals an improvement by more than one order of magnitude.
Chem Ing Tech, 2008
nahme des CO 2 in der Atmosphäre erscheint bei weiter steigenden CO 2 -Emissionen nicht möglich, ... more nahme des CO 2 in der Atmosphäre erscheint bei weiter steigenden CO 2 -Emissionen nicht möglich, gleich aus wel-cher Quelle das CO 2 kommt. Nur deutliche CO 2 -Einsparungen führen zu einer Trendumkehr.
Applied Spectroscopy, Jun 1, 2012
Monitoring of chemical reactors is key to optimizing yield and efficiency of chemical transformat... more Monitoring of chemical reactors is key to optimizing yield and efficiency of chemical transformation processes. Aside from tracking pressure and temperature, the measurement of the chemical composition is essential in this context. We present an infrared difference spectroscopy approach for determining the reactant (cyclooctene) and product (cyclooctane) concentrations during a catalytic hydrogenation reaction in the solvent cyclohexane, which is present in large excess. Subtracting the spectrum of the pure solvent from the reactor mixture spectra yields infrared (IR) spectra, which can ultimately be evaluated using a curve-fitting procedure based on spectral soft modeling. An important feature of our evaluation approach is that the calibration only requires recording the pure component spectra of the reactants, products, and solvent. Hence, no time-consuming preparation of mixtures for calibration is necessary. The IR concentration results are in good agreement with gas chromatography measurements.
Journal of Raman Spectroscopy, Apr 1, 2015
AbstractThe ultrafast vibrational dynamics of the room-temperature ionic liquid 1-ethyl-3-methyli... more AbstractThe ultrafast vibrational dynamics of the room-temperature ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate, [EMIm][EtSO4] are studied using time-resolved femtosecond coherent anti-Stokes Raman scattering spectroscopy. Raman coherences are excited around 1040 and 1400 cm−1, the vibrational dephasing of the modes in the fingerprint region is monitored as a function of time. The results indicate that beating pairs of Raman modes typically correspond to either the cation or the anion. The assignment of mixed mode pairs is not unambiguous and, moreover, the beating patterns of such possible mixed pairs are very weak. Interestingly, no evidence for interionic vibrational energy transfer is found despite the fact that the interactions in the ion pair are characterized by strong hydrogen bonds. Copyright © 2015 John Wiley & Sons, Ltd.
Applied Physics B Lasers and Optics, 2011
In this paper we report the investigation of the laser-induced breakdown and ignition behaviour o... more In this paper we report the investigation of the laser-induced breakdown and ignition behaviour of methane/air and dimethyl ether (DME)/air mixtures. Moreover, the optical emission from the induced plasma is utilized for determining the mixture composition quantitatively by means of laser-induced breakdown spectroscopy (LIBS). To the best of the authors' knowledge, LIBS and laser ignition of DME have not been reported in literature before. The technique under investigation is finally employed for combustion diagnostics in laminar as well as turbulent flames. In the laminar premixed and non-premixed flames the LIBS spectra allow spatially resolved measurements of the equivalence ratio and enable studying the mixing of gases provided through the burner with the surrounding room air. In addition, the breakdown threshold of the applied laser pulse energy yields an estimate for the local temperature. In the turbulent cases single-shot LIBS spectra are recorded at fixed position allowing the derivation of local statistical fluctuations of the equivalence ratio in partially premixed jet flames. The results show that laser-induced breakdowns have a strong potential for flame diagnostics and, under suitable conditions, for the ignition of combustible mixtures.
Kiefer, J. (University of Aberdeen, Scotland, UK) ; Gao, Y. ; Seeger, T. (Friedrich-Alexander-Uni... more Kiefer, J. (University of Aberdeen, Scotland, UK) ; Gao, Y. ; Seeger, T. (Friedrich-Alexander-Universitat Erlangen-N%C3%BCrnberg, Erlangen, Germany) ; Kliewer, Christopher Jesse ; Patterson, Brian D. ; Farrow, Roger L. ; Settersten, Thomas B. ... ; Conference: Proposed for presentation ...
