533 research outputs found
Estimates of rates for dissociative recombination of NO + e via various mechanisms
We estimate rates for the dissociative recombination (DR) of NO +
e. Although accurate excited state potential energy curves for the excited
states of the neutral are not available, we estimate that the 1 {\Phi}
and the 1 {\Pi} states of the neutral may intersect the ground state
cation potential energy surface near its equilibrium geometry. Using fixed
nuclei scattering calculations we estimate the rate for direct DR via these
states and find it to be significant. We also perform approximate calculations
of DR triggered by the indirect mechanism, which suggest that the indirect DR
rate for NO is insignificant compared to the direct rate.Comment: Submitted to Phys Rev
Effect of growth conditions on optical properties of CdSe/ZnSe single quantum dots
In this work, we have investigated the optical properties of two samples of
CdSe quantum dots by using submicro-photoluminescence spectroscopy. The effect
of vicinal-surface GaAs substrates on their properties has been also assessed.
The thinner sample, grown on a substrate with vicinal surface, includes only
dots with a diameter of less than 10 nm (type A islands). Islands of an average
diameter of about 16 nm (type B islands) that are related to a phase transition
via a Stranski-Krastanow growth process are also distributed in the thicker
sample grown on an oriented substrate. We have studied the evolution of
lineshapes of PL spectra for these two samples by improving spatial resolution
that was achieved using nanoapertures or mesa structures. It was found that the
use of a substrate with the vicinal surface leads to the suppression of
excitonic PL emitted from a wetting layer.Comment: 2pages, 2 figures, Proceedings of International Conference On
Superlattices Nano-Structures And Nano-Devices, July, Toulouse, France, to
appear in the special issue of Physica
"Ich habe ja was gegen die Wegwerfkultur, aber âŠ": eine empirische Analyse der Einflussfaktoren auf die Diskrepanz zwischen Einstellung und Verhalten am Beispiel von KonsumgĂŒtern
GemÀà der Statistikbehörde Eurostat gehört Deutschland europaweit zu den Spitzenreitern in puncto MĂŒllverursachung. Zahlreiche verbraucherpolitische MaĂnahmen zielen daher auf VerhaltensĂ€nderungen der BĂŒrger und BĂŒrgerinnen ab. Solche MaĂnahmen basieren - hĂ€ufig wohl implizit - auf der Theorie des geplanten Verhaltens (Ajzen 1991), die einen Zusammenhang zwischen der Einstellung, der Intention und dem Verhalten einer Person unterstellt. Eine empirische und verbraucherpolitische SchwĂ€che dieser Theorie sind die regelmĂ€Ăig feststellbaren Diskrepanzen: So werden trotz kritischer Diskussion um die deutsche Wegwerfkultur und des groĂen öffentlichen Interesses an einer Reduzierung der Ressourcenverschwendung umweltbewusste Einstellungen von den Verbrauchern kaum in tatsĂ€chliches Verhalten ĂŒbertragen. Diese Diskrepanzen zwischen Einstellung, Intention
und Verhalten sind in diesem Beitrag Gegenstand einer theoriegeleiteten empirischen Studie, deren Ergebnisse abschlieĂend kritisch gewĂŒrdigt und daraus resultierende Handlungsempfehlungen genannt werden
"Es war einmal ... das Leben!"
Der Titel dieses Beitrag hat dreierlei Bedeutung: So orientiert er sich erstens
an der gleichnamigen französischen TV-Zeichentrickserie, in der komplexe
Sachverhalte zum menschlichen Körper anschaulich und unterhaltsam erzÀhlt
werden. Zweitens geht es um â das Leben! Dessen KomplexitĂ€t gilt es als
Verbraucher erfolgreich zu bewÀltigen und zu gestalten. Und drittens handelt
es sich bei âEs war einmalâ um eine typische Einleitung fĂŒr MĂ€rchen, Sagen
und Legenden, also Geschichten. Vom GeschichtenerzÀhlen (Storytelling) und
dessen Rolle in der Verbraucherbildung handelt dieser Beitrag
Initial coupling and reaction progression of directly deposited biradical graphene nanoribbon monomers on iodine-passivated versus pristine Ag(111)
The development of widely applicable methods for the synthesis of C-C-bonded nanostructures on inert and insulating surfaces is a challenging yet rewarding milestone in the field of on-surface synthesis. This would enable studies of nearly unperturbed covalent nanostructures with unique electronic properties as graphene nanoribbons (GNR) and Ï-conjugated 2D polymers. The prevalent Ullmann-type couplings are almost exclusively carried out on metal surfaces to lower the temperature required for initial dehalogenation well below the desorption threshold. To overcome the necessity for the activation of monomers on the target surface, we employ a recently developed Radical Deposition Source (RaDeS) for the direct deposition of radicals onto inert surfaces for subsequent coupling by addition reactions. The radicals are generated en route by indirect deposition of halogenated precursors through a heated reactive tube, where the dehalogenation reaction proceeds. Here, we use the ditopic 6,11-diiodo-1,2,3,4-tetraphenyltriphenylene (DITTP) precursor that afforded chevron-like GNR on Au(111) via