K-line X-ray fluorescence from highly charged iron ions under dense astrophysical plasma conditions

In the present work, we report an investigation of plasma environment effects on the atomic parameters associated with the K-vacancy states in highly charged iron ions within the astrophysical context of accretion disks around black holes. More particularly, the sensitivity of K-line X-ray fluorescence parameters (wavelengths, radiative transition probabilities, and Auger rates) in Fe XVII–Fe XXV ions has been estimated for plasma conditions characterized by an electron temperature ranging from 10⁵ to 10⁷ K and an electron density ranging from 10¹⁸ to 10²² cm⁻³. In order to do this, relativistic multiconfiguration Dirac-Fock atomic structure calculations have been carried out by considering a time averaged Debye-Hückel potential for both the electron–nucleus and electron–electron interactions.     

Syntheses and characterization of different zinc(II) oxide nano-structures from direct thermal decomposition of 1D coordination polymers

Three zinc(II) nitrite coordination polymers, [Zn(4-bpdb)(NO₂)₂]_n (1), {[Zn(3-bpdb)(NO₂)]·0.5H₂O}_n (2) and [Zn(3-bpdh)(NO₂)₂]_n (3), 4-bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene, 3-bpdb = 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene and 3-bpdh = 2,5-bis(3-pyridyl)-3,4-diaza-2,4-hexadiene} were prepared and characterized by elemental analyses and IR spectroscopy. Compound 3 was structurally characterized by single-crystal X-ray diffraction and is one-dimensional polymer with coordination environments of distorted octahedral, ZnN₂O₄. The thermal stabilities of compounds 1–3 were studied by thermal gravimetric (TG) and differential thermal analyses (DTA). Direct calcination of the compounds 1–3 at 600 °C under air atmospheres yields different morphologies of nano-sized ZnO.     

Variability in the Formation and Framework Polymorphism of Metal-organic Frameworks based on Yttrium(III) and the Bifunctional Organic Linker 2,5-Dihydroxyterephthalic Acid

Depending on the solvothermal reaction conditions, we obtained three different metal-organic frameworks with yttrium(III) as metal component and 2,5-dihdyroxyterepthalic acid (H₄dhtp) as bifunctional organic linker: Y₂(H₂dhtp)₃(dmf)₄ · (dmf)₂ (CPO-29) contains dinuclear, paddle-wheel like inorganic secondary building units (SBUs) connected by the organic linker to a network with α-Po topology, while Y₂(H₂dhtp)(dhtp)(dmf)₂ (CPO-30) and Y₂(H₂dhtp)(dhtp)(dmf)₂(H₂O)₂ · (H₂O)₄ (CPO-31) contain one-dimensional inorganic SBUs that differ in how the half- and fully deprotonated ligands are connected to and arranged around them. Only the carboxylic acid groups of the organic linker are deprotonated in CPO-29, while CPO-30 and CPO-31 contain both 2,5-dihydroxyterephthalate (H₂dhtp^2–) linkers and fully deprotonated 2,5-dioxidoterephthalate (dhtp^4–) linkers. All three compounds contain large volumes filled with solvent, but we were able to demonstrate permanence of porosity only for CPO-30. Variable temperature powder X-ray diffraction reveals that CPO-29 and CPO-31 undergo discontinuous phase transitions upon heating, and the flexibility of the framework structure indicated by these might be the reason for the inability to access the pore volume. Desolvated CPO-30 and CPO-31 are polymorphs, whose network structures differ in whether the H₂dhtp^2– and dhtp^4– linkers are located in cis or trans arrangement around the inorganic SBU.     

An SIS waveguide heterodyne receiver for 600 HGz-635 GHz

A waveguide SIS heterodyne receiver using a Nb/AlO_X/Nb junction has been built for astronomical observations of molecular transitions in the frequency range 600 GHz - 635 GHz, and has been successfully used at the Caltech Submillimeter Observatory (CSO). We report double sideband (DSB) receiver noise temperatures as low as 245 K at 600 GHz -610 GHz, and near 300 K over the rest of the bandwidth. These results confirm that SIS quasiparticle mixers work well at submillimeter-wave frequencies corresponding to photon energies of at least 90% of the superconductor energy gap. In addition, we have systematically investigated the effect on the receiver performance of the overlap between first-order and second-order photon steps of opposite sign at these frequencies. The receiver noise increases by as much as 40% in the region of overlap. We infer potential limitations for operating submillimeter-wave Nb/AlO_x/Nb mixers.     

Effects of brain polarization on reaction times and pinch force in chronic stroke

Background  

Previous studies showed that anodal transcranial DC stimulation (tDCS) applied to the primary motor cortex of the affected hemisphere (M1_{affected hemisphere}) after subcortical stroke transiently improves performance of complex tasks that mimic activities of daily living (ADL). It is not known if relatively simpler motor tasks are similarly affected. Here we tested the effects of tDCS on pinch force (PF) and simple reaction time (RT) tasks in patients with chronic stroke in a double-blind cross-over Sham-controlled experimental design.     

Surface Modified Single Molecules Free-Diffusion Evidenced by Fluorescence Correlation Spectroscopy

We report on the free diffusion of single molecule near an interface studied using fluorescence correlation spectroscopy. In particular, we show that the chemical nature of the substrate may modify the free diffusion of a widely used molecule (rhodamine 6G), thus inducing unexpected effects in fluorescence correlation spectroscopy measurements. Our results show a strong influence, up to a few micrometer from the interface, of the surface polarity. This effect is assessed through the relative weight of the two dimensions diffusion process observed close to the surface. Our results are discussed in terms of competition between surface-solvent, solvent-molecule and molecule-surface specific interactions.     

