383 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
Charge transfer crystallites as molecular electrical dopants
Ground-state integer charge transfer is commonly regarded as the basic mechanism of molecular electrical doping in both, conjugated polymers and oligomers. Here, we demonstrate that fundamentally different processes can occur in the two types of organic semiconductors instead. Using complementary experimental techniques supported by theory, we contrast a polythiophene, where molecular p-doping leads to integer charge transfer reportedly localized to one quaterthiophene backbone segment, to the quaterthiophene oligomer itself. Despite a comparable relative increase in conductivity, we observe only partial charge transfer for the latter. In contrast to the parent polymer, pronounced intermolecular frontier-orbital hybridization of oligomer and dopant in 1:1 mixed-stack co-crystallites leads to the emergence of empty electronic states within the energy gap of the surrounding quaterthiophene matrix. It is their Fermi–Dirac occupation that yields mobile charge carriers and, therefore, the co-crystallites—rather than individual acceptor molecules—should be regarded as the dopants in such systems
X ray standing waves reveal lack of OH termination at hydroxylated ZnO 0001 surfaces
The vertical adsorption distances of the planar conjugated organic molecule 3,4,9,10 perylenetetracarboxylic diimide PTCDI on hydroxylated ZnO 0001 , determined with the x ray standing wave technique XSW , are at variance with adsorption geometries simulated with density functional theory for surface structure models that consider terminating OH, whereas good agreement is found for PTCDI in direct contact with the topmost Zn layer. The consequential assignment of OH to subsurface sites is supported by additional, independent XSW and energy scanned photoelectron diffraction data and calls for a reconsideration of the prevalent surface models with important implications for the understanding of ZnO 0001 surface
Energy Flow in the Hadronic Final State of Diffractive and Non-Diffractive Deep-Inelastic Scattering at HERA
An investigation of the hadronic final state in diffractive and
non--diffractive deep--inelastic electron--proton scattering at HERA is
presented, where diffractive data are selected experimentally by demanding a
large gap in pseudo --rapidity around the proton remnant direction. The
transverse energy flow in the hadronic final state is evaluated using a set of
estimators which quantify topological properties. Using available Monte Carlo
QCD calculations, it is demonstrated that the final state in diffractive DIS
exhibits the features expected if the interaction is interpreted as the
scattering of an electron off a current quark with associated effects of
perturbative QCD. A model in which deep--inelastic diffraction is taken to be
the exchange of a pomeron with partonic structure is found to reproduce the
measurements well. Models for deep--inelastic scattering, in which a
sizeable diffractive contribution is present because of non--perturbative
effects in the production of the hadronic final state, reproduce the general
tendencies of the data but in all give a worse description.Comment: 22 pages, latex, 6 Figures appended as uuencoded fil
A Search for Selectrons and Squarks at HERA
Data from electron-proton collisions at a center-of-mass energy of 300 GeV
are used for a search for selectrons and squarks within the framework of the
minimal supersymmetric model. The decays of selectrons and squarks into the
lightest supersymmetric particle lead to final states with an electron and
hadrons accompanied by large missing energy and transverse momentum. No signal
is found and new bounds on the existence of these particles are derived. At 95%
confidence level the excluded region extends to 65 GeV for selectron and squark
masses, and to 40 GeV for the mass of the lightest supersymmetric particle.Comment: 13 pages, latex, 6 Figure
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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