4,249 research outputs found
On the photoionization of the outer electrons in noble gas endohedral atoms
We demonstrate the prominent modification of the outer shell photoionization
cross-section in noble gas (NG) endohedral atoms NG@F under the action of the
fullerene F electron shell. This shell leads to two important effects, namely
to strong enhancement of the cross-section due to fullerenes shell polarization
under the action of the incoming electromagnetic wave and to prominent
oscillation of this cross-section due to the reflection of the photoelectron
from NG by the F shell.
All but He noble gas atoms are considered. The polarization of the fullerene
shell is expressed via the total photoabsorption cross-section of F. The
reflection of the photoelectron is taken into account in the frame of the
so-called bubble potential that is a spherical zero --thickness potential.
It is assumed in the derivations that NG is centrally located in the
fullerene. It is assumed also, in accord with the existing experimental data,
that the fullerenes radius R is much bigger than the atomic radius and the
thickness of the fullerenes shell . These assumptions permit, as it was
demonstrated recently, to present the NG@F photoionization cross-section as a
product of the NG cross-section and two well defined calculated factors.Comment: 19 pages, 9 figure
Bond Energies of Nitrogen and Phosphorous Hydrides and Fluorides
Recent measurements of bond energies in the N-Hn and P-Hn systems by photoionization mass spectrometry are compared with modern ab initio calculations and a semi-empirical theory. Good agreement is noted, providing confirmation for the level of accuracy of the ab initio calculations, and for the essential correctness of the semi-empirical parametrization. However, the N-Fn and P-Fn systems, also measured, are currently beyond the capabilities of such high quality ab initio calculations, and the trend s observed in the bond energica indicate that other parametrizations are necessary in the semi-empirical approach
Advanced stratified charge rotary aircraft engine design study
A technology base of new developments which offered potential benefits to a general aviation engine was compiled and ranked. Using design approaches selected from the ranked list, conceptual design studies were performed of an advanced and a highly advanced engine sized to provide 186/250 shaft Kw/HP under cruise conditions at 7620/25,000 m/ft altitude. These are turbocharged, direct-injected stratified charge engines intended for commercial introduction in the early 1990's. The engine descriptive data includes tables, curves, and drawings depicting configuration, performance, weights and sizes, heat rejection, ignition and fuel injection system descriptions, maintenance requirements, and scaling data for varying power. An engine-airframe integration study of the resulting engines in advanced airframes was performed on a comparative basis with current production type engines. The results show airplane performance, costs, noise & installation factors. The rotary-engined airplanes display substantial improvements over the baseline, including 30 to 35% lower fuel usage
Value at Risk models with long memory features and their economic performance
We study alternative dynamics for Value at Risk (VaR) that incorporate a slow moving component and information on recent aggregate returns in established quantile (auto) regression models. These models are compared on their economic performance, and also on metrics of first-order importance such as violation ratios. By better economic performance, we mean that changes in the VaR forecasts should have a lower variance to reduce transaction costs and should lead to lower exceedance sizes without raising the average level of the VaR. We find that, in combination with a targeted estimation strategy, our proposed models lead to improved performance in both statistical and economic terms
Linear-scaling time-dependent density-functional theory beyond the Tamm-Dancoff approximation: Obtaining efficiency and accuracy with in situ optimised local orbitals.
