bluuebunny/crossref_metadata_2025_split huggingface dataset:
Molecular Dynamics in Side‐Group Polymers with and without Liquid Crystalline Phases from 2H NMR
Authors: Leisen J. , Ohlemacher A. , Boeffel C. , Spiess H. W. 
Published: July 2012

AbstractSide‐group liquid crystalline polymers were investigated by 1D and 2D solid state 2H NMR methods in the glassy state. The side‐group dynamics of liquid crystalline systems were compared to those of amorphous systems of the same molecular architecture. The onset of fast rotational motion of the phenylene rings additionally to phenylene flips was detected at considerably lower absolute and reduced temperature in the liquid crystalline systems indicating the influence of the ordered state on local dynamics. In 2D exchange spectra, the lineshapes are particularly sensitive towards possible slow motion of the flip axis itself which could be envisaged to fluctuate within the orientational distribution of the mesogen. However, the experimental spectra for the investigated side‐group liquid crystalline polymers show no indication of the characteristic features, thereby proving that the flip axis is rigid within ±3° even after a large number of flips.
Organization and Desorganization of Short Range Structures in Swallow‐Tailed Liquid Crystals
Authors: Kresse Horst , Heinemann Susanne , Paschke Reinhard , Weissflog Wolfgang 
Published: July 2012

AbstractSwallow‐tailed liquid crystals are antiparallel oriented in the short range‐order. The organization and destruction of this order was studied by addition of other swallow‐tailed molecules, molecules with lateral substitution, a double swallow‐tailed and a rod‐like component. Induction and destruction of liquid crystalline phases was investigated by phase diagrams and dielectric methods. This survey contains new experimental data as well as for comparison already published results. It is a good example for the importance of geometrical relations for the formation of the liquid crystalline state.
Monodisperse Liquid Crystalline Oligomer Systems with Controlled Dipolar Structure
Authors: Festag R. , Wendorff J. H. , Birkel S. , Eisenbach C. D. 
Published: July 2012

AbstractMonodisperse oligomer systems characterized by a well defined dipolar structure were synthesized: the longitudinal dipole components were either parallel or antiparallel to each other. Such systems were obtained employing a step‐by‐step reaction using protective groups which are selectively cleavable. We investigated the influence of the dipole structure of the molecule on the phase and the dielectric behavior, combining computer simulations and experimental investigations. In addition we analyzed networks formed by such oligomers.
Stabilization and Destabilization of Thermotropic Liquid Crystalline Phases by Hydrogen Bondings
Authors: Kresse Horst , Dorscheid Claudia , Szulzewski Ilka , Frank Robert , Paschke Reinhard 
Published: July 2012

AbstractSix phase diagrams of liquid crystalline acids E and a phenole F mixed with derivatives of pyridine P, nicotinic acid Ni and pyrrolidine Py were investigated. Congruent melting compounds of the type PE and NiE were detected. Stabilization of the nematic and induction of a smectic A phase was observed in the P/E and P/F systems whereas in Py/E mixture an extremely strong destabilization of the nematic phase takes place. In the Ni/E diagrams a weak destabilization of the nematic phase was found. The results are explained by associates formed by hydrogen bridges which show a more or less strong deviation from the rod‐like molecular shape.
Liquid Crystalline Perylene Derivatives: Orientation and Phase Variation of Discotic Dyes
Authors: Pressner D. , Gültner Chr. , Spieß H. W. , Müllen K. 
Published: July 2012

AbstractDiscotic liquid‐crystalline perylene derivatives can be oriented in the magnetic field far below their clearing temperatures as shown by 2H solid‐state n.m.r. spectroscopy. A mechanical orientation of these materials was achieved by drawing fibers, which were investigated by X‐ray diffraction. In addition, it is shown that not only the melting and clearing temperatures, but also the structure of the mesophases depend sensitively on the substitution pattern. The type of liquid crystalline phase can be varied from discotic to smectic.
Discotic Liquid Crystals — A New Class of Fast Photoconductors
Authors: Adam D. , Haarer D. , Closs F. , Frey T. , Funhoff D. , Siemensmeyer K. , Schuhmacher P. , Ringsdorf H. 
Published: July 2012

