Showing papers by "Tilmann D. Märk published in 2009"
TL;DR: In this article, the Tofwerk orthogonal acceleration reflectron time-of-flight (TOF-MS) was used to measure VOCs at ultra-low concentrations (as low as a few pptv) under high mass resolution (as high as 6000m/Δm in the V-mode) with a mass range of beyond 100,000 amu.
514 citations
TL;DR: In this article, a modified hollow cathode (HC) ion source and drift tube design is used for the PTR+SRI-MS, which allows for the easy and fast switching between H 3 O +, NO + and O 2 + ions produced in high purity and in large quantities in this source.
210 citations
TL;DR: The existence of doubly charged intermediates in doped helium droplets and their role in subsequent ion–molecule reactions has so far been ignored; these intermediates provide a compelling rationale for previous observations of hydrogen loss from clusters of organic molecules and biomolecules.
Abstract: Helium nanodroplets, which contain some 10 to 10 atoms, provide an avenue for important new experiments. In a lowpressure environment, the droplets cool within microseconds to 0.37 K by evaporation of weakly bound helium atoms. Molecules captured in collisions with a superfluid droplet will quickly aggregate in the droplet s interior into novel, often metastable structures. These “personal nanocryostats” may be used to explore chemical reactions. For example, M ller et al. reported that ionization initiates complete hydrolysis of cesium–water complexes within the droplets. Herein we report ion–molecule reactions between C60 and small water clusters. Water is an integral part of biomolecular organization; its bioactivity can be further understood by characterization of its function at the C60–H2O interface. [4] C60 is hydrophobic; its hard-core radius of 0.5 nm is close to the crossover point beyond which the breakage of hydrogen bonds becomes unavoidable. Molecular dynamics simulations show that fullerenes strongly bind to single and doublestrand DNA; addition of hydrated C60 to drinking water has been found to mitigate damage of ethanol to brain cells of rats without causing any adverse biological effects. In the present work, helium droplets were doped with C60 and water, and subsequently ionized by electron impact ionization. The interpretation of experimental results was aided by ab initio Hartree–Fock calculations. Two observations stand out: Firstly, the weak interaction between neutral C60 and water extends to the cationic system. Desorption of entire water clusters rather than evaporative loss of water molecules occurs for certain water cluster sizes. Secondly, C60OH + is a major product ion. We postulate that this ion results from doubly charged [C60(H2O)] 2+ intermediates that form by charge transfer from a primary He ion. The existence of doubly charged intermediates in doped helium droplets and their role in subsequent ion–molecule reactions has so far been ignored; these intermediates provide a compelling rationale for previous observations of hydrogen loss from clusters of organic molecules and biomolecules. We first summarize results obtained by ionization of helium droplets doped with water (either H2O or D2O) but no C60 . In agreement with a previous report, [11] electron impact ionization results in a prominent series of protonated water cluster ions. Unprotonated water cluster ions are observed with a 10 % abundance relative to the protonated cluster ions. Unprotonated ions are not observable upon electron impact or multiphoton ionization of bare water clusters, but they occur if water clusters are complexed with heavy rare-gas atoms. These trends are well understood—the ground state of (H2O)2 + corresponds to the proton-transferred isomer OH–H3O , and its dissociation to OH + H3O + is energetically much more facile than dissociation to H2O + H2O . Direct ab initio dynamics studies of water clusters show that vertical ionization is followed by one or more barrierless proton transfer reactions within 100 fs; solvent reorganization leads to a highly excited cluster ion and ejection of the OH radical within sub-picoseconds; enough energy remains for the evaporation of several more water molecules. The composition of cluster ions changes drastically when helium droplets are co-doped with C60 . The most prominent ion series in Figure 1 arises from C60(D2O)n , n = 0, 1, 2. Dehydrogenated ions, that is, ions with the stoichiometry C60(D2O)n 1OD + are also observed, while the abundance of protonated ions is weak. For a quantitative analysis, we fitted the distribution of C60 isotopologues by sets of four Gaussians with fixed ratios of amplitudes computed from the 1.11 % natural abundance of [*] Prof. Dr. O. Echt Department of Physics, University of New Hampshire Durham, NH 03824 (USA) Fax: (+ 1)603-862-2998 E-mail: olof.echt@unh.edu Homepage: http://www.physics.unh.edu/
46 citations
TL;DR: The potential to identify CWAs with a high level of confidence in complex chemical environments, where multiple threat agents and interferents could also be present in trace amounts, thereby reducing the risk of false positives is demonstrated.