the usual two-staged reaction comprised of monomer-coupling into covalent polymers and subsequent formation of an extended GNR by intramolecular cyclodehydrogenation (CDH). As a model system for inert surfaces, we use Ag(111) passivated with a closed monolayer of chemisorbed iodine that behaves in an inert manner with respect to dehalogenation reactions and facilitates the progressive coupling of radicals into extended covalent structures. We deposit the DITTP-derived biradicals onto both iodine-passivated and pristine Ag(111) surfaces. While on the passivated surface, we directly observe the formation of covalent polymers, on pristine Ag(111) organometallic intermediates emerge instead. This has decisive consequences for the further progression of the reaction: heating the organometallic chain directly on Ag(111) results in complete desorption, whereas the covalent polymer on iodine-passivated Ag(111) can be transformed into the GNR. Yet, the respective CDH proceeds directly on Ag(111) after thermal desorption of the iodine passivation. Accordingly, future work is aimed at the further development of approaches for the complete synthesis of GNR on inert surfaces
Competitive metalâcoordination of hexaaminotriphenylene on Cu(111) by intrinsic copper versus extrinsic nickel adatoms
The interplay between selfâassembly and surface chemistry of 2,3,6,7,10,11âhexaminotriphenylene (HATP) on Cu(111) was complementarily studied by highâresolution ScanningâTunnelingâMicroscopy (STM) and Xâray Photoelectron Spectroscopy (XPS) under ultraâhigh vacuum conditions. To shed light on competitive metalâcoordination, comparative experiments were carried out on pristine and nickelâcovered Cu(111). Directly after room temperature deposition of HATP onto pristine Cu(111) selfâassembled aggregates were observed by STM, while XPS indicated nonâdeprotonated amino groups. Annealing up to 200 °C activated the progressive single deprotonation of all amino groups as indicated by chemical shifts of both N 1s and C 1s core levels in the XP spectra. This enabled the formation of topologically versatile Ïâd conjugated coordination networks with intrinsic copper adatoms. The basic motif of these networks was a metalâorganic trimer, where three HATP molecules were coordinated by Cu3 clusters, as corroborated by accompanying Density Functional Theory (DFT) simulations. Additional deposition of more reactive nickel atoms resulted in both chemical and structural changes with deprotonation and formation of bis(diimino)âNi bonded networks already at room temperature. Even though fused hexagonal pores were observed, extended honeycomb networks remained elusive, as tentatively explained by a restricted reversibility of these metalâorganic bonds
Modelling of interactions of polar and nonpolar pollutants with soil minerals and soil organic matter
Environmental pollution of soils by organic contaminants such as pesticides is one of the serious problems of our civilization. Contaminants can undergo various physical, chemical and biological transformation processes in soils governing behaviour, distribution, and fate of organic species in environment and subsequent environmental risks. Mechanistic understanding of molecular interactions of organic pollutants with main soil components represents a key factor for estimating the behaviour of contaminants in soils. Molecular modelling offers an opportunity to investigate and characterize various details of these interactions at molecular level providing specifications, which are difficult to obtain at the experimental level. This work represents a comprehensive overview of our investigations of the molecular interactions of organic contaminants with selected soil components. Particularly, we focused on the characterization of the structure and the surface complexation of the phenoxyacetic acid derivatives (herbicides MCPA and 2,4-D) and typical soil minerals such as clay minerals (kaolinite and montmorillonite) and iron oxyhydroxides (goethite and lepidocrocite). Further, interactions of several representative nonpolar polycyclic aromatic hydrocarbons (e.g. naphthalene, anthracene, pyrene, and phenanthrene) with iron oxyhydroxides were modelled, as well. It was found that in case of polar species, hydrogen bonds and electrostatic interactions play an important role in the formation of the surface complexes. In case of nonpolar PAHs, dispersion forces dominate in the planar stacking of the PAHs molecules on mineral surfaces. Another study focused at a complex 3D model representing humic substances firstly, featuring polar hydrophilic and nonpolar hydrophobic domains and also a nanopore SOM structure. This model was taken to simulate trapping and interactions of MCPA (polar) and naphthalene (nonpolar) species inside of the nanopore. It was found that MCPA is preferentially stabilized close to polar functional groups (carboxyl) whereas naphthalene interacts mostly with nonpolar aliphatic chains through dispersion interactions
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