AbstractWe showed for the first time that discotic liquid crystals are well suited for a new class of fast photoconducting materials. Due to their spontaneous orientation and their dynamical fluctuations in the mesophase, they show exceptionally high mobilities of 1·10−3 cm2/Vs, about two to three orders of magnitude higher than those obtained for conventional amorphous polymers. Further on, the Gaussian transport (for holes in the mesophase) is remarkable, which is characterized by the existence of a conduction band and the absence of trapping states. In contrast, the charge carrier transport in amorphous photoconductors is generally trap‐dominated which limits technical properties, leading to low carrier mobilities and undesirable memory effects. Measurements of the transient photocurrents, using the time‐of‐flight technique, have been carried out in the discotic liquid crystalline photoconductor hexapentyloxytriphenylene. The carrier mobilities and the underlying transport mechanisms both in the crystalline phase and in the mesophase are discussed in the framework of ideal intrinsic (Gaussian) transport, multiple shallow trapping, and deep trapping transport, respectively.
Surface Ordering and Finite‐size Effects at the First Order Phase Transition SmA‐SmC in Liquid‐crystalline Free‐standing Films
Authors: Kraus I. , Pieranski P. , Demikhov E. , Stegemeyer H. , Goodby J. , Slaney A. , Osipov M. 
Published: July 2012

AbstractHigh precision optical reflectivity measurements and optical image analyses are applied to investigate the first order SmA‐SmC* phase transition in free‐standing films. By decreasing the thickness of the films the critical point is observed, where the first order phase transition disappears. A mean‐field theory describing the process of surface ordering in smectic membranes is presented.
Separation of Enantiomers in a Monolayer of Racemic 3‐Hexadecyl‐oxy‐propane‐1,2‐diol
Authors: Rietz Roland , Brezesinski Gerald , Möhwald Helmuth 
Published: July 2012

AbstractMonolayers of a racemic mixture and of the pure S enantiomer of 3‐hexadecyloxy‐propane‐1,2‐diol have been characterized by thermodynamic measurements at the air/water interface as well as by fluorescence microscopy. The critical temperature Tc, the limiting molecular area at high pressure and the pressure πc corresponding to the observed main phase transition, derived from the isotherms, slightly depend on chiral purity. Above πc domains with a polygonal shape appear. However, if the domains are compressed shape instabilities occur. While the spirals of the enantiomer turn only in one direction, the spirals arising from one domain of the racemate exhibit opposite handedness. This behavior is explained by a separation of enantiomers on the molecular level in the condensed state of the racemate. A phase diagram is suggested.
Complex Salts of TCNQ with Substituted Phenyl Pyridines and Their Characterization by Scanning Tunneling Microscopy
Authors: Magonov S. N. , Bar G. , Cantow H.‐J. , Kabbeck‐Kupijai D. , Koßmehl G. 
Published: July 2012

AbstractNew conductive complex salts of tetracyanoquinodimethane (TCNQ) with substituted phenylpyridines have been prepared and characterized by optical microscopy, conductivity measurements, X‐ray powder diagrams and scanning tunneling microscopy (STM). The molecular stacking, which is responsible for the one‐dimensional conductivity, was revealed from the diffraction data and STM images. The analysis of the STM images was based on the results obtained on a model compound, the complex salt of TCNQ and quinolinium. Although in STM the surface partial electron density is measured, several crystallographic parameters of the molecular arrangement have been determined from these images.
Detection of Toxic Emissions from Incinerators
Authors: Tanada Tony N. , Velazquez Jon , Hemmi Naoki , Cool Terrill A. 
Published: July 2012

AbstractContinuous monitoring techniques are needed for assurance that the stack gases of hazardous and municipal waste incinerators do not emit unacceptable levels of dangerous compounds. The development of suitable on‐line “real‐time” monitors may help answer current public concerns regarding the safety of thermal processing methods and may simplify the permitting process for new facilities. Recent research is reviewed which suggests that resonance ionization mass spectrometry is capable of real‐time monitoring of wide classes of toxic molecules at the part‐per‐billion level against complex backgrounds of interferant species. Recent advances in laser technology promise to make on‐line repetitive multispecies monitoring of hazardous emissions a practical reality.
Application of Single Molecule Detection to DNA Sequencing and Sizing
Authors: Ambrose W. Patrick , Goodwin Peter M. , Jett James H. , Johnson Mitchell E. , Martin John C , Marrone Babetta L. , Schecker Jay A. , Wilkerson Charles W. , Keller Richard A. , Haces Alberto , Shih Po Jen , Harding John D. 
Published: July 2012

