423 research outputs found
Fission studies with 140 MeV -Particles
Binary fission induced by 140 MeV -particles has been measured for
Ag, La, Ho and Au targets. The measured
quantities are the total kinetic energies, fragment masses, and fission cross
sections. The results are compared with other data and systematics. A minimum
of the fission probability in the vicinity is observed.Comment: 4 figures, 2 table
Exploring the Bonding of Large Hydrocarbons on Noble Metals: Diindoperylene on Cu(111), Ag(111), and Au(111)
We present a benchmark study for the adsorption of a large pi-conjugated
organic molecule on different noble metal surfaces, which is based on X-ray
standing wave (XSW) measurements and density functional theory calculations
with van der Waals (vdW) interactions. The bonding distances of
diindenoperylene on Cu(111), Ag(111), and Au(111) surfaces (2.51 A, 3.01 A, and
3.10 A, respectively) determined with the normal incidence XSW technique are
compared with calculations. Excellent agreement with the experimental data,
i.e. deviations less than 0.1 A, is achieved using the Perdew-Burke-Ernzerhof
functional with vdW interactions that include the collective response of
substrate electrons (PBE+vdW^{surf} method). Noteworthy, the calculations show
that the vdW contribution to the adsorption energy increases in the order
Au(111) < Ag(111) < Cu(111).Comment: 6 pages, 4 figures, accepted by Phys. Rev.
First-principles study of the dipole layer formation at metal-organic interfaces
We study the dipole layer formed at metal-organic interfaces by means of
first-principles calculations. Interface dipoles are monitored by calculating
the work function change of Au, Ag, Al, Mg and Ca surfaces upon adsorption of a
monolayer of PTCDA (3,4,9,10-perylene-tetra-carboxylic-di-anhydride), perylene
or benzene molecules. Adsorption of PTCDA leads to pinning of the work function
for a range of metal substrates. It gives interface dipoles that compensate for
the difference in the clean metal work functions, leading to a nearly constant
work function. In contrast, adsorption of benzene always results in a decrease
of the work function, which is relatively constant for all metal substrates.
Both effects are found in perylene, where adsorption on low work function
metals gives work function pinning, whereas adsorption on high work function
metals gives work function lowering. The work function changes upon adsorption
are analyzed and interpreted in terms of two competing effects. If the molecule
and substrate interact weakly, the molecule pushes electrons into the surface,
which lowers the work function. If the metal work function is sufficiently low
with respect to the unoccupied states of the molecule, electrons are donated
into these states, which increases the binding and the work function.Comment: 18 pages, 12 figures, 5 table
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A diuranium carbide cluster stabilized inside a C80 fullerene cage.
Unsupported non-bridged uranium-carbon double bonds have long been sought after in actinide chemistry as fundamental synthetic targets in the study of actinide-ligand multiple bonding. Here we report that, utilizing Ih(7)-C80 fullerenes as nanocontainers, a diuranium carbide cluster, U=C=U, has been encapsulated and stabilized in the form of UCU@Ih(7)-C80. This endohedral fullerene was prepared utilizing the Krätschmer-Huffman arc discharge method, and was then co-crystallized with nickel(II) octaethylporphyrin (NiII-OEP) to produce UCU@Ih(7)-C80·[NiII-OEP] as single crystals. X-ray diffraction analysis reveals a cage-stabilized, carbide-bridged, bent UCU cluster with unexpectedly short uranium-carbon distances (2.03 Å) indicative of covalent U=C double-bond character. The quantum-chemical results suggest that both U atoms in the UCU unit have formal oxidation state of +5. The structural features of UCU@Ih(7)-C80 and the covalent nature of the U(f1)=C double bonds were further affirmed through various spectroscopic and theoretical analyses
Lithium distribution across the membrane of motoneurons in the isolated frog spinal cord
Lithium sensitive microelectrodes were used to investigate the transmembrane distribution of lithium ions (Li+) in motoneurons of the isolated frog spinal cord. After addition of 5 mmol·l–1 LiCl to the bathing solution the extracellular diffusion of Li+ was measured. At a depth of 500 m, about 60 min elapsed before the extracellular Li+ concentration approached that of the bathing solution. Intracellular measurements revealed that Li+ started to enter the cells soon after reaching the motoneuron pool and after up to 120 min superfusion, an intra — to extracellular concentration ratio of about 0.7 was obtained. The resting membrane potential and height of antidromically evoked action potentials were not altered by 5 mmol·l–1 Li+
Elastic scattering and breakup of 17^F at 10 MeV/nucleon
Angular distributions of fluorine and oxygen produced from 170 MeV 17^F
incident on 208^Pb were measured. The elastic scattering data are in good
agreement with optical model calculations using a double-folding potential and
parameters similar to those obtained from 16^O+208^Pb. A large yield of oxygen
was observed near \theta_lab=36 deg. It is reproduced fairly well by a
calculation of the (17^F,16^O) breakup, which is dominated by one-proton
stripping reactions. The discrepancy between our previous coincidence
measurement and theoretical predictions was resolved by including core
absorption in the present calculation.Comment: 9 pages, 5 figure
Breakup of F on Pb near the Coulomb barrier
Angular distributions of oxygen produced in the breakup of F incident
on a Pb target have been measured around the grazing angle at beam
energies of 98 and 120 MeV. The data are dominated by the proton stripping
mechanism and are well reproduced by dynamical calculations. The measured
breakup cross section is approximately a factor of 3 less than that of fusion
at 98 MeV. The influence of breakup on fusion is discussed.Comment: 7 pages, 8 figure
Ionization potentials of crystalline organic thin films: Position dependence due to molecular shape and charge redistribution
In addition to electronic polarization or charge redistribution, the shape of
neutral conjugated molecules yields position-dependent ionization potentials
and electron affinities in organic thin films. Self-consistent I(n) and A(n)
are computed in each layer n of 10-layer films of prototypical organics on a
metal. The depth dependence of I(n) is discussed at surfaces of anthracene, C60
and PTCDA. The shape contribution can be substantial, up to 0.5 eV, and comes
primarily from charge-quadrupole interactions.Comment: 13 pages, 4 figures, Accepted in Chem. Phys. Letter
Pentacene perfluoropentacene bilayers on Au 111 and Cu 111 Impact of organic metal coupling strength on molecular structure formation
As crucial element in organic opto electronic devices, heterostructures are of pivotal importance. In this context, a comprehensive study of the properties on a simplified model system of a donor acceptor D A bilayer structure is presented, using ultraviolet photoelectron spectroscopy UPS , X ray photoelectron spectroscopy XPS , low energy electron diffraction LEED and normal incidence X ray standing wave NIXSW measurements. Pentacene PEN as donor and perfluoropentacene PFP as acceptor material are chosen to produce bilayer structures on Au 111 and Cu 111 by sequential monolayer deposition of the two materials. By comparing the adsorption behavior of PEN PFP bilayers on such weakly and strongly interacting substrates, it is found that i the adsorption distance of the first layer PEN or PFP indicates physisorption on Au 111 , ii the characteristics of the bilayer structure on Au 111 are almost independent of the deposition sequence, and hence, iii in both cases a mixed bilayer is formed on the Au substrate. This is in striking contrast to PFP PEN bilayers on Cu 111 , where strong chemisorption pins PEN molecules to the metal surface and no intermixing is induced by subsequent PFP deposition. The results illustrate the strong tendency of PEN and PFP molecules to mix, which has important implications for the fabrication of PEN PFP heterojunction
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