Probing ultrafast C-Br bond fission in the UV photochemistry of bromoform with core-to-valence transient absorption spectroscopy.

UV pump-extreme UV (XUV) probe femtosecond transient absorption spectroscopy is used to study the 268 nm induced photodissociation dynamics of bromoform (CHBr3). Core-to-valence transitions at the Br(3d) absorption edge (∼70 eV) provide an atomic scale perspective of the reaction, sensitive to changes in the local valence electronic structure, with ultrafast time resolution. The XUV spectra track how the singly occupied molecular orbitals of transient electronic states develop throughout the C-Br bond fission, eventually forming radical Br and CHBr2 products. Complementary ab initio calculations of XUV spectral fingerprints are performed for transient atomic arrangements obtained from sampling excited-state molecular dynamics simulations. C-Br fission along an approximately CS symmetrical reaction pathway leads to a continuous change of electronic orbital characters and atomic arrangements. Two timescales dominate changes in the transient absorption spectra, reflecting the different characteristic motions of the light C and H atoms and the heavy Br atoms. Within the first 40 fs, distortion from C3v symmetry to form a quasiplanar CHBr2 by the displacement of the (light) CH moiety causes significant changes to the valence electronic structure. Displacement of the (heavy) Br atoms is delayed and requires up to ∼300 fs to form separate Br + CHBr2 products. We demonstrate that transitions between the valence-excited (initial) and valence + core-excited (final) state electronic configurations produced by XUV absorption are sensitive to the localization of valence orbitals during bond fission. The change in valence electron-core hole interaction provides a physical explanation for spectral shifts during the process of bond cleavage

Ultrafast valley-resolved carrier dynamics in group IV semiconductors

Attosecond transient absorption spectroscopy at the M4,5-edge of Ge following ultrafast photoexcitation reveals valley-resolved hot electron and hole relaxation, carrier recombination and trapping in Ge and Si-Ge alloy in unprecedented clarity and simultaneously

Vacancy-Mediated Magnetism in Pure Copper Oxide Nanoparticles

Room temperature ferromagnetism (RTF) is observed in pure copper oxide (CuO) nanoparticles which were prepared by precipitation method with the post-annealing in air without any ferromagnetic dopant. X-ray photoelectron spectroscopy (XPS) result indicates that the mixture valence states of Cu1+ and Cu2+ ions exist at the surface of the particles. Vacuum annealing enhances the ferromagnetism (FM) of CuO nanoparticles, while oxygen atmosphere annealing reduces it. The origin of FM is suggested to the oxygen vacancies at the surface/or interface of the particles. Such a ferromagnet without the presence of any transition metal could be a very good option for a class of spintronics

Potential therapeutic applications of microbial surface-activecompounds

Numerous investigations of microbial surface-active compounds or biosurfactants over the past two decades have led to the discovery of many interesting physicochemical and biological properties including antimicrobial, anti-biofilm and therapeutic among many other pharmaceutical and medical applications. Microbial control and inhibition strategies involving the use of antibiotics are becoming continually challenged due to the emergence of resistant strains mostly embedded within biofilm formations that are difficult to eradicate. Different aspects of antimicrobial and anti-biofilm control are becoming issues of increasing importance in clinical, hygiene, therapeutic and other applications. Biosurfactants research has resulted in increasing interest into their ability to inhibit microbial activity and disperse microbial biofilms in addition to being mostly nontoxic and stable at extremes conditions. Some biosurfactants are now in use in clinical, food and environmental fields, whilst others remain under investigation and development. The dispersal properties of biosurfactants have been shown to rival that of conventional inhibitory agents against bacterial, fungal and yeast biofilms as well as viral membrane structures. This presents them as potential candidates for future uses in new generations of antimicrobial agents or as adjuvants to other antibiotics and use as preservatives for microbial suppression and eradication strategies

Characterization of Polysulfide Radicals Present in an Ether-Based Electrolyte of a Lithium-Sulfur Battery during Initial Discharge Using in Situ X-Ray Absorption Spectroscopy Experiments and First-Principles Calculations

The presence and role of polysulfide radicals in the electrochemical processes of lithium sulfur (Li-S) batteries is currently being debated. Here, first-principles interpretations of measured X-ray absorption spectra (XAS) of Li-S cells are leveraged with an ether-based electrolyte. Unambiguous evidence is found for significant quantities of polysulfide radical species (LiS3, LiS4, and LiS5), including the trisulfur radical anion S3-, present after initial discharge to the first discharge plateau, as evidenced by a low energy shoulder in the S K-edge XAS below 2469 eV. This feature is not present in the XAS of cells at increased depth of discharge, which, by our analysis, exhibit increasing concentrations of progressively shorter polysulfide dianions. Through a combination of first-principles molecular dynamics and associated interpretation of in situ XAS of Li-S cells, atomic level insights into the chemistries are provided that underlie the operation and stability of these batteries. Calculated and measured sulfur K-edge X-ray absorption spectra (XAS) and schematic of a lithium-sulfur (Li-S) cell with ether-based electrolyte are reported. The XAS is obtained through the lithium anode probing the electrolyte beyond it. The signature of polysulfide radicals is found as a low-energy shoulder below 2469 eV after discharging to 2.25 V. This feature disappears at greater depths of discharge

