Dr. rer. nat. Wolfgang Christen
Profil
Forschungsthemen6
Bestimmung der Translationstemperatur bei der Hochdruck-Molekularstrahlexpansion kleiner Moleküle (I)
Quelle ↗Förderer: DFG Sachbeihilfe Zeitraum: 07/2006 - 06/2007 Projektleitung: Dr. rer. nat. Wolfgang Christen
Bestimmung der Translationstemperatur bei der Hochdruck-Molekularstrahlexpansion kleiner Moleküle (II)
Quelle ↗Förderer: DFG Sachbeihilfe Zeitraum: 01/2008 - 06/2009 Projektleitung: Dr. rer. nat. Wolfgang Christen
Erschließung von Zitationen in verteilten Open Access-Repositories
Quelle ↗Förderer: DFG Sachbeihilfe Zeitraum: 03/2009 - 07/2013 Projektleitung: Dr. rer. nat. Wolfgang Christen
Improved measurements of sensitive climate indicators
Quelle ↗Zeitraum: 12/2014 - 08/2017 Projektleitung: Dr. rer. nat. Wolfgang Christen
Kondensationsprozesse in der Nähe des kritischen Punktes
Quelle ↗Förderer: DFG Eigene Stelle (Sachbeihilfe) Zeitraum: 04/2010 - 09/2012 Projektleitung: Dr. rer. nat. Wolfgang Christen
Untersuchungen zur Reaktivität von Molekülclustern bei hohen Energie- und Teilchendichten mittels Cluster-Oberflächen-Stößen
Quelle ↗Förderer: DFG Sachbeihilfe Zeitraum: 11/2002 - 04/2006 Projektleitung: Dr. rer. nat. Wolfgang Christen
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Publikationen25
Top 25 nach Zitationen — Quelle: OpenAlex (BAAI/bge-m3 embedded für Matching).
The Journal of Physical Chemistry A · 76 Zitationen · DOI
Computer simulations and experiments have been performed to quantify the effects of nozzle shape and skimmer placement on high-density supersonic jets. It is shown that the on axis beam intensity achieved is much higher than intensity achieved using standard sonic nozzles. Changes in skimmer design and positioning are required to allow this intense jet to propagate in a typical supersonic beam setup.
The Journal of Physical Chemistry A · 62 Zitationen · DOI
This paper addresses the interaction of molecular cluster ions with a solid surface in the kinetic energy range of 1−100 eV/molecule. We report experimental results on the energy acquisition by the cluster following its impact on the target, the size distribution and the time scale of cluster fragmentation, and first examples of chemical reactions induced by cluster impact. In particular we show that for a p-type diamond film and moderate collision energies the elasticity of the cluster-surface impact is surprisingly high: The intact cluster recoils with typically 75% of its collision energy. Once, however, the clusters have acquired sufficient internal energy they will shatter, mostly to monomers. In the case of protonated ammonia cluster ions this shattering of clusters upon surface impact is shown to be faster than 80 ps. It provides evidence that the technique of cluster impact allows an ultrafast energy redistribution within superheated cluster ions prior to their fragmentation. The feasibility of this fascinating new approach to femtosecond chemistry is demonstrated with impact-induced chemical reactions of iodomethane clusters to molecular iodine and of trifluoromethane clusters to molecular fluorine. The detected reaction yields are surprisingly high, even for the small cluster sizes investigated so far (n < 16).
The Journal of Chemical Physics · 61 Zitationen · DOI
Measurements of the collisional energy transfer of size and energy-selected ammonia cluster ions (NH3)nH+, n=1–10, impacting a silicon wafer coated with p-type diamond film are reported. The transfer from translational energy of the incident cluster ions to kinetic energy of intact scattered cluster ions has been studied as a function of impact energy, surface composition, and size of the impinging cluster cations. For low impact energies (&lt;2.5 eV/molecule), cluster ions scattered off the target surface lost most of their initial kinetic energy, while for higher impact energies the elasticity of the cluster–surface collision is surprisingly high: Typically 75% of the impact kinetic energy is retained by the scattered parent clusters. Larger cluster ions are scattered less elastically and a large fraction of them shatter to small(est) fragments. The molecular dynamics simulations examine the two energy disposal regimes, deep inelasticity and shattering. Deep inelastic scattering occurs already below the lowest impact energies probed by the experiment. At higher collision energies, the energy loss continues to increase but a point is reached where most clusters shatter. Those few clusters that rebound intact have lost a disproportionately low fraction of their initial energy. The simulations also explore the cluster size effects, the role of the attraction to the surface, and the importance of the anisotropic forces between the molecules in the cluster. The experimental results and the simulations are discussed using the hard cube model with special reference to collective effects.