AbstractWe are developing a laser‐based technique for the rapid sequencing of 40‐kb or larger fragments of DNA at a rate of 100 to 1000 bases per second. Our approach relies on fluorescent labeling of the bases in a single fragment of DNA, attachment of this labeled DNA fragment to a support, movement of the supported DNA into a flowing sample stream, and detection of the individual, fluorescently labeled bases by laser‐induced fluorescence as they are cleaved from the DNA fragment by an exonuclease. The ability to sequence large fragments of DNA will reduce significantly the amount of subcloning and the number of overlapping sequences required to assemble megabase segments of sequence information.We are also applying our sensitive fluorescence detection to sizing of DNA fragments. Large, fluorescently stained restriction fragments of lambda phage DNA are sized by passing individual fragments through a focused, continuous‐wave laser beam in an ultrasensitive flow cytometer at a rate of ∼60 fragments per second. The size of the fluorescence burst emitted by each stained fragment as it passes through the laser beam is measured in one millisecond. We have demonstrated flow cytometric sizing of DNA fragments in a ∼0.1‐pg sample of a restriction digest of lambda DNA in 164 seconds with sizing accuracy better than 98%.
Flow and Particle Diagnostics. Multi‐Parameter and Multi‐Point Measurements
Authors: Hanson R. K. , Baer D. S. , Mcmillin B. K. , Arroyo P. 
Published: July 2012

AbstractLaser‐based diagnostic techniques, developed primarily within the combustion community, offer considerable promise for nonintrusive measurements in gaseous flows relevant to industry. In this paper, we review two diagnostic methods under development in our laboratory. The first technique allows simultaneous measurement of multiple parameters through analysis of the shape and shift of fully resolved absorption lineshapes. The light source is a tunable, narrow‐linewidth semiconductor diode laser, applied in the present case to either water vapor (1.39 microns) at combustion gas temperatures (to 1700 K) or to argon (800 nm) in an atmospheric pressure plasma (at temperatures to 8000 K). Parameters inferred include, for water: temperature, pressure, velocity and species density; for argon, the parameters are electron density, kinetic temperature and the population temperature for the 4 s levels of argon. The second diagnostic technique is planar laser‐induced fluorescence (PL1F), which allows instantaneous measurements of a single parameter at multiple points in the flowfield. The example application to be discussed is single‐shot temperature imaging in a model scramjet flowfield using a two laser‐two camera imaging scheme.
Flow Field Diagnostics in Industrial Devices
Authors: Köhler J. , Lawrenz W. , Meier F. , Meinhardt P. , Stolz W. , Bloss W. H. 
Published: July 2012

AbstractA new 2‐dimensional time resolved (200 frames/s) flow field measuring technique (PIV‐Movie) has been developed and applied to quantitative cycle resolved measurements of the instationary flow field within the combustion chamber of a transparent square piston engine. A conventional Particle Image Velocimetry (PIV) setup was modified using a double pulsed copper vapor laser (200 Hz, pulse separation 33 microseconds) and a rotating drum camera, which was used to record the Mie‐scattered signals from seeding particles. The circumferential velocity of the drum of the camera causes an image shifting of the two exposures taken with a double pulse. Because the copper vapor laser can be run continuously in the double pulse mode a series of up to 70 double pulsed images per individual engine cycle could be recorded on photographic film. These film data were evaluated with respect to both magnitude and direction of individual flow vectors in the flow field.
Temperature Measurements in High Pressure Combustion
Authors: Stricker W. , Woyde M. , Lückerath R. , Bergmann V. 
Published: July 2012

AbstractWe report on a fundamental study of N2 and H2 CARS thermometry in a high‐pressure, premixed methane/air flame. Based on CARS spectra of high quality, models were developed for spectral shape calculations in order to determine accurate temperatures. Rotational Raman spectra were used to measure an independent reference temperature. The limits of applicability were shown. Single‐pulse N2 CARS thermometry on a subscale ramjet combustor with 3 bar pressure inside the combustion chamber was performed.
Temperature Measurement by UV‐Degenerate Four Wave Mixing
Authors: Ewart P. 
Published: July 2012