Characterization of Polysulfide Radicals Present in an Ether-Based Electrolyte of a Lithium-Sulfur Battery during Initial Discharge Using in Situ X-Ray Absorption Spectroscopy Experiments and First-Principles Calculations

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. The presence and role of polysulfide radicals in the electrochemical processes of lithium sulfur (Li-S) batteries is currently being debated. Here, first-principles interpretations of measured X-ray absorption spectra (XAS) of Li-S cells are leveraged with an ether-based electrolyte. Unambiguous evidence is found for significant quantities of polysulfide radical species (LiS3, LiS4, and LiS5), including the trisulfur radical anion S3 -, present after initial discharge to the first discharge plateau, as evidenced by a low energy shoulder in the S K-edge XAS below 2469 eV. This feature is not present in the XAS of cells at increased depth of discharge, which, by our analysis, exhibit increasing concentrations of progressively shorter polysulfide dianions. Through a combination of first-principles molecular dynamics and associated interpretation of in situ XAS of Li-S cells, atomic level insights into the chemistries are provided that underlie the operation and stability of these batteries. Calculated and measured sulfur K-edge X-ray absorption spectra (XAS) and schematic of a lithium-sulfur (Li-S) cell with ether-based electrolyte are reported. The XAS is obtained through the lithium anode probing the electrolyte beyond it. The signature of polysulfide radicals is found as a low-energy shoulder below 2469 eV after discharging to 2.25 V. This feature disappears at greater depths of discharge

Dissociation Dynamics and Electronic Structures of Highly Excited Ferrocenium Ions Studied by Femtosecond XUV Absorption Spectroscopy.

The dissociation dynamics of ferrocene are explored following strong field ionization using femtosecond time-resolved extreme ultraviolet (XUV) transient absorption spectroscopy. Employing transitions in the vicinity of the iron 3p (M2,3) edge, the dissociation is monitored from the point of view of the iron atom. With low strong field pump intensities (≈2 × 1013 W cm-2), only ferrocenium cations are produced, and their iron 3p absorption spectrum is reported. It very closely resembles the 3p spectrum of atomic Fe+ ions but is red-shifted by 0.8 eV. With the aid of time-dependent density functional theory calculations, the spectrum is assigned to a combination of doublet and quartet spin states of ferrocenium ions. Ionization with more intense strong field pump pulses (≥6 × 1013 W cm-2) leads predominantly to the prompt production of ferrocenium ions that dissociate to give the spectral signature of bare Fe+ ions within 240 ± 80 fs. Within the temporal resolution of the experiment (≈40 fs), no spectral intermediates are observed, suggesting that the dissociation process occurs directly from the excited ferrocenium ion and that the bonds between the iron center and both cyclopentadienyl rings are broken almost simultaneously in an asynchronous concerted decay process. No evidence of slower dissociation channels is observed at a pump-probe delay of 250 ps, suggesting that all energy is very rapidly routed into dissociative states

Ultrafast transient absorption at the Germanium M4,5-edge to measure electron and hole dynamics

Extreme ultraviolet (XUV) transient absorption at the germanium M4,5-edge simultaneously measures electron and hole dynamics over 1.5 ps with few-femtosecond resolution. In the analysis, time-dependent density functional theory (TD-DFT) will be compared with experimental data

Ultrafast transient absorption at the Germanium M4,5-edge to measure electron and hole dynamics

Extreme ultraviolet (XUV) transient absorption at the germanium M -edge simultaneously measures electron and hole dynamics over 1.5 ps with few-femtosecond resolution. In the analysis, time-dependent density functional theory (TD-DFT) will be compared with experimental data. 4,

Femtosecond x-ray spectroscopy of an electrocyclic ring-opening reaction.

The ultrafast light-activated electrocyclic ring-opening reaction of 1,3-cyclohexadiene is a fundamental prototype of photochemical pericyclic reactions. Generally, these reactions are thought to proceed through an intermediate excited-state minimum (the so-called pericyclic minimum), which leads to isomerization via nonadiabatic relaxation to the ground state of the photoproduct. Here, we used femtosecond (fs) soft x-ray spectroscopy near the carbon K-edge (~284 electron volts) on a tabletop apparatus to directly reveal the valence electronic structure of this transient intermediate state. The core-to-valence spectroscopic signature of the pericyclic minimum observed in the experiment was characterized, in combination with time-dependent density functional theory calculations, to reveal overlap and mixing of the frontier valence orbital energy levels. We show that this transient valence electronic structure arises within 60 ± 20 fs after ultraviolet photoexcitation and decays with a time constant of 110 ± 60 fs