511 research outputs found
High-order harmonic generation from Rydberg states at fixed Keldysh parameter
Because the commonly adopted viewpoint that the Keldysh parameter
determines the dynamical regime in strong field physics has long been
demonstrated to be misleading, one can ask what happens as relevant physical
parameters, such as laser intensity and frequency, are varied while is
kept fixed. We present results from our one- and fully three-dimensional
quantum simulations of high-order harmonic generation (HHG) from various bound
states of hydrogen with up to 40, where the laser intensities and the
frequencies are scaled from those for in order to maintain a fixed
Keldysh parameter for all . We find that as we increase
while keeping fixed, the position of the cut-off scales in well
defined manner. Moreover, a secondary plateau forms with a new cut-off,
splitting the HHG plateau into two regions. First of these sub-plateaus is
composed of lower harmonics, and has a higher yield than the second one. The
latter extends up to the semiclassical cut-off. We find that this
structure is universal, and the HHG spectra look the same for all
when plotted as a function of the scaled harmonic order. We investigate the
-, - and momentum distributions to elucidate the physical mechanism
leading to this universal structure
Phase-dependent interference fringes in the wavelength scaling of harmonic efficiency
We describe phase-dependent wavelength scaling of high-order harmonic
generation efficiency driven by ultra-short laser fields in the mid-infrared.
We employ both numerical solution of the time-dependent Schr\"{o}dinger
equation and the Strong Field Approximation to analyze the fine-scale
oscillations in the harmonic yield in the context of channel-closing effects.
We show, by varying the carrier-envelope phase, that the amplitude of these
oscillations depend strongly on the number of returning electron trajectories.
Furthermore, the peak positions of the oscillations vary significantly as a
function of the carrier-envelope phase. Owing to its practical applications, we
also study the wavelength dependence of harmonic yield in the "single-cycle"
limit, and observe a smooth variation in the wavelength scaling originating
from the vanishing fine-scale oscillations.Comment: 5 pages, 4 figure
Dielectronic Recombination of Fe XV forming Fe XIV: Laboratory Measurements and Theoretical Calculations
We have measured resonance strengths and energies for dielectronic
recombination (DR) of Mg-like Fe XV forming Al-like Fe XIV via N=3 -> N' = 3
core excitations in the electron-ion collision energy range 0-45 eV. All
measurements were carried out using the heavy-ion Test Storage Ring at the Max
Planck Institute for Nuclear Physics in Heidelberg, Germany. We have also
carried out new multiconfiguration Breit-Pauli (MCBP) calculations using the
AUTOSTRUCTURE code. For electron-ion collision energies < 25 eV we find poor
agreement between our experimental and theoretical resonance energies and
strengths. From 25 to 42 eV we find good agreement between the two for
resonance energies. But in this energy range the theoretical resonance
strengths are ~ 31% larger than the experimental results. This is larger than
our estimated total experimental uncertainty in this energy range of +/- 26%
(at a 90% confidence level). Above 42 eV the difference in the shape between
the calculated and measured 3s3p(^1P_1)nl DR series limit we attribute partly
to the nl dependence of the detection probabilities of high Rydberg states in
the experiment. We have used our measurements, supplemented by our
AUTOSTRUCTURE calculations, to produce a Maxwellian-averaged 3 -> 3 DR rate
coefficient for Fe XV forming Fe XIV. The resulting rate coefficient is
estimated to be accurate to better than +/- 29% (at a 90% confidence level) for
k_BT_e > 1 eV. At temperatures of k_BT_e ~ 2.5-15 eV, where Fe XV is predicted
to form in photoionized plasmas, significant discrepancies are found between
our experimentally-derived rate coefficient and previously published
theoretical results. Our new MCBP plasma rate coefficient is 19-28% smaller
than our experimental results over this temperature range
Emergence of structure in mouse embryos: Structural Entropy morphometry applied to digital models of embryonic anatomy
We apply an information-theoretic measure to anatomical models of the Edinburgh Mouse Atlas Project. Our goal is to quantify the anatomical complexity of the embryo and to understand how this quantity changes as the organism develops through time. Our measure, Structural Entropy, takes into account the geometrical character of the intermingling of tissue types in the embryo. It does this by a mathematical process that effectively imagines a point-like explorer that starts at an arbitrary place in the 3D structure of the embryo and takes a random path through the embryo, recording the sequence of tissues through which it passes. Consideration of a large number of such paths yields a probability distribution of paths making connections between specific tissue types, and Structural Entropy is calculated from this (mathematical details are given in the main text). We find that Structural Entropy generally decreases (order increases) almost linearly throughout developmental time (4–18 days). There is one `blip’ of increased Structural Entropy across days 7–8: this corresponds to gastrulation. Our results highlight the potential for mathematical techniques to provide insight into the development of anatomical structure, and also the need for further sources of accurate 3D anatomical data to support analyses of this kind
Morphometric and morphological evaluation of mastoid emissary canal using cone-beam computed tomography
Objectives: This study aimed to determine mastoid emissary canal’s (MEC) and mastoid foramen (MF) prevalence and morphometric characteristics on cone-beam computed tomography (CBCT) images to underline its clinical significance and discuss its surgical consequences. Methods: In the retrospective analysis, two oral and maxillofacial radiologists analyzed the CBCT images of 135 patients (270 sides). The biggest MF and MEC were measured in the images evaluated in MultiPlanar Reconstruction (MPR) views. The MF and MEC mean diameters were calculated. The mastoid foramina number was recorded. The prevalence of MF was studied according to gender and side of the patient. Results: The overall prevalence of MEC and MF was 119 (88.1%). The prevalence of MEC and MF is 55.5% in females and 44.5% in males. MEC and MF were identified as bilateral in 80 patients (67.20%) and unilateral in 39 patients (32.80%). The mean diameter of MF was 2.4 ± 0.9 mm. The mean height of MF was 2.3 ± 0.9. The mean diameter of the MEC was 2.1 ± 0.8, and the mean height of the MEC was 2.1 ± 0.8. There is a statistical difference between the genders (p = 0.043) in foramen diameter. Males had a significantly larger mean diameter of MF in comparison to females. Conclusion: MEC and MF must be evaluated thoroughly if the surgery is contemplated. Radiologists and surgeons should be aware of mastoid emissary canal morphology, variations, clinical relevance, and surgical consequences while operating in the suboccipital and mastoid areas to avoid unexpected and catastrophic complications. CBCT may be a reliable imaging diagnostic technique
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Dielectronic Recombination In Active Galactic Nuclei
XMM-Newton and Chandra observations of active galactic nuclei (AGN) show rich spectra of X-ray absorption lines. These observations have detected a broad unresolved transition array (UTA) between ˜ 15-17 Å. This is attributed to inner-shell photoexcitation of M-shell iron ions. Modeling these UTA features is currently limited by uncertainties in the low-temperature dielectronic recombination (DR) data for M-shell iron. In order to resolve this issue, and to provide reliable iron M-shell DR data for plasma modeling, we are carrying out a series of laboratory measurements using the heavy-ion Test Storage Ring (TSR) at the Max-Plank-Institute for Nuclear Physics in Heidelberg, Germany. Currently, laboratory measurements of low temperature DR can only be performed at storage rings. We use the DR data obtained at TSR, to calculate rate coefficients for plasma modeling and to benchmark theoretical DR calculations. Here we report our recent experimental results for DR of Fe XIV forming Fe XIII
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