Abstract: This work demonstrates for the first time the potential of using recent developments in proton transfer reaction mass spectrometry for the rapid detection and identification of chemical warfare agents (CWAs) in real-time. A high-resolution (m/Δm up to 8000) and high-sensitivity (∼50 cps/ppbv) proton transfer reaction time-of-flight mass spectrometer (PTR-TOF 8000 from Ionicon Analytik GmBH) has been successfully used to detect a number of CWA simulants at room temperature; namely dimethyl methylphosphonate, diethyl methylphosphonate, diisopropyl methylphosphonate, dipropylene glycol monomethyl ether and 2-chloroethyl ethyl sulfide. Importantly, we demonstrate in this paper the potential to identify CWAs with a high level of confidence in complex chemical environments, where multiple threat agents and interferents could also be present in trace amounts, thereby reducing the risk of false positives. Instantaneous detection and identification of trace quantities of chemical threats using proton transfer reaction mass spectrometry could form the basis for a timely warning system capability with greater precision and accuracy than is currently provided by existing analytical technologies. Copyright © 2009 John Wiley & Sons, Ltd.
43 citations
TL;DR: In this paper, electron impact ionization studies with the amino acid valine in different environments, i.e., (i) isolated in the gas phase, (ii) embedded in superfluid helium droplets and (iii) co-embedded with water in liquid helium.
Abstract: Here we report electron impact ionization studies with the amino acid valine in different environments, i.e., (i) isolated in the gas phase, (ii) embedded in superfluid helium droplets and (iii) co-embedded with water in superfluid helium droplets. Mass spectra are presented for all three environments for which changes in the fragmentation pattern of valine upon ionization are investigated. Comparison is made with previous electron impact ionization and photoionization studies with valine in the gas phase which confirms the fragile nature of this amino acid. Embedding valine in cold superfluid helium droplets leads to the formation of highly abundant protonated valine clusters. Co-embedding water with valine in helium droplets reduces fragmentation of valine.
38 citations
TL;DR: A simple classical model is proposed to estimate ionic radii from n(s) and finds that radii of halide anions in helium are nearly twice as large as in alkali halide crystals, indicating the formation of an anion bubble due to the repulsive forces that derive from the exchange interaction.
Abstract: Helium nanodroplets are doped with SF(6), C(4)F(8), CCl(4), C(6)H(5)Br, CH(3)I, and I(2). Upon interaction with free electrons a variety of positively and negatively charged cluster ions X(+/-)He(n) are observed where X(+/-) = F(+/-), Cl(+/-), Br(+/-), I(+), I(2) (+), or CH(3)I(+). The yield of these ions versus cluster size n drops at characteristic sizes n(s) that range from n(s) = 10.2+/-0.6 for F(+) to n(s) = 22.2+/-0.2 for Br(-). n(s) values for halide anions are about 70% larger than for the corresponding cations. The steps in the ion yield suggest closure of the first solvation shell. We propose a simple classical model to estimate ionic radii from n(s). Assuming the helium density in the first solvation shell equals the helium bulk density one finds that radii of halide anions in helium are nearly twice as large as in alkali halide crystals, indicating the formation of an anion bubble due to the repulsive forces that derive from the exchange interaction. In spite of the simplicity of our model, anion radii derived from it agree within approximately 10% with values derived from the mobility of halide anions in superfluid bulk helium, and with values computed by quantum Monte Carlo methods for X(-)He(n) cluster anions.
34 citations
TL;DR: Distributions of larger argon cluster ions containing up to 60 atoms closely resemble distributions observed upon electron impact or photoionization of bare argon clusters; caging and evaporative cooling provided by the helium matrix do not suffice to quench fragmentation of the nascent argon Cluster ions.