We present a solution of the full time-dependent density-functional theory (TDDFT) eigenvalue equation in the linear response formalism exhibiting a linear-scaling computational complexity with system size, without relying on the simplifying Tamm-Dancoff approximation (TDA). The implementation relies on representing the occupied and unoccupied subspaces with two different sets of in situ optimised localised functions, yielding a very compact and efficient representation of the transition density matrix of the excitation with the accuracy associated with a systematic basis set. The TDDFT eigenvalue equation is solved using a preconditioned conjugate gradient algorithm that is very memory-efficient. The algorithm is validated on a small test molecule and a good agreement with results obtained from standard quantum chemistry packages is found, with the preconditioner yielding a significant improvement in convergence rates. The method developed in this work is then used to reproduce experimental results of the absorption spectrum of bacteriochlorophyll in an organic solvent, where it is demonstrated that the TDA fails to reproduce the main features of the low energy spectrum, while the full TDDFT equation yields results in good qualitative agreement with experimental data. Furthermore, the need for explicitly including parts of the solvent into the TDDFT calculations is highlighted, making the treatment of large system sizes necessary that are well within reach of the capabilities of the algorithm introduced here. Finally, the linear-scaling properties of the algorithm are demonstrated by computing the lowest excitation energy of bacteriochlorophyll in solution. The largest systems considered in this work are of the same order of magnitude as a variety of widely studied pigment-protein complexes, opening up the possibility of studying their properties without having to resort to any semiclassical approximations to parts of the protein environment.T.J.Z. acknowledges the support of EPSRC under Grant No. EP/J017639/1 and the ARCHER eCSE programme. M.C.P. and P.D.H. acknowledge the support of EPSRC under Grant No. EP/J015059/1. The underlying data of this publication can be accessed via the following persistent URI: https://www.repository.cam.ac.uk/handle/1810/251293This is the author accepted manuscript. The final version is available from AIP via http://dx.doi.org/10.1063/1.493628
Self-similar magnetoresistance of Fibonacci ultrathin magnetic films
We study numerically the magnetic properties (magnetization and
magnetoresistance) of ultra-thin magnetic films (Fe/Cr) grown following the
Fibonacci sequence. We use a phenomenological model which includes Zeeman,
cubic anisotropy, bilinear and biquadratic exchange energies. Our physical
parameters are based on experimental data recently reported, which contain
biquadratic exchange coupling with magnitude comparable to the bilinear
exchange coupling. When biquadratic exchange coupling is sufficiently large a
striking self-similar pattern emerges.Comment: 5 pages, 5 EPS figures, REVTeX, accepted for publication in Phys.
Rev.
Investigation of lunar surface chemical contamination by LEM descent engine and associated equipment Final report
Lunar surface contamination from LEM rocket exhaust - methods of minimizing sample contaminatio
Quantum Relaxation of Magnetisation in Magnetic Particles
At temperatures below the magnetic anisotropy energy, monodomain magnetic
systems (small particles, nanomagnetic devices, etc.) must relax quantum
mechanically. This quantum relaxation must be mediated by the coupling to both
nuclear spins and phonons (and electrons if either particle or substrate is
conducting. We analyze the effect of each of these couplings, and then combine
them. Conducting systems can be modelled by a "giant Kondo" Hamiltonian, with
nuclear spins added in as well. At low temperatures, even microscopic particles
on a conducting substrate (containing only spins) will have their
magnetisation frozen over millenia by a combination of electronic dissipation
and the "degeneracy blocking" caused by nuclear spins. Raising the temperature
leads to a sudden unblocking of the spin dynamics at a well defined
temperature. Insulating systems are quite different. The relaxation is strongly
enhanced by the coupling to nuclear spins. At short times the magnetisation of
an ensemble of particles relaxes logarithmically in time, after an initial very
fast decay; this relaxation proceeds entirely via the nuclear spins. At longer
times phonons take over, but the decay rate is still governed by the
temperature-dependent nuclear bias field acting on the particles - decay may be
exponential or power-law depending on the temperature. The most surprising
feature of the results is the pivotal role played by the nuclear spins. The
results are relevant to any experiments on magnetic particles in which
interparticle dipolar interactions are unimportant. They are also relevant to
future magnetic device technology.Comment: 30 pages, RevTex, e:mail , Submitted to J.Low
Temp.Phys. on 1 Nov. 199
Giant resonances of endohedral atoms
We demonstrate for the first time that the effect of fullerene shell upon
photoionization of the "caged" atom in an endohedral can result in formation of
Giant Endohedral Resonances or GER. This is illustrated by the concrete case of
Xe@C60 photoionization cross-section that exhibits at 17 eV a powerful
resonance with total oscillator strengths of about 25. The prominent
modification of the 5p6 electron photoionization cross-section of Xe@C60 takes
place due to strong fullerene shell polarization under the action of the
incoming electromagnetic wave and oscillation of this cross-section due to the
reflection of the photoelectron from Xe by the C60. These two factors transform
the smoothly decreasing 5p6 cross-section of Xe into a rather complex curve
with a powerful maximum for Xe@C60, with the oscillator strength being equal to
25! We present also the results for the dipole angular anisotropy parameter
that is strongly affected by the reflection of the photoelectron waves but not
modified by C60 polarization. The polarization of C60 is expressed via its
total photoabsorption cross-section. The reflection of the photoelectron is
taken into account in the frame of the so-called bubble potential that is a
spherical delta-type potential.Comment: 7pages, 3 figures, 14 references,some previous references are remove
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