623 research outputs found
Macroscopic effects in attosecond pulse generation
We examine how the generation and propagation of high-order harmonics in a
partly ionized gas medium affect their strength and synchronization. The
temporal properties of the resulting attosecond pulses generated in long gas
targets can be significantly influenced by macroscopic effects, in particular
by the intensity in the medium and the degree of ionization. Under some
conditions, the use of gas targets longer than the absorption length can lead
to the generation of self-compressed attosecond pulses. We show this effect
experimentally, using long argon-filled gas cells as generating medium.Comment: 5 pages 4 figure
Probing single-photon ionization on the attosecond time scale
We study photoionization of argon atoms excited by attosecond pulses using an
interferometric measurement technique. We measure the difference in time delays
between electrons emitted from the and from the shell, at
different excitation energies ranging from 32 to 42 eV. The determination of
single photoemission time delays requires to take into account the measurement
process, involving the interaction with a probing infrared field. This
contribution can be estimated using an universal formula and is found to
account for a substantial fraction of the measured delay.Comment: 4 pages, 4 figures, under consideratio
Intensity Dependence of Laser-Assisted Attosecond Photoionization Spectra
We study experimentally the influence of the intensity of the infrared (IR)
probe field on attosecond pulse train (APT) phase measurements performed with
the RABITT method (Reconstruction of Attosecond Beating by Interference in
Two-Photon Transitions). We find that if a strong IR field is applied, the
attosecond pulses will appear to have lower-than-actual chirp rates. We also
observe the onset of the streaking regime in the breakdown of the weak-field
RABITT conditions. We perform a Fourier-analysis of harmonic and sideband
continuum states and show that the mutual phase relation of the harmonics can
be extracted from higher Fourier components.Comment: preprint to article in Laser Physics Nikolai B. Delone memorial issue
11 pages, 9 figures, published online 15 July 200
A novel and practical screening tool for the detection of silent myocardial infarction in patients with type 2 diabetes
Silent myocardial infarction (MI) is a prevalent finding in patients with type 2 diabetes and is associated with significant mortality and morbidity. Late gadolinium enhancement (LGE) by cardiovascular magnetic resonance (CMR) is the most validated technique for detection of silent MI but is time consuming, costly and requires administration of intravenous contrast. We therefore planned to develop a simple and low cost population screening tool to identify those at highest risk of silent MI validated against the CMR reference standard.100 asymptomatic patients with type 2 diabetes underwent electrocardiogram (ECG), echocardiography, biomarker assessment and CMR at 3.0T including assessment of left ventricular ejection fraction and LGE. Global longitudinal strain (GLS) from 2 and 4 chamber cines was measured using feature tracking.17/100 patients with no history of cardiovascular disease had silent MI defined by LGE in an infarct pattern on CMR. Only 4 silent MI patients had Q waves on ECG. Patients with silent MI were older (65 vs 60, p=0.05), had lower E/A ratio (0.75 vs 0.89, p=0.004), lower GLS (-15.2% vs -17.7%, p=0.004) and higher NT-proBNP (106ng/L vs 52ng/L, p=0.003). A combined risk score derived from these 4 factors had an area under the receiver operating characteristic (ROC) curve of 0.823 (0.734-0.892), P<0.0001. A score of ?3/5 had 82% sensitivity and 72% specificity for silent MI.Using measures that can be derived in an outpatient clinic setting, we have developed a novel screening tool for the detection of silent MI in type 2 diabetes. The screening tool had significantly superior diagnostic accuracy than current ECG criteria for the detection of silent MI in asymptomatic patients
Coherent Electron Scattering Captured by an Attosecond Quantum Stroboscope
The basic properties of atoms, molecules and solids are governed by electron
dynamics which take place on extremely short time scales. To measure and
control these dynamics therefore requires ultrafast sources of radiation
combined with efficient detection techniques. The generation of extreme
ultraviolet (XUV) attosecond (1 as = 10-18 s) pulses has, for the first time,
made direct measurements of electron dynamics possible. Nevertheless, while
various applications of attosecond pulses have been demonstrated
experimentally, no one has yet captured or controlled the full three
dimensional motion of an electron on an attosecond time scale. Here we
demonstrate an attosecond quantum stroboscope capable of guiding and imaging
electron motion on a sub-femtosecond (1 fs = 10-15 s) time scale. It is based
on a sequence of identical attosecond pulses which are synchronized with a
guiding laser field. The pulse to pulse separation in the train is tailored to
exactly match an optical cycle of the laser field and the electron momentum
distributions are detected with a velocity map imaging spectrometer (VMIS).
