15,403 research outputs found
Computational design of metal-supported molecular switches: Transient ion formation during light- and electron-induced isomerisation of azobenzene
In molecular nanotechnology, a single molecule is envisioned to act as the
basic building block of electronic devices. Such devices may be of special
interest for organic photovoltaics, data storage, and smart materials. However,
more often than not the molecular function is quenched upon contact with a
conducting support. Trial-and-error-based decoupling strategies via molecular
functionalisation and change of substrate have in many instances proven to
yield unpredictable results. The adsorbate-substrate interactions that govern
the function can be understood with the help of first-principles simulation.
Employing dispersion-corrected Density-Functional Theory (DFT) and linear
expansion Delta-Self-Consistent-Field DFT, the electronic structure of a
prototypical surface-adsorbed functional molecule, namely azobenzene adsorbed
to (111) single crystal facets of copper, silver and gold, is investigated and
the main reasons for the loss or survival of the switching function upon
adsorption are identified. The light-induced switching ability of a
functionalised derivative of azobenzene on Au(111) and azobenzene on Ag(111)
and Au(111) is assessed based on the excited-state potential energy landscapes
of their transient molecular ions, which are believed to be the main
intermediates of the experimentally observed isomerisation reaction. We provide
a rationalisation of the experimentally observed function or lack thereof that
connects to the underlying chemistry of the metal-surface interaction and
provides insights into general design strategies for complex light-driven
reactions at metal surfaces.Comment: 14 pages, 5 figures, submitted to J. Phys. Condens. Matte
Bistability loss as key feature in azobenzene (non-)switching on metal surfaces
Coinage metal adsorbed azobenzene is investigated as prototypical molecular
switch. It is shown that switching capabilities are not just lost due to
excited state quenching, but already due to changes in the ground state
energetics. Electron demanding coadsorbates are suggested as strategy to regain
the switching function.Comment: 8 pages, 3 figure
Assessing computationally efficient isomerization dynamics: Delta-SCF density-functional theory study of azobenzene molecular switching
We present a detailed comparison of the S0, S1 (n -> \pi*) and S2 (\pi ->
\pi*) potential energy surfaces (PESs) of the prototypical molecular switch
azobenzene as obtained by Delta-self-consistent-field (Delta-SCF)
Density-Functional Theory (DFT), time-dependent DFT (TD-DFT) and approximate
Coupled Cluster Singles and Doubles (RI-CC2). All three methods unanimously
agree in terms of the PES topologies, which are furthermore fully consistent
with existing experimental data concerning the photo-isomerization mechanism.
In particular, sum-method corrected Delta-SCF and TD-DFT yield very similar
results for S1 and S2, when based on the same ground-state exchange-correlation
(xc) functional. While these techniques yield the correct PES topology already
on the level of semi-local xc functionals, reliable absolute excitation
energies as compared to RI-CC2 or experiment require an xc treatment on the
level of long-range corrected hybrids. Nevertheless, particularly the
robustness of Delta-SCF with respect to state crossings as well as its
numerical efficiency suggest this approach as a promising route to dynamical
studies of larger azobenzene-containing systems.Comment: 25 pages, 6 figure
Optimal gradual annuitization : quantifying the costs of switching to annuities
We compute the optimal dynamic asset allocation policy for a retiree with Epstein-Zin utility. The retiree can decide how much he consumes and how much he invests in stocks, bonds, and annuities. Pricing the annuities we account for asymmetric mortality beliefs and administration expenses. We show that the retiree does not purchase annuities only once but rather several times during retirement (gradual annuitization). We analyze the case in which the retiree is restricted to buy annuities only once and has to perform a (complete or partial) switching strategy. This restriction reduces both the utility and the demand for annuities
Deferred Annuities and Strategic Asset Allocation
We derive the optimal portfolio choice and consumption pattern over the lifecycle for households facing labor income, capital market, and mortality risk. In addition to stocks and bonds, households also have access to deferred annuities. Deferred annuities offer a hedge against mortality risk and provide similar benefits as Social Security. We show that a considerable fraction of wealth should be annuitized to skim the return enhancing mortality credit. The remaining liquid wealth (stocks and bonds) is used to hedge labor income risk during work life and to earn the equity premium. We find a marginal difference between a strategy involving deferred annuities and one where the investor can purchase immediate life annuities.
Proving the correctness of the flight director program EADIFD, volume 1
EADIFD is written in symbolic assembly language for execution on the C4000 airborne computer. It is a subprogram of an aircraft navigation and guidance program and is used to generate pitch and roll command signals for use in terminal airspace. The proof of EADIFD was carried out by an inductive assertion method consisting of two parts, a verification condition generator and a source language independent proof checker. With the specifications provided by NASA, EADIFD was proved correct. The termination of the program is guaranteed and the program contains no instructions that can modify it under any conditions
Safety management of a complex R and D ground operating system
A perspective on safety program management was developed for a complex R&D operating system, such as the NASA-Lewis Research Center. Using a systems approach, hazardous operations are subjected to third-party reviews by designated-area safety committees and are maintained under safety permit controls. To insure personnel alertness, emergency containment forces and employees are trained in dry-run emergency simulation exercises. The keys to real safety effectiveness are top management support and visibility of residual risks
Investigation of statistical techniques to select optimal test levels for spacecraft vibration tests Final report, 1 Nov. 1969 - 31 Oct. 1970
Statistical techniques for selecting optimal test levels for vibration tests of spacecraft hardwar
A chain rule for the expected suprema of Gaussian processes
The expected supremum of a Gaussian process indexed by the image of an index
set under a function class is bounded in terms of separate properties of the
index set and the function class. The bound is relevant to the estimation of
nonlinear transformations or the analysis of learning algorithms whenever
hypotheses are chosen from composite classes, as is the case for multi-layer
models
Interpretation of X-ray Absorption Spectroscopy in the Presence of Surface Hybridization
X-ray absorption spectroscopy yields direct access to the electronic and
geometric structure of hybrid inorganic-organic interfaces formed upon
adsorption of complex molecules at metal surfaces. The unambiguous
interpretation of corresponding spectra is challenged by the intrinsic
geometric flexibility of the adsorbates and the chemical interactions with the
interface. Density-functional theory (DFT) calculations of the extended
adsorbate-substrate system are an established tool to guide peak assignment in
X-ray photoelectron spectroscopy (XPS) of complex interfaces. We extend this to
the simulation and interpretation of X-ray absorption spectroscopy (XAS) data
in the context of functional organic molecules on metal surfaces using
dispersion-corrected DFT calculations within the transition potential approach.
On the example of X-ray absorption signatures for the prototypical case of
2H-porphine adsorbed on Ag(111) and Cu(111) substrates, we follow the two main
effects of the molecule/surface interaction on XAS: (1) the substrate-induced
chemical shift of the 1s core levels that dominates in physisorbed systems and
(2) the hybridization-induced broadening and loss of distinct resonances that
dominates in more chemisorbed systems.Comment: 13 pages, 4 figure
- …