Abstract: Electron impact ionization of argon clusters embedded in helium droplets is investigated. Superior mass resolution makes it possible to distinguish between nominally isobaric cluster ions. An abundance maximum for ArHe12+ is unambiguously confirmed; the spectra also prove the formation of Ar2Hen+ complexes that had been claimed to fragment into pure Ar2+. Distributions of larger argon cluster ions containing up to 60 atoms closely resemble distributions observed upon electron impact or photoionization of bare argon clusters; caging and evaporative cooling provided by the helium matrix do not suffice to quench fragmentation of the nascent argon cluster ions. Intriguing abundance anomalies are observed in distributions of argon cluster ions that contain water, nitrogen or oxygen impurities. The strong abundance of Ar55H2O+, Ar54O2+ and Ar54N2+ contrasts with the virtual absence of slightly larger cluster ions containing the corresponding impurities. The features are probably related to enhanced cluster ion stability upon closure of the second icosahedral shell but the difference in magic numbers (54 versus 55) and the well-known reactivity of charged argon-nitrogen complexes suggest structural differences.
32 citations
TL;DR: In this article, a joint experimental and theoretical investigation with the aim to make direct comparisons between experimental results and theoretical results is presented. But the results are limited to the case of water molecules surrounding biological molecules during irradiation with heavy particles (atoms, ions).
31 citations
TL;DR: The complete freezing of dissociation intermediates in TNT embedded in the droplet is explained by the particular mechanisms of DEA in nitrobenzenes, which is characterized by complex rearrangement processes in the transient negative ion (TNI) prior to decomposition.
Abstract: Electron attachment to the explosive trinitrotoluene (TNT) embedded in Helium droplets (TNT@He) generates the non-decomposed complexes (TNT)(n)(-), but no fragment ions in the entire energy range 0-12 eV. This strongly contrasts the behavior of single TNT molecules in the gas phase at ambient temperatures, where electron capture leads to a variety of different fragmentation products via different dissociative electron attachment (DEA) reactions. Single TNT molecules decompose by attachment of an electron at virtually no extra energy reflecting the explosive nature of the compound. The complete freezing of dissociation intermediates in TNT embedded in the droplet is explained by the particular mechanisms of DEA in nitrobenzenes, which is characterized by complex rearrangement processes in the transient negative ion (TNI) prior to decomposition. These mechanisms provide the condition for effective energy withdrawal from the TNI into the dissipative environment thereby completely suppressing its decomposition.
30 citations
TL;DR: The effect of incident electrons on acetic acid clusters is explored for the first time and a particularly intriguing contrast between the monomer and cluster anions is that helium atoms are seen attached to the latter whereas no evidence of helium atom attachment is found for the monomers.
Abstract: The effect of incident electrons on acetic acid clusters is explored for the first time. The acetic acid clusters are formed inside liquid helium nanodroplets and both cationic and anionic products ejected into the gas phase are detected by mass spectrometry. The cation chemistry (induced by electron ionization at 100 eV) is dominated by production of protonated acetic acid (Ac) clusters, Ac(n)H(+), although some fragmentation is also observed. In the case of anion production (at 2.8 eV electron energy) there is a clear distinction between the monomer and the clusters. For the monomer the dominant product is the dehydrogenated species, [Ac-H](-), whereas for the clusters both the parent anion, Ac(n)(-), and the dehydrogenated species, [Ac(n)-H](-), have similar abundances. A particularly intriguing contrast between the monomer and cluster anions is that helium atoms are seen attached to the latter whereas no evidence of helium atom attachment is found for the monomer. This surprising observation is attributed to the formation of acyclic (head-to-tail) acetic acid clusters in helium nanodroplets, which have more favourable electronic properties for binding helium atoms. The acyclic clusters represent a local minimum on the potential energy surface and in the case of the dimer this is distinct from the cyclic isomer (the global minimum) identified in gas phase experiments.
29 citations
TL;DR: A review of the status of calculations of absolute electron-impact ionization cross sections using the Deutsch-Mark (DM) formalism for a variety of targets ranging from groundstate atoms to atoms in excited states to molecules, free radicals and clusters, and to positive and negative ions can be found in this paper.
