Enhanced X-ray emission arising from laser-plasma confinement by a strong transverse magnetic field.

We analyze, using experiments and 3D MHD numerical simulations, the dynamic and radiative properties of a plasma ablated by a laser (1 ns, 10[Formula: see text]-10[Formula: see text] W/cm[Formula: see text]) from a solid target as it expands into a homogeneous, strong magnetic field (up to 30 T) that is transverse to its main expansion axis. We find that as early as 2 ns after the start of the expansion, the plasma becomes constrained by the magnetic field. As the magnetic field strength is increased, more plasma is confined close to the target and is heated by magnetic compression. We also observe that after [Formula: see text] ns, the plasma is being overall shaped in a slab, with the plasma being compressed perpendicularly to the magnetic field, and being extended along the magnetic field direction. This dense slab rapidly expands into vacuum; however, it contains only [Formula: see text] of the total plasma. As a result of the higher density and increased heating of the plasma confined against the laser-irradiated solid target, there is a net enhancement of the total X-ray emissivity induced by the magnetization.

Nucleophilic versus Electrophilic Activation of Hydrogen Peroxide over Zr-Based Metal-Organic Frameworks.

Zr-based metal-organic frameworks (Zr-MOF) UiO-66 and UiO-67 catalyze thioether oxidation in nonprotic solvents with unprecedentedly high selectivity toward corresponding sulfones (96-99% at ca. 50% sulfide conversion with only 1 equiv of H2O2). The reaction mechanism has been investigated using test substrates, kinetic, adsorption, isotopic (18O) labeling, and spectroscopic tools. The following facts point out a nucleophilic character of the peroxo species responsible for the superior formation of sulfones: (1) nucleophilic parameter XNu = 0.92 in the oxidation of thianthrene 5-oxide and its decrease upon addition of acid; (2) sulfone to sulfoxide ratio of 24 in the competitive oxidation of methyl phenyl sulfoxide and p-Br-methyl phenyl sulfide; (3) significantly lower initial rates of methyl phenyl sulfide oxidation relative to methyl phenyl sulfoxide (kS/kSO = 0.05); and (4) positive slope ρ = +0.42 of the Hammett plot for competitive oxidation of p-substituted aryl methyl sulfoxides. Nucleophilic activation of H2O2 on Zr-MOF is also manifested by their capability of catalyzing epoxidation of electron-deficient C═C bonds in α,ß-unsaturated ketones accompanied by oxidation of acetonitrile solvent. Kinetic modeling on methyl phenyl sulfoxide oxidation coupled with adsorption studies supports a mechanism that involves the interaction of H2O2 with Zr sites with the formation of a nucleophilic oxidizing species and release of water followed by oxygen atom transfer from the nucleophilic oxidant to sulfoxide that competes with water for Zr sites. The nucleophilic peroxo species coexists with an electrophilic one, ZrOOH, capable of oxygen atom transfer to nucleophilic sulfides. The predominance of nucleophilic activation of H2O2 over electrophilic one is, most likely, ensured by the presence of weak basic sites in Zr-MOFs identified by FTIR spectroscopy of adsorbed CDCl3 and quantified by adsorption of isobutyric acid.

Laboratory unraveling of matter accretion in young stars.

Accretion dynamics in the formation of young stars is still a matter of debate because of limitations in observations and modeling. Through scaled laboratory experiments of collimated plasma accretion onto a solid in the presence of a magnetic field, we open a first window on this phenomenon by tracking, with spatial and temporal resolution, the dynamics of the system and simultaneously measuring multiband emissions. We observe in these experiments that matter, upon impact, is ejected laterally from the solid surface and then refocused by the magnetic field toward the incoming stream. This ejected matter forms a plasma shell that envelops the shocked core, reducing escaped x-ray emission. This finding demonstrates one possible structure reconciling current discrepancies between mass accretion rates derived from x-ray and optical observations, respectively.

Ion pinhole imaging diagnostics on fast ion source in femtosecond laser plasma of cluster targets.