AbstractThe derivation of temperatures of gas phase media from spectra generated by DFWM, induced by frequency scanned narrow linewidth lasers, is briefly reviewed. The technique of multiplex or broadband DFWM is described, by which spectra, and hence temperatures, may be obtained by a single shot of a novel broadband “modeless” laser. Experimental measurements of broadband DFWM lineshapes from OH are briefly described and compared with theoretical predictions. Results of temperature measurements in premixed methane/air flames using broadband DFWM spectra are reported. Systematic errors arising from the effect of absorption are discussed, improvements to and applications of the technique to turbulent media are briefly considered.
Detection of Polyatomic Molecules using Infrared Degenerate Four‐Wave Mixing
Authors: Germann G. J. , Mcilroy A. , Dreier T. , Farrow R. L. , Rakestraw D. J. 
Published: July 2012

AbstractDegenerate four‐wave mixing detection of methane and acetylene is demonstrated using rotational‐vibrational transitions for resonance enhancement. High resolution spectra (<0.007 cm−1) of methane and acetylene, near 3000 and 3300 cm−1 respectively, have been acquired using a single‐longitudinal‐mode optical parametric oscillator. These experiments suggest that infrared degenerate four‐wave mixing provides a new diagnostic capability for the sensitive detection of polyatomic molecules with high temporal and spatial resolution.
Application of Laser‐Induced Fluorescence and CARS for Combustion Diagnostics
Authors: Aldén M. , Bengtsson P.‐E. , Georgiev N. , Löfström C. , Martinsson L. , Neij H. 
Published: July 2012

AbstractLaser diagnostic techniques have during the last decade been successfully applied for diagnostics of combustion processes. The main advantages with these techniques are that they are inherently non‐intrusive and that spatially as well as temporally resolved information can be achieved. The current paper will exemplify two different laser spectroscopic techniques: Laser‐Induced Fluorescence (LIF), and Coherent anti‐Stokes Raman Scattering (CARS), which are currently the most used laser techniques for temperature and species concentration measurements in combustion systems.
2D‐Diagnostics in Industrial Devices
Authors: Arnold A. , Bräumer A. , Buschmann A. , Decker M. , Dinkelacker F. , Heitzmann T. , Orth A. , Schäfer M. , Sick V. , Wolfrum J. 
Published: July 2012

AbstractThe high spectral brightness of tunable high power excimer lasers allows the two‐ and three‐dimensional application of techniques like laser‐induced fluorescence (LIF), Mie and Rayleigh scattering for industrial applications. The construction of these lasers allows easy transportation and installation to perform measurements at combustion facilities which can not be moved. In combination with suitable filters and gated image‐intensified CCD cameras techniques are now available to measure multi‐dimensional distributions of temperatures and concentrations. Simultaneous measurements of temperature fields and hydroxyl radical distributions were performed to study the influence of turbulence on large premixed natural gas flames. A combination of temperature and nitric oxide concentration measurements yielded information about the correlations between NO formation and burner design in domestic gas burners. LIF and Mie scattering of fuel and hydroxyl radicals were used for measurements in IC engine simulators, Otto and Diesel engines.
Laser Surface Processing and Diagnostics
Authors: Preuß S. , Stuke M. 
Published: July 2012

AbstractLaser processing has become a powerful tool for surface modification and is applied successfully in many areas. In this article we focus on laser ablation as a very attractive way of surface patterning. We report both applications and mechanistic studies. In the first part of this paper we describe a two‐step method for the patterning of technical polymer surfaces by laser ablation. With this process, micron or even submicron structures can be obtained in a simple way. By doping polymers with photochromic compounds the feasibility of this process is improved drastically. In the second part we report the ablation of lithium niobate (LiNbO3), polyimide (PI), poly(tetrafluoroethylene) (PTFE, teflon) and poly(methylmethacrylate) (PMMA) by 0.5 ps UV excimer laser pulses at 248 nm. Time‐resolved measurements were carried out with pulse pairs of variable delay in the range — 200 ps to +200 ps. The ablation rate is very sensitive to the time delay between the two pulses and contains information about the dynamics of the process. For LiNbO3, efficient shielding is observed within a few picoseconds. For polyimide, excited state absorption is found to decrease the ablation rate for delay times below 30 ps. For PTFE and PMMA, and the total fluence just above threshold, the ablation rate versus time delay shows an autocorrelation type behavior with a full width at half maximum below 400 fs, since two‐photon absorption dominates the ablation process. A simple ablation model based on the assumption, that the absorbed volumetric energy density has to exceed a certain threshold value, will be presented. It describes the observed ablation rates for PTFE precisely.
Time Resolved Trace Analysis of Exhaust Gas by Means of Laser Mass Spectrometry
Authors: Weickhardt C. , Boesl U. 
Published: July 2012