This technique has enabled us to guide ionized electrons back to their parent
ion and image the scattering event. We envision that coherent electron
scattering from atoms, molecules and surfaces captured by the attosecond
quantum stroboscope will complement more traditional scattering techniques
since it provides high temporal as well as spatial resolution.Comment: 6 pages, 4 figure
Attosecond electron spectroscopy using a novel interferometric pump-probe technique
We present an interferometric pump-probe technique for the characterization
of attosecond electron wave packets (WPs) that uses a free WP as a reference to
measure a bound WP. We demonstrate our method by exciting helium atoms using an
attosecond pulse with a bandwidth centered near the ionization threshold, thus
creating both a bound and a free WP simultaneously. After a variable delay, the
bound WP is ionized by a few-cycle infrared laser precisely synchronized to the
original attosecond pulse. By measuring the delay-dependent photoelectron
spectrum we obtain an interferogram that contains both quantum beats as well as
multi-path interference. Analysis of the interferogram allows us to determine
the bound WP components with a spectral resolution much better than the inverse
of the attosecond pulse duration.Comment: 5 pages, 4 figure
Myocardial Extracellular Volume Estimation by CMR Predicts Functional Recovery Following Acute MI
Objectives: In the setting of reperfused acute myocardial infarction (AMI), the authors sought to compare prediction of contractile recovery by infarct extracellular volume (ECV), as measured by T1-mapping cardiac magnetic resonance (CMR), with late gadolinium enhancement (LGE) transmural extent. Background: The transmural extent of myocardial infarction as assessed by LGE CMR is a strong predictor of functional recovery, but accuracy of the technique may be reduced in AMI. ECV mapping by CMR can provide a continuous measure associated with the severity of tissue damage within infarcted myocardium. Methods: Thirty-nine patients underwent acute (day 2) and convalescent (3 months) CMR scans following AMI. Cine imaging, tissue tagging, T2-weighted imaging, modified Look-Locker inversion T1 mapping natively and 15 min post–gadolinium-contrast administration, and LGE imaging were performed. The ability of acute infarct ECV and acute transmural extent of LGE to predict convalescent wall motion, ejection fraction (EF), and strain were compared per-segment and per-patient. Results: Per-segment, acute ECV and LGE transmural extent were associated with convalescent wall motion score (p < 0.01; p < 0.01, respectively). ECV had higher accuracy than LGE extent to predict improved wall motion (area under receiver-operating characteristics curve 0.77 vs. 0.66; p = 0.02). Infarct ECV ≤0.5 had sensitivity 81% and specificity 65% for prediction of improvement in segmental function; LGE transmural extent ≤0.5 had sensitivity 61% and specificity 71%. Per-patient, ECV and LGE correlated with convalescent wall motion score (r = 0.45; p < 0.01; r = 0.41; p = 0.02, respectively) and convalescent EF (p < 0.01; p = 0.04). ECV and LGE extent were not significantly correlated (r = 0.34; p = 0.07). In multivariable linear regression analysis, acute infarct ECV was independently associated with convalescent infarct strain and EF (p = 0.03; p = 0.04), whereas LGE was not (p = 0.29; p = 0.24). Conclusions: Acute infarct ECV in reperfused AMI can complement LGE assessment as an additional predictor of regional and global LV functional recovery that is independent of transmural extent of infarction
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