MASS SPECTRA OF SULFUR COMPOUNDS. ASSESSMENT OF 1, X (X=2,3,4,5) HYDROGEN TRANSFER IN DIALKYL SULFIDES AND DISULFIDES UNDER ELECTRON IMPACT. A NOVEL 1,5-H SHIFT

Abstract

The transfer of hydrogen atoms from the gamma carbon of dialkyl disulfides upon electron impact is presented and its occurrence is formally shown by the electron ionization mass spectrum of 1-(2′,2′-dideuteriocyclohexyl)-2,3-dithiapent-1,1′-ene (4b). Also, the spectrum of 1-(2′,2′-dideuteriocyclohexyl)-2-thiahex-1-ene (3b) is analyzed in terms of hydrogen/deuterium transfer, where it is absent. This result is compared with selected mass spectral data of eighteen other dialkylthianes. Evidence is put forth to indicate that dialkyl sulfides are prone to undergo only 1,3-H shift upon electron inpact, whereas in dithianes 1,3- and 1,5-Hydrogen transfer take place. The evidence collected suggests that neither 1,2- nor 1,4-hydrogen transfer occurs in both sulfur derivatives.     

Silicon clusters produced by femtosecond laser ablation: non-thermal emission and gas-phase condensation

Neutral silicon clusters Si_n (up to n=7) and their cations Si_n⁺ (up to n=10) have been produced by femtosecond laser ablation of bulk silicon in vacuum and investigated using time-of-flight mass spectrometry. Two populations of the Si_n⁺ clusters with different velocity and abundance distributions in the ablation plume have been clearly distinguished. Possible mechanisms of cluster formation (Coulomb explosion, gas-phase condensation, phase explosion) are discussed. PACS 52.38.Mf; 61.46.+w; 79.20.Ds     

Electronic mechanism of ion expulsion under UV nanosecond laser excitation of silicon: Experiment and modeling

We present experimental and modeling studies of UV nanosecond pulsed laser desorption and ablation of (111) bulk silicon. The results involve a new approach to the analysis of plume formation dynamics under high-energy photon irradiation of the semiconductor surface. Non-thermal, photo-induced desorption has been observed at low laser fluence, well below the melting threshold. Under ablation conditions, the non-thermal ions also have a high concentration. The origin of these ions is discussed on the basis of electronic excitation of Si surface states associated with the Coulomb explosion mechanism. We present a model describing dynamics of silicon target excitation, heating and charge-carrier transport. PACS 52.38.Mf; 68.34.Tj; 68.35.Rh; 79.20.Ds     

Intercedenol A and B, Two New Triterpenoids from the Sea Cucumber Mensamaria intercedens

Two new triterpenoids named intercedenol A and B were isolated from the acid hydrolysate of the crude glycoside fraction. The structures were elucidated by ESI-MS and NMR spectrum.     

Hyperbolic conservation laws on manifolds. An error estimate for finite volume schemes

Following Ben-Artzi and LeFloch, we consider nonlinear hyperbolic conservation laws posed on a Riemannian manifold, and we establish an L1-error estimate for a class of finite volume schemes allowing for the approximation of entropy solutions to the initial value problem. The error in the L1 norm is of order h1/4 at most, where h represents the maximal diameter of elements in the family of geodesic triangulations. The proof relies on a suitable generalization of Cockburn, Coquel, and LeFloch＇s theory which was originally developed in the Euclidian setting. We extend the arguments to curved manifolds, by taking into account the effects to the geometry and overcoming several new technical difficulties.     

Gheorghe Ţiţeica and the origins of affine differential geometry

We describe the historical and ideological context that brought to the fore the study of a centro-affine invariant that subsequently received much attention. The invariant was introduced by ?i?eica in 1907, and this discovery has been viewed by many as a consequence of Klein's Erlangen program. We thus present the starting point of affine differential geometry, as it was discovered by ?i?eica after his years in the Ph.D. program in Paris (1896–1899) under the guidance of Gaston Darboux.     

Citrinal A,a novel tricyclic derivative of citrinin,from an algicolous fungus Penicillium sp.i-1-1

Citrinal A(1),a novel tricyclic compound with a rare tetrahydro-2H-benzofuro[7-b][1,4]dioxin-9(3H)-one skeleton,along with two known related compounds,citrinin(2) and 2,3,4-trimethyl-5,7-dihydroxy-2,3-dihydrobenzofuran(3) were isolated from an algicolous fungus Penicillium sp.i-1-1.The structure and stereochemistry of 1 were determined by comprehensive spectral and biogenic analysis.Its cytotoxic effects on the A-549 and HL-60 cell lines were evaluated.     

A general continuum approach to describe fast electronic transport in pulsed laser irradiated materials: The problem of Coulomb explosion

We present a continuum model, based on a drift-diffusion approach, aimed at describing the dynamics of electronic excitation, heating, and charge-carrier transport in different materials (metals, semiconductors, and dielectrics) under femtosecond and nanosecond pulsed laser irradiation. The laser-induced charging of the targets is investigated at laser intensities above the material removal threshold. It is demonstrated that, for near-infrared femtosecond irradiation, charging of dielectric surfaces causes a sub-picosecond electrostatic rupture of the superficial layers, alternatively called Coulomb explosion (CE), while this effect is strongly inhibited for metals and semiconductors as a consequence of superior carrier transport properties. On the other hand, application of the model to UV nanosecond pulsed laser interaction with bulk silicon has pointed out the possibility of Coulomb explosion in semiconductors. For such regimes a simple analytical theory for the threshold laser fluence of CE has been developed, showing results in agreement with the experimental observations. Various related aspects concerning the possibility of CE depending on different irradiation parameters (fluence, wavelength and pulse duration) and material properties are discussed. This includes the temporal and spatial dynamics of charge-carrier generation in non-metallic targets and evolution of the reflection and absorption characteristics. PACS  79.20.Ds, 52.50.Jm     

Synchrotron-based FTIR microspectroscopy for the mapping of photo-oxidation and additives in acrylonitrile–butadiene–styrene model samples and historical objects

Synchrotron-based Fourier transform infrared micro-spectroscopy (SR-μFTIR) was used to map photo-oxidative degradation of acrylonitrile–butadiene–styrene (ABS) and to investigate the presence and the migration of additives in historical samples from important Italian design objects. High resolution (3 × 3 μm²) molecular maps were obtained by FTIR microspectroscopy in transmission mode, using a new method for the preparation of polymer thin sections. The depth of photo-oxidation in samples was evaluated and accompanied by the formation of ketones, aldehydes, esters, and unsaturated carbonyl compounds. This study demonstrates selective surface oxidation and a probable passivation of material against further degradation. In polymer fragments from design objects made of ABS from the 1960s, UV-stabilizers were detected and mapped, and microscopic inclusions of proteinaceous material were identified and mapped for the first time.