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l-Phenylalanine functionalized silver nanoparticles: Photocatalytic and nonlinear optical applications
No full textRestricted Access.An extensive study on the behavior of l-Phenylalanine capped silver nanoparticles (Phe-Ag NPs) in the aqueous phase and in a sol–gel thin film showed different UV/Vis, Transmission Electron Microscope (TEM), Dynamic Light Scattering and Zeta potential profiles. Scanning Electron Microscope (SEM) images of the samples in the sol gel film showed Ag embedded in the SiO2 matrix. Surface Enhanced Raman Spectra (SERS) confirmed that both in the aqueous media and in the sol gel film, the attachment of Phe to the Ag NP surface was through the benzene ring, with the sol–gel film showing a better enhancement. Photocatalytic degradation of crystal violet was measured spectrophotometrically using Phe-Ag NPs as a nanocatalyst under visible light illumination. Intensity-dependent nonlinear optical absorption of Phe-Ag measured using the open aperture Z-scan technique revealed that the material is an efficient optical limiter with potential applications
Quantum defect theory description of weakly bound levels and Feshbach resonances in LiRb
No full textOpen AccessThe multichannel quantum defect theory (MQDT) in combination with the frame transformation (FT) approach is applied to model the Fano–Feshbach resonances measured for 7Li87Rb and 6Li87Rb Marzok et al (2009 Phys. Rev. A 79 012717). The MQDT results show a level of accuracy comparable to that of previous models based on direct, fully numerical solutions of the the coupled channel Schrödinger equations. Here, energy levels deduced from 2-photon photoassociation (PA) spectra for 7Li85Rb are assigned by applying the MQDT approach, obtaining the bound state energies for the coupled channel problem. Our results confirm that MQDT yields a compact description of PA observables as well as the Fano–Feshbach resonance positions and widths
Govind Swarup: Radio astronomer, innovator par excellence and a wonderfully inspiring leader
No full textOpen Acces
An analysis of the halo and relic radio emission from Abell 3376 from Murchison widefield array observations.
No full textOpen AccessWe have carried out multiwavelength observations of the nearby (z = 0.046) rich, merging galaxy cluster Abell 3376 with the Murchison Widefield Array (MWA). As a part of the GaLactic and Extragalactic All-sky MWA Survey, this cluster was observed at 88, 118, 154, 188, and 215 MHz. The known radio relics, towards the eastern and western peripheries of the cluster, were detected at all the frequencies. The relics, with a linear extent of ∼1 Mpc each, are separated by ∼2 Mpc. Combining the current observations with those in the literature, we have obtained the spectra of these relics over the frequency range 80–1400 MHz. The spectra follow power laws, with α = −1.17 ± 0.06 and −1.37 ± 0.08 for the west and east relics, respectively (S∝να). Assuming the break frequency to be near the lower end of the spectrum we estimate the age of the relics to be ∼0.4 Gyr. No diffuse radio emission from the central regions of the cluster (halo) was detected. The upper limit on the radio power of any possible halo that might be present in the cluster is a factor of 35 lower than that expected from the radio power and X-ray luminosity correlation for cluster haloes. From this we conclude that the cluster halo is very extended (>500 kpc) and/or most of the radio emission from the halo has decayed. The current limit on the halo radio power is a factor of 10 lower than the existing upper limits with possible implications for models of halo formation
Empirical covariance modeling for 21 cm power spectrum estimation: A method demonstration and new limits from early Murchison widefiel array 128-tile data
No full textOpen AccessThe separation of the faint cosmological background signal from bright astrophysical foregrounds remains one of the most daunting challenges of mapping the high-redshift intergalactic medium with the redshifted 21 cm line of neutral hydrogen. Advances in mapping and modeling of diffuse and point source foregrounds have improved subtraction accuracy, but no subtraction scheme is perfect. Precisely quantifying the errors and error correlations due to missubtracted foregrounds allows for both the rigorous analysis of the 21 cm power spectrum and for the maximal isolation of the “EoR window” from foreground contamination. We present a method to infer the covariance of foreground residuals from the data itself in contrast to previous attempts at a priori modeling. We demonstrate our method by setting limits on the power spectrum using a 3 h integration from the 128-tile Murchison Widefield Array. Observing between 167 and 198 MHz, we find at 95% confidence a best limit of Δ2(k)<3.7×104 mK2 at comoving scale k=0.18 h Mpc−1 and at z=6.8, consistent with existing limits
GLEAM: The galactic and extragalactic all-sky MWA survey
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)GLEAM, the GaLactic and Extragalactic All-sky MWA survey, is a survey of the entire radio sky south of declination + 25° at frequencies between 72 and 231 MHz, made with the MWA using a drift scan method that makes efficient use of the MWA's very large field-of-view. We present the observation details, imaging strategies, and theoretical sensitivity for GLEAM. The survey ran for two years, the first year using 40-kHz frequency resolution and 0.5-s time resolution; the second year using 10-kHz frequency resolution and 2 s time resolution. The resulting image resolution and sensitivity depends on observing frequency, sky pointing, and image weighting scheme. At 154 MHz, the image resolution is approximately 2.5 × 2.2/cos (δ + 26.7°) arcmin with sensitivity to structures up to ~ 10° in angular size. We provide tables to calculate the expected thermal noise for GLEAM mosaics depending on pointing and frequency and discuss limitations to achieving theoretical noise in Stokes I images. We discuss challenges, and their solutions, that arise for GLEAM including ionospheric effects on source positions and linearly polarised emission, and the instrumental polarisation effects inherent to the MWA's primary beam
Reduced ionic contaminations in CdSe quantum dot dispersed ferroelectric liquid crystal and its applications.
No full textRestricted Access.Octadecylamine capped cadmium selenide quantum dots (CdSe QDs) were dispersed in the ferroelectric liquid crystal (FLC) FELIX 16/100. The QD dispersed FLC system was investigated on the planar anchored cell. Addition of specific concentration of the QDs in the pure FLC induces a new relaxation mode along with the Goldstone relaxation mode. QDs assisted quantum fluctuations are probably responsible for the existence of this new relaxation mode in the QDs dispersed FLC system. The ionic contaminations associated with the FLC materials were trapped on the surface of QDs due to the ion-trapping character of QDs. The trapping of ionic contaminations was confirmed by the a.c. conductivity measurement. The physical properties of the pure and dispersed FLC were carried out as a function of doping concentration of QDs, temperature and frequency
