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Bandyopadhyay B.,University of DelhiDelhi | Schleicher D.R.G.,University of Concepcion
Physical Review D - Particles, Fields, Gravitation and Cosmology

The reionization of helium describes the transition from its singly ionized state to a doubly ionized state in the intergalactic medium. This process is important for the thermal evolution of the intergalactic medium and influences the mean free path of photons with energies above 54.4 eV. While it is well known that helium reionization is mostly driven by the contribution of energetic quasars at z<6, we study here how helium reionization proceeds if there is an additional contribution due to the annihilation of dark matter. We explore the effects of different dark matter profiles for the dark matter clumping factor, which can significantly enhance the annihilation rate at late times. We find that the presence of dark matter annihilation enhances the He++ abundance at early stages where it would be zero within the standard model, and it can further increase during structure formation, reflecting the increase of the dark matter clumping factor. The latter is, however, degenerate with the buildup of the quasar contribution, and we therefore expect no significant changes at late times. We expect that future studies of the He+ Lyman α forest may help to assess whether the evolution is consistent with the contribution from quasars alone, or if an additional component may be required. © 2015 American Physical Society. Source

Vijayarangamuthu K.,University of DelhiDelhi | Rath S.,University of DelhiDelhi
International Journal of Applied Ceramic Technology

Sensing of low concentrations of two nitroaromatic compounds, 1,2-dinitrotoluene and 2-nitrophenol, is presented. The sensing mechanism is based on surface-enhanced Raman scattering (SERS) using nanostructured tin oxide as the SERS-active substrate. The SnOx nanostructures are synthesized by a simple solgel method and doped with Ag and Au. The Raman signal of a low concentration of the analyte, otherwise extremely weak, becomes significant when the analytes are attached to these substrates. Doping of SnOx nanopowders with Ag and Au leads to a further increase in the Raman intensities. This study demonstrates the scope of ceramic-metal nanocomposites as convenient solid-state SERS sensors for low-level detection. © 2014 The American Ceramic Society. Source

Yadav A.,University of DelhiDelhi | Mathur P.,University of DelhiDelhi
Inorganica Chimica Acta

Abstract Copper(II) complexes of a N-octylated bisbenzimidazolyl ligand are synthesized and characterized. These complexes carry out the oxidative dealkylation of 2,4,6-tri-tertbutylphenol (TTBP) using molecular oxygen to 2,6-ditertbutylbenzoquinone and 4,6-di-tertbutylbenzoquinone. The oxidation proceeds via a phenoxyl radical species detected spectrophotometrically, and by EPR. A reactive copper(II)-dioxygen species is involved that carries out the oxidation reaction. A comparison of the rates of formation of the 4,6-di-tert-butylbenzoquinone versus 2,6-di-tert-butylbenzoquinone suggests that alternative pathway may exist for the formation of the para-quinone derivative. Isolation of an intermediate 4,4′-peroxybis(2,4,6-tri-tert-butylcyclohexa-2,5-dienone species and its structural characterization supports the above contention. © 2015 Elsevier B.V. Source

Lumb S.,University of Delhi | Munjal D.,University of DelhiDelhi | Prasad V.,University of DelhiDelhi
Physica Scripta

Exponential cosine screened Coulomb potential (ECSCP) has been widely used in various branches of physics e.g., solid-state physics, nuclear physics and plasma physics. The atomic photoionization processes under plasma shielding can serve as an efficient tool for study of plasma properties in various environments ranging from nano-scale devices to astrophysical objects. In the present study, ECSCP has been used to characterize a dense quantum plasma and its effect on the spectrum of an atom encaged in a spherical box has been investigated. The work has further been extended to study the response of such a system to a periodic laser field. Photoexcitation and ionization probabilities of the system have been studied as a function of applied laser field parameters using the non-perturbative Floquet technique. As the Floquet method requires exact energy values and oscillator strengths, the spectrum of confined system has been calculated using Bernstein-polynomial method. The variation of energy spectrum and oscillator strengths with screening as well as confinement parameters has also been explored. © 2015 The Royal Swedish Academy of Sciences. Source

Kotla S.K.R.,University of DelhiDelhi | Choudhary D.,University of DelhiDelhi | Tiwari R.K.,Chapman University | Verma A.K.,University of DelhiDelhi | Verma A.K.,Jawaharlal Nehru UniversityDelhi
Tetrahedron Letters

Abstract The Rh(III)-catalyzed oxidative coupling of 2-aryl-imidazopyridines with internal alkynes via double C-H activation has been described. This approach provides straightforward access to highly functionalized polycyclic imidazopyridines in good to excellent yields. © 2015 Elsevier Ltd. Source

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