Abstract: Collisions of electrons with matter causing ionization are among the most fundamental processes in collision physics. The knowledge of cross sections for electron‐impact ionization is of basic importance to our understanding of collision physics and critical to many applications such as low‐temperature processing plasmas, fusion edge plasmas, gas discharges, planetary, stellar, and cometary atmospheres, radiation chemistry, mass spectrometry, and chemical analysis. While much progress has been made in the experimental determination of cross sections for atomic and molecular targets, rigorous quantum mechanical calculations of ionization cross sections are scarce and exist only for some simple atoms in their electronic ground state. The need to incorporate ionization cross sections for these targets in modeling codes in many applications has stimulated a renewed interest in the use of less rigorous approaches to the calculation of ionization cross section ranging from simplistic additivity rules to semirigorous methods that incorporate aspects of established collision theories and some quantum mechanically calculated target properties. Here we present a review of the status of calculations of absolute electron‐impact ionization cross sections using the Deutsch–Mark (DM) formalism for a variety of targets ranging from ground‐state atoms to atoms in excited states to molecules, free radicals and clusters, and to positive and negative ions. The main emphasis is on demonstrating the versatility of the DM formalism as its range of applicability has been extended over the years, both in terms of range of impact energies covered and range of target species studied. Extensive comparisons will be made with available experimental data and, to the extent possible, with results from other cross‐section calculations to demonstrate the accuracy, reliability, and predictive potential of the DM formalism.
TL;DR: The possible use of PTR-MS in waste managements and composting plants both for the real time monitoring of volatile emissions and, after calibration with olfactometric assessments, for the instrumental determination of odour concentration is described.
TL;DR: The remarkable instability of RDX to electron attachment with virtually thermal electrons reflects the highly explosive nature of this compound.
Abstract: Low energy electron attachment to gas phase royal demolition explosive (RDX) (and RDX-A3) has been performed by means of a crossed electron-molecular beam experiment in an electron energy range from 0 to 14 eV with an energy resolution of ∼70 meV. The most intense signals are observed at 102 and 46 amu and assigned to C2H4N3O2− and NO2−, respectively. Anion efficiency curves of 16 anions have been measured. Product ions are observed mainly in the low energy region, near 0 eV arising from surprisingly complex reactions associated with multiple bond cleavages and structural and electronic rearrangement. The remarkable instability of RDX to electron attachment with virtually thermal electrons reflects the highly explosive nature of this compound. The present results are compared to other explosive aromatic nitrocompounds studied in our laboratory recently.
TL;DR: In this paper, electron impact ionization of helium nano-droplets containing several 104 He atoms and doped with CCl4 or SF6 molecules is studied with high-mass resolution.
TL;DR: In this paper, the yields of (C60)2 - and C60D2O- anions reveal another important factor, namely depletion of the anion signal by dissociation which is energetically more facile than autodetachment.
Abstract: Helium nanodroplets, formed in a supersonic expansion, are doped with C60 in a pickup cell. In some experiments, they are co-doped with water. Electrons are attached to the doped droplets; the yield of anions is recorded as a function of electron energy. The C60 - yield extends to much higher energies than in experiments involving isolated, hot fullerenes; we attribute the difference to the low temperature of the neutral precursors and the efficient cooling of the nascent anions by the helium droplet, which quench thermally activated autodetachment. The yields of (C60)2 - and C60D2O- anions reveal another important factor, namely depletion of the anion signal by dissociation which is energetically more facile than autodetachment.
TL;DR: Experimental evidence is obtained for the existence of the most weakly bound member of a new class of centrosymmetric radical cations in which H(2) bridges two identical main group elements, He-H- H-He(+) (see picture), although in a metastable, electronically excited state.
Abstract: Survival of the weakest: The existence of a new class of centrosymmetric radical cations in which H2 bridges two identical main group elements was recently proposed in this journal by Uggerud and co-workers. By growing complexes inside helium nanodroplets at subkelvin temperatures, we obtained experimental evidence for the existence of the most weakly bound member of this class, He-H-H-He+ (see picture), although in a metastable, electronically excited state.
