328 research outputs found
Ten-dimensional wave packet simulations of methane scattering
We present results of wavepacket simulations of scattering of an oriented
methane molecule from a flat surface including all nine internal vibrations. At
a translational energy up to 96 kJ/mol we find that the scattering is almost
completely elastic. Vibrational excitations when the molecule hits the surface
and the corresponding deformation depend on generic features of the potential
energy surface. In particular, our simulation indicate that for methane to
dissociate the interaction of the molecule with the surface should lead to an
elongated equilibrium C--H bond length close to the surface.Comment: RevTeX 15 pages, 3 eps figures: This article may be found at
http://link.aip.org/link/?jcp/109/1966
Energy dissipation and scattering angle distribution analysis of the classical trajectory calculations of methane scattering from a Ni(111) surface
We present classical trajectory calculations of the rotational vibrational
scattering of a non-rigid methane molecule from a Ni(111) surface. Energy
dissipation and scattering angles have been studied as a function of the
translational kinetic energy, the incidence angle, the (rotational) nozzle
temperature, and the surface temperature. Scattering angles are somewhat
towards the surface for the incidence angles of 30, 45, and 60 degree at a
translational energy of 96 kJ/mol. Energy loss is primarily from the normal
component of the translational energy. It is transfered for somewhat more than
half to the surface and the rest is transfered mostly to rotational motion. The
spread in the change of translational energy has a basis in the spread of the
transfer to rotational energy, and can be enhanced by raising of the surface
temperature through the transfer process to the surface motion.Comment: 8 pages REVTeX, 5 figures (eps
Bond breaking in vibrationally excited methane on transition metal catalysts
The role of vibrational excitation of a single mode in the scattering of
methane is studied by wave packet simulations of oriented CH4 and CD4 molecules
from a flat surface. All nine internal vibrations are included. In the
translational energy range from 32 up to 128 kJ/mol we find that initial
vibrational excitations enhance the transfer of translational energy towards
vibrational energy and increase the accessibility of the entrance channel for
dissociation. Our simulations predict that initial vibrational excitations of
the asymmetrical stretch (nu_3) and especially the symmetrical stretch (nu_1)
modes will give the highest enhancement of the dissociation probability of
methane.Comment: 4 pages REVTeX, 2 figures (eps), to be published in Phys. Rev. B.
(See also arXiv:physics.chem-ph/0003031). Journal version at
http://publish.aps.org/abstract/PRB/v61/p1565
Generalized iterated wreath products of cyclic groups and rooted trees correspondence
Consider the generalized iterated wreath product where . We
prove that the irreducible representations for this class of groups are indexed
by a certain type of rooted trees. This provides a Bratteli diagram for the
generalized iterated wreath product, a simple recursion formula for the number
of irreducible representations, and a strategy to calculate the dimension of
each irreducible representation. We calculate explicitly fast Fourier
transforms (FFT) for this class of groups, giving literature's fastest FFT
upper bound estimate.Comment: 15 pages, to appear in Advances in the Mathematical Science
Untangling free carrier and exciton dynamics in layered hybrid perovskites using ultrafast optical and terahertz spectroscopy
\ua9 2024 The Author(s). Published by IOP Publishing Ltd.Layered hybrid perovskites (LPKs) are promising as alternatives or additives to 3D metal halide perovskites for optoelectronic applications including photovoltaic cells, LEDs and lasers due to their increased stability. However, high exciton binding energies in these materials mean that excitons are the majority species under the operating conditions of many devices. Although the efficiency of devices that incorporate LPKs has been increasing, much is still unknown about the interplay of excitons and free charge-carriers in these materials, which is vital information for understanding how optoelectronic properties dictate device efficiency. In this work, we employ optical pump/THz probe spectroscopy (OPTP) and visible transient absorption spectroscopy (TAS) to analyse the optoelectronic properties and charge-carrier dynamics of phenylethylammonium lead iodide (PEA)2PbI4. By combining these techniques, we are able to disentangle the contributions from excitons and free charge-carriers. We observe fast cooling of free charge-carriers and exciton formation on a timescale of ∼400 fs followed by slower bimolecular recombination of residual free charge-carriers with a rate constant k 2 ∼ 109 cm3s−1. Excitons recombine via two monomolecular processes with lifetimes t 1 ∼ 11 ps and t2 ∼ 83 ps. Furthermore, we detect signatures of exciton-phonon coupling in the transient absorption kinetic traces. These findings provide new insight into the interplay between free charge-carriers and excitons as well as a possible mechanism to further understand the charge-carrier dynamics in LPKs
Perovskite-perovskite tandem photovoltaics with optimized bandgaps
We demonstrate four and two-terminal perovskite-perovskite tandem solar cells
with ideally matched bandgaps. We develop an infrared absorbing 1.2eV bandgap
perovskite, , that can deliver 14.8 %
efficiency. By combining this material with a wider bandgap
material, we reach monolithic two
terminal tandem efficiencies of 17.0 % with over 1.65 volts open-circuit
voltage. We also make mechanically stacked four terminal tandem cells and
obtain 20.3 % efficiency. Crucially, we find that our infrared absorbing
perovskite cells exhibit excellent thermal and atmospheric stability,
unprecedented for Sn based perovskites. This device architecture and materials
