19,340 research outputs found
Submicron metal powders produced by ball milling with grinding aids
In ball milling metal powders to submicron size, various salts are more effective as grinding aids than conventional surfactants. Absolute ethyl alcohol is used as the grinding liquid
Theory of Auger decay by laser-dressed atoms
We devise an ab initio formalism for the quantum dynamics of Auger decay by
laser-dressed atoms which are inner-shell ionized by extreme ultraviolet (XUV)
light. The optical dressing laser is assumed to be sufficiently weak such that
ground-state electrons are neither excited nor ionized by it. However, the
laser has a strong effect on continuum electrons which we describe in
strong-field approximation with Volkov waves. The XUV light pulse has a low
peak intensity and its interaction is treated as a one-photon process. The
quantum dynamics of the inner-shell hole creation with subsequent Auger decay
is given by equations of motion (EOMs). For this paper, the EOMs are simplified
in terms of an essential-states model which is solved analytically and averaged
over magnetic subshells. We apply our theory to the M_4,5 N_1 N_2,3 Auger decay
of a 3d hole in a krypton atom. The orbitals are approximated by scaled
hydrogenic wave functions. A single attosecond pulse produces 3d vacancies
which Auger decay in the presence of an 800nm laser with an intensity of 10^13
W / cm^2. We compute the Auger electron spectrum and assess the convergence of
the various quantities involved.Comment: 19 pages, 7 figures, 1 table, RevTeX
Ramsey method for Auger-electron interference induced by an attosecond twin pulse
We examine the archetype of an interference experiment for Auger electrons:
two electron wave packets are launched by inner-shell ionizing a krypton atom
using two attosecond light pulses with a variable time delay. This setting is
an attosecond realization of the Ramsey method of separated oscillatory fields.
Interference of the two ejected Auger-electron wave packets is predicted,
indicating that the coherence between the two pulses is passed to the Auger
electrons. For the detection of the interference pattern an accurate
coincidence measurement of photo- and Auger electrons is necessary. The method
allows one to control inner-shell electron dynamics on an attosecond timescale
and represents a sensitive indicator for decoherence.Comment: 5 pages, 5 figures, RevTeX4.1, revise
User's guide to the Mission Analysis Evaluation and Space Trajectory Operations program (MAESTRO)
The MAESTRO system is a mission analysis tool designed to present to the user information necessary to make the various decisions required in the design and execution of a spaceflight mission. The system was designed so that it can be used in both the pre-launch mission planning phase of a mission and during the flight as an in-flight decision making tool. A description of each of the following modes is presented: (1) trajectory propagation mode; (2) retro-fire determination mode; (3) midcourse analysis determination mode; (4) Monte Carlo mode; (5) verification mode; (6) orbit stability mode; and (7) post injection trim mode. A description of the inputs necessary to run the program mode is given along with a sample case
Strength and High-Temperature Stability of Dispersion Strengthened Nickel-MgO Alloys
Strength and high-temperature stability of dispersion strengthened nickel-magnesium oxide alloy
Treatment of failed articular cartilage reconstructive procedures of the knee: A systematic review
Background: Symptomatic articular cartilage lesions of the knee are common and are being treated surgically with increasing frequency. While many studies have reported outcomes following a variety of cartilage restoration procedures, few have investigated outcomes of revision surgery after a failed attempt at cartilage repair or reconstruction. Purpose: To investigate outcomes of revision cartilage restoration procedures for symptomatic articular cartilage lesions of the knee following a previously failed cartilage reconstructive procedure. Study Design: Systematic review; Level of evidence, 4. Methods: A literature search was performed by use of the PubMed, EMBASE, and MEDLINE/Ovid databases for relevant articles published between 1975 and 2017 that evaluated patients undergoing revision cartilage restoration procedure(s) and reported outcomes using validated outcome measures. For studies meeting inclusion criteria, relevant information was extracted. Results: Ten studies met the inclusion criteria. Lesions most commonly occurred in the medial femoral condyle (MFC) (52.8%), with marrow stimulation techniques (MST) the index procedure most frequently performed (70.7%). Three studies demonstrated inferior outcomes of autologous chondrocyte implantation (ACI) following a previous failed cartilage procedure compared with primary ACI. One study comparing osteochondral allograft (OCA) transplant following failed microfracture (MFX) with primary OCA transplant demonstrated similar clinical outcomes and graft survival at midterm follow-up. No studies reported outcomes following osteochondral autograft transfer (OAT) or newer techniques. Conclusion: This systematic review of the literature reporting outcomes following revision articular cartilage restoration procedures (most commonly involving the MFC) demonstrated a high proportion of patients who underwent prior MST. Evidence is sufficient to suggest that caution should be taken in performing ACI in the setting of prior MST, likely secondary to subchondral bone compromise. OCA appears to be a good revision treatment option even if the subchondral bone has been violated from prior surgery or fracture. </jats:sec
F-theory GUTs with U(1) Symmetries: Generalities and Survey
We study the structure of SU(5) F-theory GUT models that engineer additional
U(1) symmetries. These are highly constrained by a set of relations observed by
Dudas and Palti (DP) that originate from the physics of 4D anomaly
cancellation. Using the DP relations, we find a general tension between
unification and the suppression of dimension 5 proton decay when one or more
U(1)'s are PQ symmetries and hypercharge flux is used to break the SU(5) GUT
group. We then specialize to spectral cover models, whose global completions in
F- theory we know how to construct. In that setting, we provide a technical
derivation of the DP relations, construct spectral covers that yield all
possible solutions to them, and provide a complete survey of spectral cover
models for SU(5) GUTs that exhibit two U(1) symmetries.Comment: 27 pages plus 5 appendices (70 pages total) ; v2 references adde
The Environmental Kuznets Curve Under a New framework: Role of Social Capital in Water Pollution
We advance a case for an inclusion of social capital in the environmental Kuznets curve analysis using highly disaggregated data on water pollution in Louisiana. A social capital index and other variables are used in parametric and spatial panel regression models to explain water pollution dynamics.Environmental Economics and Policy,
Quantum interference in attosecond transient absorption of laser-dressed helium atoms
We calculate the transient absorption of an isolated attosecond pulse by
helium atoms subject to a delayed infrared (\ir) laser pulse. With the central
frequency of the broad attosecond spectrum near the ionization threshold, the
absorption spectrum is strongly modulated at the sub-\ir-cycle level. Given
that the absorption spectrum results from a time-integrated measurement, we
investigate the extent to which the delay-dependence of the absorption yields
information about the attosecond dynamics of the atom-field energy exchange. We
find two configurations in which this is possible. The first involves multi
photon transitions between bound states that result in interference between
different excitation pathways. The other involves the modification of the bound
state absorption lines by the IR field, which we find can result in a sub-cycle
time dependence only when ionization limits the duration of the strong field
interaction
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