In a recent report, Uggerud and co-workers (A. Krapp et al., Chem. Eur. J.2008, 14, 4028) proposed the existence of a new class of radical cations in which a dihydrogen bridges two identical main group elements. Upon electron impact ionization of helium nanodroplets doped with one or more H2 molecules we observe various HexHy+ cluster ions, including He2H2+, which would belong to the proposed class of radical cations. Mass-analyzed kinetic energy scans reveal that the ion is metastable; it dissociates in the field-free region of the mass spectrometer. One reaction is into HeH2+ + He with a low kinetic energy release of 15±4 meV. Surprisingly, another unimolecular reaction is observed, into HeH+ + HeH (or He + H). The probability of this reaction is an order of magnitude higher, and the average kinetic energy release is four times larger. These findings suggest the presence of a metastable electronically excited state; they are consistent with the proposed linear, centrosymmetric ion structure of He-H-H-He+.
TL;DR: Dissociative electron attachment to all three isomers of dichlorobenzene has been investigated using a crossed electron-molecular beam apparatus with a new temperature-regulated effusive molecular beam source and strong enhancement of the negative ion production with the gas temperature at low electron energies has been observed.
Abstract: Dissociative electron attachment to all three isomers of dichlorobenzene has been investigated in the electron energy range from 0 to 2 eV and in the gas temperature range from 391 to 696 K using a crossed electron-molecular beam apparatus with a new temperature-regulated effusive molecular beam source. In the case of the dissociative electron attachment channel Cl−/1,2-dichlorobenzene and Cl−/1,4-dichlorobenzene, strong enhancement of the negative ion production with the gas temperature at low electron energies has been observed. The low-energy peak increases dramatically when the gas temperature is raised from 391 to 696 K. Activation energies for dissociative electron attachment of (482 ± 20) meV for 1,2-dichlorobenzene and (59 ± 20) meV for 1,4-dichlorobenzene have been determined. For the resonance at (0.49 ± 0.03) eV in 1,2-dichlorobenzene and (0.32 ± 0.03) eV in 1,4-dichlorobenzene, no dependence of the cross sections on the gas temperature has been observed. In the case of the dissociative electro...
TL;DR: Surface-induced interactions of the projectile ion C2D4+ with room-temperature stainless steel, carbon highly oriented pyrolytic graphite (HOPG), and two different types of diamond surfaces were investigated over the range of incident energies.
01 Jan 2009
TL;DR: Crossed-beam experiments revealed that attachment of a free electron to the cyclobutane pyrimidine dimers c,s-DMT<>DMT and c,a- DMT leads to the formation of dimer radical anions with the lifetime of at least 80 micros, thus showing that the latter are much more stable than previously believed.
01 Feb 2009-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: Sticking coefficients of deuterium from CD 2 + are quantified on fusion relevant plasma sprayed tungsten and carbon fibre composite in the incident energy range from about 0-100 eV as mentioned in this paper.
Abstract: Sticking coefficients of deuterium from CD 2 + are quantified on fusion relevant plasma sprayed tungsten and carbon fibre composite in the incident energy range from about 0–100 eV. The samples that were cut from ASDEX-Upgrade tiles are exposed to a beam of CD 2 + of specific incident energy, Einc, in the tandem mass spectrometer BESTOF in Innsbruck. Nuclear reaction analysis is performed ex-situ at IPP Garching for the quantification of deuterium content. The deuterium content difference measured on a spot before and after ion-beam exposure of the sample is assigned to the above mentioned species of hydrocarbon molecules sticking on the surface, allowing the calculation of the sticking probability of a specific deuterated molecular ion. The sticking coefficient, S, is found to depend on the incident energy and shows a maximum of about S ∼ 0.4 around Einc = 30 eV on CFC and about S ∼ 0.1 near Einc = 20 eV in case of PSW.
02 May 2009
TL;DR: In this paper, electron attachment to gas phase nitrobenzene, all three isomers of mononitrotoluene and 2,4,6 trinitrotoluanene is studied by means of two crossed electron-molecular beam experiments.
Abstract: Electron attachment to gas phase nitrobenzene, all three isomers of mononitrotoluene and 2,4,6‐trinitrotoluene is studied by means of two crossed electron‐molecular beam experiments We point out the formation of long‐lived metastable parent anions and the most abundant anions produced via dissociative electron attachment (DEA) The experimental results are supported by quantum‐chemical calculations, to determine the electronic configuration of selected molecular orbitals or the electrostatic potential mapped on an isosurface of the total electron density to find preferential sites of electron attachment