set will enable 'all perovskite' thin film solar cells to reach the highest
efficiencies in the long term at the lowest costs
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Homeostatic dysregulation proceeds in parallel in multiple physiological systems
An increasing number of aging researchers believes that multi-system physiological dysregulation may be a key biological mechanism of aging, but evidence of this has been sparse. Here, we used biomarker data on nearly 33,000 individuals from four large datasets to test for the presence of multi-system dysregulation. We grouped 37 biomarkers into six a priori groupings representing physiological systems (lipids, immune, oxygen transport, liver function, vitamins, and electrolytes), then calculated dysregulation scores for each system in each individual using statistical distance. Correlations among dysregulation levels across systems were generally weak but significant. Comparison of these results to dysregulation in arbitrary ‘systems’ generated by random grouping of biomarkers showed that a priori knowledge effectively distinguished the true systems in which dysregulation proceeds most independently. In other words, correlations among dysregulation levels were higher using arbitrary systems, indicating that only a priori systems identified distinct dysregulation processes. Additionally, dysregulation of most systems increased with age and significantly predicted multiple health outcomes including mortality, frailty, diabetes, heart disease, and number of chronic diseases. The six systems differed in how well their dysregulation scores predicted health outcomes and age. These findings present the first unequivocal demonstration of integrated multi-system physiological dysregulation during aging, demonstrating that physiological dysregulation proceeds neither as a single global process nor as a completely independent process in different systems, but rather as a set of system-specific processes likely linked through weak feedback effects. These processes – probably many more than the six measured here – are implicated in aging
Untangling free carrier and exciton dynamics in layered hybrid perovskites using ultrafast optical and terahertz spectroscopy
Layered hybrid perovskites (LPKs) are promising as alternatives or additives to 3D metal halide perovskites for optoelectronic applications including photovoltaic cells, LEDs and lasers due to their increased stability. However, high exciton binding energies in these materials mean that excitons are the majority species under the operating conditions of many devices. Although the efficiency of devices that incorporate LPKs has been increasing, much is still unknown about the interplay of excitons and free charge-carriers in these materials, which is vital information for understanding how optoelectronic properties dictate device efficiency. In this work, we employ optical pump/THz probe spectroscopy (OPTP) and visible transient absorption spectroscopy (TAS) to analyse the optoelectronic properties and charge-carrier dynamics of phenylethylammonium lead iodide (PEA)2PbI4. By combining these techniques, we are able to disentangle the contributions from excitons and free charge-carriers. We observe fast cooling of free charge-carriers and exciton formation on a timescale of ∼400 fs followed by slower bimolecular recombination of residual free charge-carriers with a rate constant k 2 ∼ 109 cm3s−1. Excitons recombine via two monomolecular processes with lifetimes t 1 ∼ 11 ps and t2 ∼ 83 ps. Furthermore, we detect signatures of exciton–phonon coupling in the transient absorption kinetic traces. These findings provide new insight into the interplay between free charge-carriers and excitons as well as a possible mechanism to further understand the charge-carrier dynamics in LPKs
A robotic wheelchair trainer: design overview and a feasibility study
<p>Abstract</p> <p>Background</p> <p>Experiencing independent mobility is important for children with a severe movement disability, but learning to drive a powered wheelchair can be labor intensive, requiring hand-over-hand assistance from a skilled therapist.</p> <p>Methods</p> <p>To improve accessibility to training, we developed a robotic wheelchair trainer that steers itself along a course marked by a line on the floor using computer vision, haptically guiding the driver's hand in appropriate steering motions using a force feedback joystick, as the driver tries to catch a mobile robot in a game of "robot tag". This paper provides a detailed design description of the computer vision and control system. In addition, we present data from a pilot study in which we used the chair to teach children without motor impairment aged 4-9 (n = 22) to drive the wheelchair in a single training session, in order to verify that the wheelchair could enable learning by the non-impaired motor system, and to establish normative values of learning rates.</p> <p>Results and Discussion</p> <p>Training with haptic guidance from the robotic wheelchair trainer improved the steering ability of children without motor impairment significantly more than training without guidance. We also report the results of a case study with one 8-year-old child with a severe motor impairment due to cerebral palsy, who replicated the single-session training protocol that the non-disabled children participated in. This child also improved steering ability after training with guidance from the joystick by an amount even greater than the children without motor impairment.</p> <p>Conclusions</p> <p>The system not only provided a safe, fun context for automating driver's training, but also enhanced motor learning by the non-impaired motor system, presumably by demonstrating through intuitive movement and force of the joystick itself exemplary control to follow the course. The case study indicates that a child with a motor system impaired by CP can also gain a short-term benefit from driver's training with haptic guidance.</p
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