Controlled pinewood fractionation with supercritical ethanol: A prerequisite toward pinewood conversion into chemicals and biofuels

The objective of this work was to investigate the ability of supercritical (SC) ethanol conditions to attack preferentially the lignin fraction against the carbohydrate fraction and their effects on the product distribution among gases, light products, bio-oils, and chars. In this study, the conversion of each pinewood component was determined by the analysis of solid residues to quantify cellulose, hemicellulose, lignin, and char contents. It is shown that, by tuning the temperature, hemicellulose and lignin are already transformed in subcritical ethanol conditions, lignin being more reactive than hemicellulose. In contrast, native wood cellulose is recalcitrant to liquefaction in SC ethanol near the critical point (T_c = 241 °C and P_c = 61 bar), but 20% of native wood cellulose is converted in SC ethanol at 280 °C. Besides, the severity of the conditions, in terms of temperature and treatment time, does not significantly influence the yields of gases, light products, and bio-oils but strongly enhances char formation. Interestingly, the increase in SC ethanol density does not change the conversion of biomass components but has a marked effect on bio-oil yield and prevents char formation. The optimum fractionation conditions to convert the lignin component, while keeping unattacked the cellulose fraction with a minimum formation of char, are dense SC ethanol, at 250 °C for 1 h, in batch conditions. However, although lignin is more reactive than hemicellulose under these conditions, these fractions are converted, in a parallel way, to around 50% and 60%, respectively.     

On-Surface Synthesis and Characterization of Acene-Based Nanoribbons Incorporating Four-Membered Rings

A bottom up method for the synthesis of unique tetracene-based nanoribbons, which incorporate cyclobutadiene moieties as linkers between the acene segments, is reported. These structures were achieved through the formal [2+2] cycloaddition reaction of ortho-functionalized tetracene precursor monomers. The formation mechanism and the electronic and magnetic properties of these nanoribbons were comprehensively studied by means of a multitechnique approach. Ultra-high vacuum scanning tunneling microscopy showed the occurrence of metal-coordinated nanostructures at room temperature and their evolution into nanoribbons through formal [2+2] cycloaddition at 475 K. Frequency-shift non-contact atomic force microscopy images clearly proved the presence of bridging cyclobutadiene moieties upon covalent coupling of activated tetracene molecules. Insight into the electronic and vibrational properties of the so-formed ribbons was obtained by scanning tunneling microscopy, Raman spectroscopy, and theoretical calculations. Magnetic properties were addressed from a computational point of view, allowing us to propose promising candidates to magnetic acene-based ribbons incorporating four-membered rings. The reported findings will increase the understanding and availability of new graphene-based nanoribbons with high potential in future spintronics.     

Excited-State Dynamics of Thienoguanosine,an Isomorphic Highly Fluorescent Analogue of Guanosine

Thienoguanosine (^thG) is an isomorphic analogue of guanosine with promising potentialities as fluorescent DNA label. As a free probe in protic solvents, ^thG exists in two tautomeric forms, identified as the H1, being the only one observed in nonprotic solvents, and H3 keto–amino tautomers. We herein investigate the photophysics of ^thG in solvents of different polarity, from water to dioxane, by combining time-resolved fluorescence with PCM/TD-DFT and CASSCF calculations. Fluorescence lifetimes of 14.5–20.5 and 7–13 ns were observed for the H1 and H3 tautomers, respectively, in the tested solvents. In methanol and ethanol, an additional fluorescent decay lifetime (≈3 ns) at the blue emission side (λ≈430 nm) as well as a 0.5 ns component with negative amplitude at the red edge of the spectrum, typical of an excited-state reaction, were observed. Our computational analysis explains the solvent effects observed on the tautomeric equilibrium. The main radiative and nonradiative deactivation routes have been mapped by PCM/TD-DFT calculations in solution and CASSCF in the gas phase. The most easily accessible conical intersection, involving an out-of plane motion of the sulfur atom in the five-membered ring of ^thG, is separated by a sizeable energy barrier (≥0.4 eV) from the minimum of the spectroscopic state, which explains the large experimental fluorescence quantum yield.     

Luminescent Graphene-Based Materials via Europium Complexation on Dipyridylpyridazine-Functionalized Graphene Sheets

Graphene-based materials exhibit outstanding physical properties and so are potentially applicable in a great variety of fields. Unlike their corresponding oxides, graphite and graphene are not prone to functionalization. Diels–Alder reactions are among the scarce reactions that they can occur without disrupting their conjugated sp² systems. Herein, the reaction between graphite and 3,6-di(2-pyridyl)-1,2,4,5-tetrazine under different conditions affords several graphene-based materials consisting of dipyridylpyridazine-functionalized few-layer graphene, multilayer graphene and graphite, the sheets of which act as ligands for the grafting of a europium complex. These three materials show strong red emission under 365 nm UV radiation. Their emitting particles can be visualized by confocal microscopy. The rich coordination chemistry of dipyridylpyridazine ligands has potential novel properties for similarly functionalized graphene-based materials grafted with other metal complexes.