The spatial configuration of the ion source generated under femtosecond laser interaction with clusters is investigated. While intense laser pulses (36 fs, 60 mJ, intensity of 4 × 1017 W/cm2) propagated in CO2 cluster (~0.22 µm in diameter) media, the shape of the obtained plasma ion source was registered for the first time by means of pinhole imaging method. The remarkable decrease in fast ion yield in the vicinity of the assumed best laser focus near the gas cluster jet axis is observed. Such observed anisotropy of the ion source is suggested to originate from the influence of the laser prepulse destroying clusters in advance to the arrival of the main pulse. The assumption is confirmed by optical shadowgraphy images of the plasma channel and is important for further development of an efficient laser-plasma-based fast ion source. Following the observed geometry of the ion source, the laser intensity limit allowing to accelerate ions to ~100 keV energy range was estimated.

Alkene Epoxidation Catalyzed by Ti-Containing Polyoxometalates: Unprecedented ß-Oxygen Transfer Mechanism.

A DFT study revealed that the mechanism of alkene epoxidation with hydrogen peroxide catalyzed by Ti-containing polyoxometalates (POMs) depends on the Ti coordination environment: For rigid and hindered Ti centers, the unprecedented ß-oxygen transfer from the titanium hydroperoxo species becomes favored over the α-oxygen one. Improving the model for catalyst description, the calculations were able to reproduce the Arrhenius activation energy values determined in kinetic studies. Unlike protonation, the possible ion-pairing between POMs and countercations has a minor effect on the electrophlicity of the catalyst and, consequently, on the activity of epoxidation.

Revealing the second harmonic generation in a femtosecond laser-driven cluster-based plasma by analyzing shapes of Ar XVII spectral lines.

We present experiments dealing with a femtosecond laser-driven cluster-based plasma, where by analyzing the nonlinear phenomenon of satellites of spectral lines of Ar XVII, we revealed the nonlinear phenomenon of the generation of the second harmonic of the laser frequency. For performing this analysis we developed new results in the theory of satellites of spectral lines. From such lineshape analysis we found, in particular, that the efficiency of converting the short (40 fs) intense (3x10¹8 W/cm²) incident laser light into the second harmonic was 2%. This result is in the excellent agreement with the 2-Dimensional Particle-In-Cell (2D PIC) simulation that we also performed. There is also an order of magnitude agreement between the thresholds for the SHG found from the line shape analysis and from the 2D PIC simulations.

Mechanism of thioether oxidation over di- and tetrameric ti centres: kinetic and DFT studies based on model Ti-containing polyoxometalates.

The oxidation of thioethers by the green oxidant aqueous H2 O2 catalysed by the tetratitanium-substituted Polyoxometalate (POM) (Bu4 N)8 [{γ-SiTi2 W10 O36 (OH)2 }2 (µ-O)2 ], as a model catalyst comprising tetrameric titanium centres, was investigated by kinetic modelling and DFT calculations. Several mechanisms of sulfoxidation were evaluated by using methyl phenyl sulfide (PhSMe) as a model substrate in the experiments and dimethyl sulfide in the calculations. The first mechanism assumes that the active hydroperoxo species forms directly through interaction of the Ti2 (µ-OH)2 group in [{γ-SiTi2 W10 O36 (OH)2 }2 (µ-O)2 ](8-) (1 D) with H2 O2 . The second mechanism includes hydrolysis of Ti-O-Ti bonds linking two γ-Keggin units in structure 1 D to produce the monomer [(γ-SiW10 Ti2 O38 H2 )(OH)2 ](4-) (1 M), followed by the formation of an active hydroperoxo species upon interaction of the Ti hydroxo group with H2 O2 . Both kinetic modelling and DFT calculations support the mechanism through the monomeric species that involves the hydrolysis step. According to the DFT studies the activation of H2 O2 by compound 1 M is preferred by 6.5 kcal mol(-1) with respect to anion 1 D due to the more flexible Ti environment of the terminal Ti hydroxo group in 1 M. The calculations also indicate that for the "monomeric" mechanism two pathways are operative: the mono- and the multinuclear pathway. In the mononuclear mechanism, the active group is the terminal TiOH group, whereas in the multinuclear path the active group is the bridging Ti2 (µ-OH) moiety. Moreover, unlike previous studies, the sulfoxidation is preferred through a ß-oxygen atom transfer from the Ti hydroperoxo group because the α-oxygen atom transfer leads to an unfavourable seven-fold coordinated Ti environment in the transition state. Finally, we have generalised these results to other Ti-containing POMs: the Ti-monosubstituted α-Keggin ion [α-PTi(OH)W11 O39 ](4-) and the dititanium-substituted sandwich-type ion [Ti2 (OH)2 As2 W19 O67 ](8-) .