Applied Spectroscopy, Dec 1, 2007
The vibrational structure of two room-temperature ionic liquids with the cation 1-ethyl-3-methyli... more The vibrational structure of two room-temperature ionic liquids with the cation 1-ethyl-3-methylimidazolium [EMIM] and the respective anions bis(trifluoromethylsulfonyl)imide [TFSI] and ethylsulfate [EtOSO(3)] is investigated. In particular, attenuated total reflection (ATR) infrared (IR) as well as Raman spectra in the spectral range from 500 to 3500 cm(-1) have been recorded and analyzed. Moreover, the depolarization ratios of the Raman lines are determined. The individual peaks are assigned to the corresponding vibrational modes of the molecules. While the CH stretching region around 3000 cm(-1) is dominating in Raman spectra, it is remarkably weak in IR spectra. Finally, the results for 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide are compared to previous studies of single ions available in the literature. This comparison shows good agreement.
Proceedings of the Combustion Institute, 2007
ABSTRACT Polarization spectroscopy in the mid-infrared (IRPS) has been applied to the detection o... more ABSTRACT Polarization spectroscopy in the mid-infrared (IRPS) has been applied to the detection of acetylene molecules making use of the asymmetric C–H stretching vibration at around 3 μm. The infrared laser pulses were produced through difference frequency generation in a LiNbO3 crystal pumped by a Nd:YAG and dye laser system. By directly probing the ro-vibrational transitions with IRPS, sensitive detection of molecules with otherwise inaccessible electronic states was realized with high temporal and spatial resolution by using a pulsed laser and a cross-beam geometry. Detection sensitivities of 2 × 1013 molecules/cm3 (10 ppm in 70 mbar gas mixture) of C2H2 were achieved using the P(1 1) line of the (0 1 0(1 1)0)–(0 0 0 00 00) band. The dependence of the IRPS signal on the pump laser fluence, acetylene mole fraction, and buffer gas pressure of Ar, N2, H2, and CO2 has been studied experimentally. The investigation demonstrates the quantitative nature of IRPS for sensitive detection of polyatomic IR active molecules. In order to fully demonstrate the technique for combustion applications, nascent acetylene molecules were measured in a low pressure methane/oxygen flame.
Biosensors, 2016
Water-in-oil (w/o) emulsions are of great interest in many areas of the life sciences, including ... more Water-in-oil (w/o) emulsions are of great interest in many areas of the life sciences, including food technology, bioprocess engineering, and pharmaceuticals. Such emulsions are complex multi-component systems and the molecular mechanisms which lead to a stable emulsion are yet to be fully understood. In this work, attenuated total reflection (ATR) infrared (IR) spectroscopy is applied to a series of w/o emulsions of an aqueous anthocyanin-rich bilberry extract dispersed in a medium chain triglyceride (MCT) oil phase. The content of the emulsifier polyglycerin-polyricinoleat (PGPR) has been varied systematically in order to investigate whether or not its concentration has an impact on the molecular stabilization mechanisms. The molecular stabilization is accessed by a careful analysis of the IR spectrum, where changes in the vibrational frequencies and signal strengths indicate alterations of the molecular environment at the water/oil interface. The results suggest that adding emulsifier in excess of 1% by weight does not lead to an enhanced stabilization of the emulsion.
Chem Ing Tech, 2009
ABSTRACT
The journal of physical chemistry. A, Jan 24, 2015
Using blends of bioethanol and gasoline as automotive fuel leads to a net decrease in the product... more Using blends of bioethanol and gasoline as automotive fuel leads to a net decrease in the production of harmful emission compared to the use of pure fossil fuel. However, fuel droplet evaporation dynamics change depending on the mixing ratio. Here we use single particle manipulation techniques to study the evaporation dynamics of ethanol/gasoline blend microdroplets. The use of an electrodynamic balance enables measurements of the evaporation of individual droplets in a controlled environment, while optical tweezers facilitate studies of the behavior of droplets inside a spray. Hence, the combination of both methods is perfectly suited to obtain a complete picture of the evaporation process. The influence of adding varied amounts of ethanol to gasoline is investigated, and we observe that droplets with a greater fraction of ethanol take longer to evaporate. Furthermore, we find that our methods are sensitive enough to observe the presence of trace amounts of water in the droplets. A...
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Papers by Johannes Kiefer