3,303 research outputs found
Non-adiabatic Effects in the Dissociation of Oxygen Molecules at the Al(111) Surface
The measured low initial sticking probability of oxygen molecules at the
Al(111) surface that had puzzled the field for many years was recently
explained in a non-adiabatic picture invoking spin-selection rules [J. Behler
et al., Phys. Rev. Lett. 94, 036104 (2005)]. These selection rules tend to
conserve the initial spin-triplet character of the free O2 molecule during the
molecule's approach to the surface. A new locally-constrained
density-functional theory approach gave access to the corresponding
potential-energy surface (PES) seen by such an impinging spin-triplet molecule
and indicated barriers to dissociation which reduce the sticking probability.
Here, we further substantiate this non-adiabatic picture by providing a
detailed account of the employed approach. Building on the previous work, we
focus in particular on inaccuracies in present-day exchange-correlation
functionals. Our analysis shows that small quantitative differences in the
spin-triplet constrained PES obtained with different gradient-corrected
functionals have a noticeable effect on the lowest kinetic energy part of the
resulting sticking curve.Comment: 17 pages including 11 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Changes of vibrational lifetimes with minor structural modification of small polyatomic molecules
Substantial changes of population lifetimes of CH-stretching modes are observed when two atoms are exchanged in CH2=CCl2 to form trans CHCl=CHCl and when three deuterons are substituted in C6H6 to form 1,3,5.-C6H3D3. The measured lifetimes are in good agreement with estimates based on Fermi resonance-mixing which is inferred from infrared and Raman spectra
A new Raman technique of superior spectral resolution
Raman-active vibrational modes are coherently excited by the transient stimulated Raman process. A subsequent delayed probe of relatively long duration interacts with the freely relaxing vibrations. Raman spectra are generated with higher resolution and more accurate peak positions than in conventional Raman spectroscopy. In liquid cyclohexane four new Raman lines were readily detected in the frequency range 2870–2920 cm−1
Optimal Topological Test for Degeneracies of Real Hamiltonians
We consider adiabatic transport of eigenstates of real Hamiltonians around
loops in parameter space. It is demonstrated that loops that map to nontrivial
loops in the space of eigenbases must encircle degeneracies. Examples from
Jahn-Teller theory are presented to illustrate the test. We show furthermore
that the proposed test is optimal.Comment: Minor corrections, accepted in Phys. Rev. Let
On-chip spectroscopy with thermally-tuned high-Q photonic crystal cavities
Spectroscopic methods are a sensitive way to determine the chemical
composition of potentially hazardous materials. Here, we demonstrate that
thermally-tuned high-Q photonic crystal cavities can be used as a compact
high-resolution on-chip spectrometer. We have used such a chip-scale
spectrometer to measure the absorption spectra of both acetylene and hydrogen
cyanide in the 1550 nm spectral band, and show that we can discriminate between
the two chemical species even though the two materials have spectral features
in the same spectral region. Our results pave the way for the development of
chip-size chemical sensors that can detect toxic substances
CO oxidation at Pd(100): A first-principles constrained thermodynamics study
The possible formation of oxides or thin oxide films (surface oxides) on late
transition metal surfaces is recently being recognized as an essential
ingredient when aiming to understand catalytic oxidation reactions under
technologically relevant gas phase conditions. Using the CO oxidation at
Pd(100) as example, we investigate the composition and structure of this model
catalyst surface over a wide range of (T,p)-conditions within a multiscale
modeling approach where density-functional theory is linked to thermodynamics.
The results show that under the catalytically most relevant gas phase
conditions a thin surface oxide is the most stable "phase" and that the system
is actually very close to a transition between this oxidic state and a reduced
state in form of a CO covered Pd(100) surface.Comment: 13 pages including 7 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
A joint time-dependent density-functional theory for excited states of electronic systems in solution
We present a novel joint time-dependent density-functional theory for the
description of solute-solvent systems in time-dependent external potentials.
Starting with the exact quantum-mechanical action functional for both electrons
and nuclei, we systematically eliminate solvent degrees of freedom and thus
arrive at coarse-grained action functionals which retain the highly accurate
\emph{ab initio} description for the solute and are, in principle, exact. This
procedure allows us to examine approximations underlying popular embedding
theories for excited states. Finally, we introduce a novel approximate action
functional for the solute-water system and compute the solvato-chromic shift of
the lowest singlet excited state of formaldehyde in aqueous solution, which is
in good agreement with experimental findings.Comment: 11 page
Graph Convolutional Networks for Model-Based Learning in Nonlinear Inverse Problems
The majority of model-based learned image reconstruction methods in medical imaging have been limited to
uniform domains, such as pixelated images. If the underlying
model is solved on nonuniform meshes, arising from a finite
element method typical for nonlinear inverse problems, interpolation and embeddings are needed. To overcome this, we
present a flexible framework to extend model-based learning
directly to nonuniform meshes, by interpreting the mesh as a
graph and formulating our network architectures using graph
convolutional neural networks. This gives rise to the proposed
iterative Graph Convolutional Newton-type Method (GCNM),
which includes the forward model in the solution of the inverse
problem, while all updates are directly computed by the network
on the problem specific mesh. We present results for Electrical
Impedance Tomography, a severely ill-posed nonlinear inverse
problem that is frequently solved via optimization-based methods,
where the forward problem is solved by finite element methods.
Results for absolute EIT imaging are compared to standard
iterative methods as well as a graph residual network. We
show that the GCNM has strong generalizability to different
domain shapes and meshes, out of distribution data as well
as experimental data, from purely simulated training data and
without transfer training
Ab initio Molecular Dynamics in Adaptive Coordinates
We present a new formulation of ab initio molecular dynamics which exploits
the efficiency of plane waves in adaptive curvilinear coordinates, and thus
provides an accurate treatment of first-row elements. The method is used to
perform a molecular dynamics simulation of the CO_2 molecule, and allows to
reproduce detailed features of its vibrational spectrum such as the splitting
of the Raman sigma+_g mode caused by Fermi resonance. This new approach opens
the way to highly accurate ab initio simulations of organic compounds.Comment: 11 pages, 3 PostScript figure
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