Efficient epoxidation of olefins by H2O2 catalyzed by iron "helmet" phthalocyanines.

High yields of epoxides were obtained in the oxidation of a large range of olefins using 1.2-2 equiv. of H2O2 in the presence of iron helmet phthalocyanines. The involvement of high-valent iron oxo species was evidenced using cryospray mass spectrometry.

Mechanistic insights into oxidation of 2-methyl-1-naphthol with dioxygen: autoxidation or a spin-forbidden reaction?

Oxidation of 2-methyl-1-naphthol (MNL) with molecular oxygen proceeds efficiently under mild reaction conditions (3 atm O(2), 60-80 °C) in the absence of any catalyst or sensitizer and produces 2-methyl-1,4-naphthoquinone (MNQ, menadione, or vitamin K(3)) with selectivity up to 80% in nonpolar solvents. (1)H NMR and (1)H,(1)H-COSY studies revealed the formation of 2-methyl-4-hydroperoxynaphthalene-1(4H)-one (HP) during the reaction course. Several mechanistic hypotheses, including conventional radical autoxidation, electron transfer mechanisms, photooxygenation, and thermal intersystem crossing (ISC), have been evaluated using spectroscopic, mass-spectrometric, spin-trapping, (18)O(2) labeling, kinetic, and computational techniques. Several facts collectively implicate that ISC contributes significantly into MNL oxidation with O(2) at elevated pressure: (i) the reaction rate is unaffected by light; (ii) C-C-coupling dimers are practically absent; (iii) the reaction is first order in both MNL and O(2); (iv) the observed activation parameters (ΔH(‡) = 8.1 kcal mol(-1) and ΔS(‡) = -50 eu) are similar to those found for the spin-forbidden oxidation of helianthrene with (3)O(2) (Seip, M.; Brauer, H.-D. J. Am. Chem. Soc.1992, 114, 4486); and (v) the external heavy atom effect (2-fold increase of the reaction rate in iodobenzene) points to spin inversion in the rate-limiting step.

Enhanced Kα output of Ar and Kr using size optimized cluster target irradiated by high-contrast laser pulses.

We observed that increasing the clusters size and laser pulse contrast can enhance the X-ray flux emitted by femtosecond-laser-driven-cluster plasma. By focusing a high contrast laser (10(-10)) on large argon clusters, high flux Kα-like X-rays (around 2.96 keV) is generated with a total flux of 2.5 × 10(11) photons/J in 4π and a conversion efficiency of 1.2 × 10-4. In the case of large Kr clusters, the best total flux for L-shell X-rays is 5.3 × 1011 photons/J with a conversion efficiency of 1.3 × 10-4 and, for the Kα X-ray (12.7 keV), it is 8 × 10(8) photons/J with a conversion efficiency of 1.6 × 10-6. Using this X-ray source, a single-shot high-performance X-ray imaging is demonstrated.

Propagation-based phase-contrast enhancement of nanostructure images using a debris-free femtosecond-laser-driven cluster-based plasma soft x-ray source and an LiF crystal detector.

We demonstrate in-line phase-contrast imaging of nanothickness foils by using a relatively large, polychromatic, debris-free femtosecond-laser-driven cluster-based plasma soft x-ray source, and a high-resolution, large dynamic range LiF crystal detector. The spatial coherence length of radiation in our setup reached a value of 5 microm on the sample plane, which is enough to observe phase-contrast enhancement in the images registered by the detector placed only a few hundred micrometers behind the object. We have developed a tabletop soft x-ray emission source, which emits radiation within a 4pi sr solid angle, and which allows one to obtain contact and propagation-based phase-contrast imaging of nanostructures with 700 nm spatial resolutions. This advance could be of utility for metrology applications.