Physics Letters B · 60 Zitationen · DOI
International Journal of Mass Spectrometry and Ion Processes · 47 Zitationen · DOI
Langmuir · 38 Zitationen · DOI
Crystallization processes under different conditions are of fundamental interest in chemistry, pharmacy, and medicine. Therefore, we have studied the formation of micro- and nanosized crystals using water-caffeine (1,3,7-trimethyl-1 H-purine-2,6(3 H,7 H)-dione) solutions under ambient conditions as a relevant model system. When droplets of an aqueous caffeine solution evaporate and eventually dry on surfaces (glass, polystyrene, and polyester), stable "coffee tabletop" rings with a perimeter of typically 3 mm are formed after 20 to 50 min. Using a micro focus X-ray beam available at the BESSY muSpot-beamline, the fine structure of different caffeine needles can be distinguished. Unexpectedly, both crystal modifications (alpha- and beta-caffeine) are present, but locally separated in these rings. Furthermore, AFM studies reveal the presence of even smaller particles on a nanometer length scale. To eliminate influences of surface irregularities from the crystallization process, acoustic levitation of liquid samples was employed. Such levitated droplets are trapped in a stable position and only surrounded by air. The solvent in an ultrasonically levitated drop evaporates completely, and the resulting crystallization of caffeine was followed in situ by synchrotron X-ray diffraction. In this case, the diffraction pattern is in accordance with pure alpha-caffeine and does not indicate the formation of the room temperature polymorph beta-caffeine. Hence, our investigations open new vistas that may lead to a controlled formation of cocrystals and novel polymorphs of micro- and nanocrystalline materials, which are of relevance for fundamental studies as well as for pharmaceutical and medical applications.
The Journal of Chemical Physics · 28 Zitationen · DOI
Pulsed, supersonic beams of pure carbon monoxide and carbon dioxide at stagnation conditions above their critical point have been investigated by time-of-flight measurements as a function of pressure and temperature. Although both molecules form clusters readily in adiabatic expansions, surprisingly large speed ratios (above 100) indicative of very low translational temperatures (below 0.1 K) have been achieved. In particular, the supersonic expansion of CO(2) at stagnation temperatures slightly above the phase transition to the supercritical state results in unprecedented cold beams. This efficient cooling is attributed to the large values of the heat capacity ratio of supercritical fluids in close vicinity of their critical point.
Crystal Growth & Design · 24 Zitationen · DOI
A systematic crystal morphology study on the pharmaceutical model compound caffeine has been conducted on different surfaces: silicon, silver, soda lime glass, and silver subsurface ion-exchanged soda-lime silicate (SIMO) glasses. The morphology of the solid caffeine deposits has been investigated using environmental scanning electron microscopy (ESEM), atomic force microscopy (AFM), and X-ray diffraction (XRD). Needle-shaped caffeine crystals have been observed by drop-casting and also by applying the rapid expansion of supercritical solutions (RESS) technique using supercritical carbon dioxide. The aspect ratio of the crystalline needles typically vary between 10 and 100, but have been observed as large as 500. The XRD data of the RESS products indicate unambiguously the presence of the thermodynamically most stable polymorph of caffeine known as the β-form. Under defined conditions we observe a unique, surface-mediated morphology for caffeine crystals with nearly perfect hexagonal shape. The relative fraction of the hexagons was seen to strongly increase especially when SIMO glasses were used. These hexagons have a distinct upper size limit depending on the solvent and substrate being used. The size distribution analysis of the hexagons yielded an average perimeter of typically 10 μm. The mechanism of the formation process of this new hexagonal motif is explained in terms of the spinodal dewetting of the thin film of caffeine solution on the surface.
