1,035 research outputs found
The determination of the infrared radiative lifetimes of a vibrationally excited neutral molecule using stimulated-emission-pumping, molecular-beam time-of-flight.
The authors present a new experimental method for measurement of collision-free infrared radiative lifetimes for single quantum states of a vibrationally excited sample. This method provides a more direct route to the infrared Einstein A coefficients than has been previously possible. Results for NO(X (2) Pi upsilon=21 and upsilon=7) are presented. Comparison to results of ab initio calculations shows excellent agreement. A controversy regarding the relative intensities of first overtone and fundamental emission intensities in NO is laid to rest. The most complete least squares analysis of existing data was carried out to derive the electric dipole moment function (EDMF) to an accuracy of +/-0.02 D between 0.9 and 1.7 Angstrom
The boundary element approach to Van der Waals interactions
We develop a boundary element method to calculate Van der Waals interactions
for systems composed of domains of spatially constant dielectric response. We
achieve this by rewriting the interaction energy expression exclusively in
terms of surface integrals of surface operators. We validate this approach in
the Lifshitz case and give numerical results for the interaction of two spheres
as well as the van der Waals self-interaction of a uniaxial ellipsoid. Our
method is simple to implement and is particularly suitable for a full,
non-perturbative numerical evaluation of non-retarded van der Waals
interactions between objects of a completely general shape.Comment: 4 pages, 4 figures, RevTe
Enhanced dispersion interaction between quasi-one dimensional conducting collinear structures
Recent investigations have highlighted the failure of a sum of terms
to represent the dispersion interaction in parallel metallic, anisotropic,
linear or planar nanostructures [J. F. Dobson, A. White, and A. Rubio, Phys.
Rev. Lett. 96, 073201 (2006) and references therein]. By applying a simple
coupled plasmon approach and using electron hydrodynamics, we numerically
evaluate the dispersion (non-contact van der Waals) interaction between two
conducting wires in a collinear pointing configuration. This case is compared
to that of two insulating wires in an identical geometry, where the dispersion
interaction is modelled both within a pairwise summation framework, and by
adding a pinning potential to our theory leading to a standard oscillator-type
model of insulating dielectric behavior. Our results provide a further example
of enhanced dispersion interaction between two conducting nanosystems compared
to the case of two insulating ones. Unlike our previous work, this calculation
explores a region of relatively close coupling where, although the electronic
clouds do not overlap, we are still far from the asymptotic region where a
single power law describes the dispersion energy. We find that strong
differences in dispersion attraction between metallic and semiconducting /
insulating cases persist into this non-asymptotic region. While our theory will
need to be supplemented with additional short-ranged terms when the electronic
clouds overlap, it does not suffer from the short-distance divergence exhibited
by purely asymptotic theories, and gives a natural saturation of the dispersion
energy as the wires come into contact.Comment: 10 pages, 5 figures. Added new extended numerical calculations, new
figures, extra references and heavily revised tex
Erratum: Observational constraints on supermassive dark stars
No abstract is available for this article
Direct evidence of terahertz emission arising from anomalous Hall effect
A detailed understanding of the different mechanisms being responsible for
terahertz (THz) emission in ferromagnetic (FM) materials will aid in designing
efficient THz emitters. In this report, we present direct evidence of THz
emission from single layer CoFeB (CoFeB) FM thin films.
The dominant mechanism being responsible for the THz emission is the anomalous
Hall effect (AHE), which is an effect of a net backflow current in the FM layer
created by the spin-polarized current reflected at the interfaces of the FM
layer. The THz emission from the AHE-based CoFeB emitter is optimized by
varying its thickness, orientation, and pump fluence of the laser beam. Results
from electrical transport measurements show that skew scattering of charge
carriers is responsible for the THz emission in the CoFeB AHE-based THz
emitter.Comment: 5 pages, 4 figure
Comparison between the first Odin-SMR, Aura MLS and CloudSat retrievals of cloud ice mass in the upper tropical troposphere
International audienceEmerging microwave satellite techniques are expected to provide improved global measurements of cloud ice mass. CloudSat, Aura MLS and Odin-SMR fall into this category and first cloud ice retrievals from these instruments are compared. The comparison is made for partial ice water columns above 12 km, following the SMR retrieval product. None of the instruments shows significant false cloud detections and a consistent view of the geographical distribution of cloud ice is obtained, but differences on the absolute levels exist. CloudSat gives the lowest values, with an overall mean of 2.12 g/m2. A comparable mean for MLS is 4.30 g/m2. This relatively high mean can be an indication of overestimation of the vertical altitude of cloud ice by the MLS retrievals. The vertical response of SMR has also some uncertainty, but this does not affect the comparison between MLS and CloudSat. SMR observations are sensitive to cloud inhomogeneities inside the footprint and some compensation is required. Results in good agreement with CloudSat, both in regard of the mean and probability density functions, are obtained for a weak compensation, while a simple characterisation of the effect indicates the need for stronger compensation. The SMR mean was found to be 1.89/2.62/4.10 g/m2 for no/selected/strongest compensation, respectively. Assumptions about the particle size distribution are a consideration for all three instruments, and constitute the dominating retrieval uncertainty for CloudSat. The comparison indicates a retrieval accuracy of about 40% (3.1±1.2 g/m2). This number is already very small compared to uncertainties of cloud ice parametrisation in atmospheric models, but can be decreased further through a better understanding of main retrieval error sources