AbstractThe large number of components and their very different and rapidly varying concentrations complicates the analysis of exhaust gases of combustion engines. Here, a new analytical technique, based on the combination of resonance enhanced multiphoton ionization, performed by pulsed tunable lasers, and time of flight mass spectrometry is presented. It allows the simultaneous detection of several trace components with concentrations ranging from 1 ppm up to several percent with a quantitative accuracy better than 10%. Time resolution of 10 ms could be achieved with this system. A preliminary list of substances detectable by this technique contains 25 substances; some of them belong to the strongest compounds inducing ozone formation.
A Bundle of Tapered Fibers Transmits 830 mJ/Pulse of 308 nm XeCl Laser Pulses
Authors: Devel A. , Pfleiderer C. , Wolfrum J. , Greulich K. O. 
Published: July 2012

AbstractImprovements of a tapered bundle of 32 tapered fibers permit the transmission of XeCl‐excimer laser pulses (308 nm) with high energy densities. More than 2000 laser pulses of 530 mJ/pulse output at the bundle's end could be transmitted without fiber damage achieving a transmission of approx. 40% using a Siemens XP 2020 XeCl excimer laser. With a SOPRA VEL XeCl excimer laser a peak output energy of 830 mJ/pulse at the bundle's end was possible. Measurements of the beam profile of the bundle were also performed. The beam profile of the near field was very inhomogeneous with an average energy density of 2250 mJ/cm2. Due to the high divergence of the radiation at the end of each fiber, the beam profile of the bundle's far field was better homogenized. An average energy density of 550 mJ/cm2 was achieved. For applications as surface treatments it is necessary to observe the surface to be objected to laser treatment on‐line during radiation. A simultaneous quality‐control integrated in the bundle is possible by means of difference reflectometry.
Collisional Narrowing and Spectral Shift in CARS Spectra of Molecular Nitrogen and Oxygen at Pressures up to 2500 bar and Temperatures of 850 K
Authors: Dreier T. , Ridder M. , Schiff G. , Suvernev A. 
Published: July 2012

AbstractVibrational‐rotational CARS spectra of molecular nitrogen and oxygen at high densities and temperatures are recorded using a scanning CARS‐spectrometer. Measurements are performed at pressures up to 2500 bar and temperatures up to 850 K. The spectral line position and halfwidth of the fully collapsed Q‐branch are determined for pure N2, O2, N2/O2 mixtures and Ar/O2 mixtures. Comparisons of the experimental data with different theoretical models for the rotational energy transfer are also presented. Three quantum mechanical approaches (MEG‐, ECS‐, PEG‐laws) are compared with a classical description of the relaxation process treating the angular momentum as a continuous variable.
Design of Multiplex Dyes
Authors: Sauer Markus , Schulz Andreas , Seeger Stefan , Wolfrum Jürgen , Arden‐Jacob Jutta , Deltau Gerhard , Drexhage Karl H. 
Published: July 2012

AbstractFluorescent dyes which are almost identical in their absorption and emission spectral characteristics but differ in fluorescence lifetime (“Multiplex Dyes”) have been developed for analytical applications. First investigations to design such dyes were carried out with rhodamines. In order to influence the excited state lifetime without changing the spectral characteristics we studied rhodamines modified with non‐conjugated substituents that promote non‐radiative transitions. We used two different methods to control the fluorescence lifetime in rhodamine dyes with rigidized amino groups: (A) Substitution with benzyl derivatives of different electron acceptor strength at the amino position. (B) Substitution of the carboxyphenyl group by phenyl groups with reduced steric requirements and varying acceptor properties.