Berichte der Bunsengesellschaft für physikalische Chemie · 21 Zitationen · DOI
Abstract Sulfur dioxide clusters prepared in seeded, pulsed, supersonic molecular beams have been scattered from single crystal surfaces under ultrahigh vacuum conditions with the aim to investigate chemical changes (e.g. fragmentation) and cluster‐surface interaction with regard to certain experimentally accessible conditions (cluster size, kinetic energy, type and temperature of the surface). First measurements are reported which show that (SO 2 ) n ( n > 3) dissociation may occur yielding both, positively and negatively charged ionization products. Possible cluster‐surface interaction mechanisms are discussed.
Supersonic Beams at High Particle Densities: Model Description beyond the Ideal Gas Approximation
2010The Journal of Physical Chemistry A · 20 Zitationen · DOI
Supersonic molecular beams constitute a very powerful technique in modern chemical physics. They offer several unique features such as a directed, collision-free flow of particles, very high luminosity, and an unsurpassed strong adiabatic cooling during the jet expansion. While it is generally recognized that their maximum flow velocity depends on the molecular weight and the temperature of the working fluid in the stagnation reservoir, not a lot is known on the effects of elevated particle densities. Frequently, the characteristics of supersonic beams are treated in diverse approximations of an ideal gas expansion. In these simplified model descriptions, the real gas character of fluid systems is ignored, although particle associations are responsible for fundamental processes such as the formation of clusters, both in the reservoir at increased densities and during the jet expansion. In this contribution, the various assumptions of ideal gas treatments of supersonic beams and their shortcomings are reviewed. It is shown in detail that a straightforward thermodynamic approach considering the initial and final enthalpy is capable of characterizing the terminal mean beam velocity, even at the liquid-vapor phase boundary and the critical point. Fluid properties are obtained using the most accurate equations of state available at present. This procedure provides the opportunity to naturally include the dramatic effects of nonideal gas behavior for a large variety of fluid systems. Besides the prediction of the terminal flow velocity, thermodynamic models of isentropic jet expansions permit an estimate of the upper limit of the beam temperature and the amount of condensation in the beam. These descriptions can even be extended to include spinodal decomposition processes, thus providing a generally applicable tool for investigating the two-phase region of high supersaturations not easily accessible otherwise.
Physical Review A · 20 Zitationen · DOI
The mathematical term for the mean flow velocity in supersonic beams of ideal gases is extended to include real gas properties. This procedure yields an explicit dependence of the flow velocity on pressure, as observed in recent experiments of free jet expansions. Applied to stagnation conditions slightly above the critical point, the model suggests that seeded high-pressure jet expansions might be suitable for slowing down virtually any molecule with maximum efficiency. Moreover, we discuss the consequence of a pressure-dependent flow velocity ${v}_{0}$ for the speed ratio $S={v}_{0}/\ensuremath{\Delta}{v}_{\ensuremath{\parallel}}$ with respect to collisional cooling and suggest to use the velocity spread $\ensuremath{\Delta}{v}_{\ensuremath{\parallel}}$ as a more nonambiguous measure of translational temperature in high-pressure jet expansions.
The Journal of Physical Chemistry A · 18 Zitationen · DOI
We introduce a straightforward experimental approach for determining the mean flow velocity of a supersonic jet with very high precision. While time measurements easily can achieve accuracies of Δt/t ≤ 10(-4), typically the absolute flight distances are much less well-defined. This causes significantly increased errors in calculations of the mean flow velocity and mean kinetic energy. The basic concept to improve on this situation is changing the flight distance in vacuo by precisely defined increments employing a linear translation stage. We demonstrate the performance of this method with a flight path that can be varied by approximately 15% with a tolerance of setting of 50 μm. In doing so, an unprecedented accurate value for the mean flow velocity of Δv/<v> < 3 × 10(-4) has been obtained without prior knowledge of the total distance. This very high precision in source pressure, temperature, and particle speed facilitates an improved energetic analysis of condensation processes in supersonic jet expansions. The technique is also of broad interest to other fields employing the strong adiabatic cooling of supersonic beams, in particular, molecular spectroscopy. In the presented case study, a thorough analysis of arrival time spectra of neutral helium implies cluster formation even at elevated temperatures.
Review of Scientific Instruments · 17 Zitationen · DOI
A compact apparatus for transferring nonvolatile particles into the gas phase and depositing them on a solid surface has been built and tested successfully. As initial experiment, solid caffeine with a vanishingly low vapor pressure has been dissolved in supercritical carbon dioxide, expanded into vacuum using a pulsed, supersonic molecular beam, and detected using a simple residual gas analyzer.