Comparison between early Odin-SMR, Aura MLS and CloudSat retrievals of cloud ice mass in the upper tropical troposphere
International audienceEmerging microwave satellite techniques are expected to provide improved global measurements of cloud ice mass. CloudSat, Aura MLS and Odin-SMR fall into this category and early cloud ice retrievals from these instruments are compared. The comparison follows the SMR retrieval product and is made for partial ice water columns above 12 km. None of the retrievals shows a significant degree of false cloud detections, the ratio between local mean values from the instruments is fairly constant and a consistent view of the geographical distribution of cloud ice is obtained. However, important differences on the absolute levels exist, where the overall mean is 9.6, 4.2 and 3.7 g m?2 for CloudSat, SMR and MLS, respectively. Assumptions about the particle size distribution (PSD) are a consideration for all three instruments and constitute the dominating retrieval uncertainty for CloudSat. The mean for CloudSat when applying the same PSD as for MLS and SMR was estimated to 6.3 g m?2. A second main consideration for MLS and SMR are the effects caused by the poorer spatial resolution: a possible vertical misplacement of retrieved values and an impact of cloud inhomogeneities. The latter effect was found to be the dominating retrieval uncertainty for SMR, giving a possible mean value range of 2.3?8.9 g m?2. The comparison indicates a common retrieval accuracy in the order of 70%. Already this number should suffice for improved validations of cloud ice parametrisation schemes in atmospheric models, but a substantially better consistency between the datasets should be attainable through an increased understanding of main retrieval error sources
Atomic Supersymmetry, Rydberg Wave Packets, and Radial Squeezed States
We study radial wave packets produced by short-pulsed laser fields acting on
Rydberg atoms, using analytical tools from supersymmetry-based quantum-defect
theory. We begin with a time-dependent perturbative calculation for
alkali-metal atoms, incorporating the atomic-excitation process. This provides
insight into the general wave packet behavior and demonstrates agreement with
conventional theory. We then obtain an alternative analytical description of a
radial wave packet as a member of a particular family of squeezed states, which
we call radial squeezed states. By construction, these have close to minimum
uncertainty in the radial coordinates during the first pass through the outer
apsidal point. The properties of radial squeezed states are investigated, and
they are shown to provide a description of certain aspects of Rydberg atoms
excited by short-pulsed laser fields. We derive expressions for the time
evolution and the autocorrelation of the radial squeezed states, and we study
numerically and analytically their behavior in several alkali-metal atoms. Full
and fractional revivals are observed. Comparisons show agreement with other
theoretical results and with experiment.Comment: published in Physical Review
Quantum mechanics/molecular mechanics modeling of drug metabolism:Mexiletine N-hydroxylation by cytochrome P450 1A2
The mechanism of cytochrome P450(CYP)-catalyzed
hydroxylation of
primary amines is currently unclear and is relevant to drug metabolism;
previous small model calculations have suggested two possible mechanisms:
direct N-oxidation and H-abstraction/rebound. We have modeled the
N-hydroxylation of (<i>R</i>)-mexiletine in CYP1A2 with
hybrid quantum mechanics/molecular mechanics (QM/MM) methods, providing
a more detailed and realistic model. Multiple reaction barriers have
been calculated at the QM(B3LYP-D)/MM(CHARMM27) level for the direct
N-oxidation and H-abstraction/rebound mechanisms. Our calculated barriers
indicate that the direct N-oxidation mechanism is preferred and proceeds
via the doublet spin state of Compound I. Molecular dynamics simulations
indicate that the presence of an ordered water molecule in the active
site assists in the binding of mexiletine in the active site, but
this is not a prerequisite for reaction via either mechanism. Several
active site residues play a role in the binding of mexiletine in the
active site, including Thr124 and Phe226. This work reveals key details
of the N-hydroxylation of mexiletine and further demonstrates that
mechanistic studies using QM/MM methods are useful for understanding
drug metabolism
Observational constraints on supermassive dark stars
Some of the first stars could be cooler and more massive than standard
stellar models would suggest, due to the effects of dark matter annihilation in
their cores. It has recently been argued that such objects may attain masses in
the 10^4--10^7 solar mass range, and that such supermassive dark stars should
be within reach of the upcoming James Webb Space Telescope. Notwithstanding
theoretical difficulties with this proposal, we argue here that some of these
objects should also be readily detectable with both the Hubble Space Telescope
and ground-based 8--10 m class telescopes. Existing survey data already place
strong constraints on 10^7 solar mass dark stars at z~10. We show that such
objects must be exceedingly rare or short-lived to have avoided detection.Comment: 6 pages, 4 figures. v3: erratum incorporate
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