The European Physical Journal D · 16 Zitationen · DOI
The Journal of Chemical Physics · 13 Zitationen · DOI
We describe a generally applicable method for the experimental determination of stationary flow conditions in pulsed supersonic beams, utilizing time-resolved electron induced fluorescence measurements of high pressure jet expansions of helium. The detection of ultraviolet photons from electronically excited helium emitted very close to the nozzle exit images the valve opening behavior-with the decided advantage that a photon signal is not affected by beam-skimmer and beam-residual gas interactions; it thus allows to conclusively determine those operation parameters of a pulsed valve that yield complete opening. The studies reveal that a "flat-top" signal, indicating constant density and commonly considered as experimental criterion for continuous flow, is insufficient. Moreover, translational temperature and mean terminal flow velocity turn out to be significantly more sensitive in testing for the equivalent behavior of a continuous nozzle source. Based on the widely distributed Even-Lavie valve we demonstrate that, in principle, it is possible to achieve quasi-continuous flow conditions even with fast-acting valves; however, the two prerequisites are a minimum pulse duration that is much longer than standard practice and previous estimates, and a suitable tagging of the appropriate beam segment.
Physica Scripta · 13 Zitationen · DOI
Attempting to improve the comprehension of supersonic molecular beams at elevated pressures we present a comparative study of thermodynamic descriptions of the terminal flow velocity in free jet expansions. As model system we choose carbon dioxide due to its widespread utilization in supercritical fluid technology. Numerical results for the thermodynamic quantities are obtained using a high accuracy equation of state explicit in the Helmholtz free energy. The influence of pressure and temperature on the beam velocity is investigated for a broad range of stagnation conditions. A consistent physical picture is obtained for calculations employing the initial and final molar enthalpies, while enormous discrepancies are found for descriptions based on the molar isobaric heat capacity or the heat capacity ratio. The deviations are particularly pronounced at the gas–liquid phase transition and in the vicinity of the critical point and can be related to the diverse assumptions of ideal gas behavior. It is shown that computations using real fluid enthalpies permit to assess the fraction of condensation in supersonic jets.
The European Physical Journal D · 12 Zitationen · DOI
The European Physical Journal D · 11 Zitationen · DOI
International Journal of Mass Spectrometry · 8 Zitationen · DOI
Review of Scientific Instruments · 8 Zitationen · DOI
We describe an experimental setup for supersonic jet expansions of supercritical fluids. It is characterized by well-defined thermodynamic values to allow systematic investigations of pressure and temperature effects on molecular beam parameters. The design permits stagnation temperatures T(0)=225-425 K with a thermal stability DeltaT(0)<30 mK and stagnation pressures p(0)=0.2-12 MPa that are measured with 0.05% precision. For optimum stability, gas reservoir, pressure transducer, and gauge amplifier are temperature-controlled, and a feedback loop permits active pressure stabilization using a pulseless syringe pump. With this approach stagnation pressures can be reproduced and kept constant to Deltap(0)<2.9 kPa. As a result, flow velocity and kinetic energy of molecular beams can be controlled with maximum accuracy.
Review of Scientific Instruments · 8 Zitationen · DOI
We present a new instrument that has been specifically designed for systematic studies of reactive cluster-surface collisions under well-defined ultrahigh-vacuum conditions. As a key feature, it incorporates a temperature-controlled, high-pressure cluster source with an unsurpassed pulse-to-pulse stability for the generation of weakly bound molecular aggregates. Impact processes can be investigated in two experimental configurations complementing one another: In the first, the exploration of intracluster reactions of accelerated, size-selected, positively or negatively charged cluster ions (1–100eV∕molecule) is accomplished, employing an inert target surface combined with angular-integrated ion detection. This arrangement uses a compact, homebuilt dual time-of-flight mass spectrometer with excellent mass resolution m∕Δm&gt;2000 and postacceleration capabilities. In the second configuration, the detailed analysis of chemical reactions between neutral clusters and a well-defined single crystal at lower kinetic energies (0.01–1eV∕molecule) is possible. Here, the surface interaction of a seeded supersonic cluster beam is probed with angular-, time-, and velocity-resolved detection of neutral reaction products using a high-performance quadrupole mass spectrometer.
The Journal of Chemical Physics · 6 Zitationen · DOI
We describe an improved experimental method for the generation of cold, directed particle bunches, and the highly accurate determination of their velocities in a pulsed supersonic beam, allowing for high-resolution experiments of atoms, molecules, and clusters. It is characterized by a pulsed high pressure jet source with high brilliance and optimum repeatability, a flight distance of few metres that can be varied with a tolerance of setting of 50 μm, and a precision in the mean flight time of particles of better than 10(-4). The technique achieves unmatched accuracies in particle velocities and kinetic energies and also permits the reliable determination of enthalpy changes with very high precision.
The Journal of Physical Chemistry C · 6 Zitationen · DOI
The reaction of Ru(10−10) with oxygen has been investigated under high-pressure conditions (≤1 bar) with thermal desorption spectroscopy (TDS), low electron energy diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and measurements of the work function changes ΔΦ, derived from ultraviolet photoelectron spectroscopy (UPS). For oxygen exposures between 0.1 and 1000 L, only chemisorption is observed independent of the sample preparation temperature TP (425 K < TP < 750 K). For exposures higher than 1000 L, chemisorption is followed by oxide formation at TP > 525 K. At exposures larger than 1000 L and temperatures below 525 K, the formation of an intermediate oxygen state as a precursor to the actual oxide is observed with XPS and TDS. It is accompanied by a perturbation of the long-range order of the Ru surface structure. Significant changes of the work function, ΔΦ, indicate that a Ru(10−10) surface exposed to oxygen at TP < 525 K resembles properties of a defect-rich Ru(0001) surface. The minimum oxygen exposures required for the formation of oxide as well as the intermediate state are found to be approximately 2 orders of magnitude smaller than on Ru(0001).
The Journal of Physical Chemistry B · 5 Zitationen · DOI
Smooth and defect-rich Ru(0001) surfaces prepared under ultrahigh-vacuum (UHV) conditions have been loaded with oxygen under high-pressure (p </= 1 bar) and low-temperature (T < 550 K) conditions. On these surfaces the CO oxidation reaction has been investigated by means of thermal desorption spectroscopy (TDS), ultraviolet photoelectron spectroscopy (UPS) and reactive molecular beam scattering (RMBS). Both surfaces are oxide-free and exhibit a high reactivity. The maximum CO/CO(2) conversion probability observed for a defect-rich Ru(0001) surface amounts to 6 x 10(-3) and is comparable to that of a surface covered with rutile RuO(2)(110) domains. RMBS experiments led to the identification of three different reaction channels. The first and second channel is related to CO adsorbing at oxygen-free defect sites and follow the Langmuir-Hinshelwood mechanism. Whereas the first reaction channel is already observed at room temperature, the second is thermally activated, contributing to the CO(2) yield only for reaction temperatures above 400 K. The third channel is due to the recombination of CO molecules with oxygen atoms located in smooth areas of the surface undisturbed by defects. This reaction channel is thermally activated as well.
The Journal of Physical Chemistry A · 4 Zitationen · DOI
Argon gas at a high pressure (∼80 bar) has been expanded using a miniaturized pulsed valve at room temperature, producing a supersonic beam of cold, large argon droplets. Atoms of silver are subsequently embedded into the droplet using the pick-up technique. The resulting Ag(n)Ar(droplet) distribution was analyzed using multiphoton laser ionization time-of-flight mass spectrometry. Besides bare metal clusters, snowballs of silver monomers and dimers encapsulated in up to 50 argon atoms have been observed. The influence of the solvent on the optical absorption of the solute was studied for embedded Ag(8) using resonant two-photon ionization in the ultraviolet. A redshift and broadening of the Ag(8)Ar(droplet) optical spectrum compared to that measured in pure [Federmann et al., Eur. Phys. J. D 1999, 9, 11] and Ar-doped helium droplets [Diederich et al., J. Chem. Phys.2002, 116, 3263] was observed, which is attributed to the interaction with the larger Ar matrix environment.
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- Name
- Dr. rer. nat. Wolfgang Christen
- Titel
- Dr. rer. nat.
- Fakultät
- Mathematisch-Naturwissenschaftliche Fakultät
- Institut
- Institut für Chemie
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- +49 30 2093-82620
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