Energy distribution curves of ultrafast laser-induced field emission and their implications for electron dynamics

Energy distribution curves of laser-induced electron pulses from a tungsten tip have been measured as a function of tip voltage and laser power. Electron emission via tunneling through and/or excitation over the surface barrier from photoexcited nonequilibrium electron distributions are clearly observed. The spectral shapes largely vary with the emission processes and are strongly affected by electron dynamics. Simulations successfully reproduce the spectra, thus allowing direct insight into the involved electron dynamics and revealing the temporal tunability of electron emission via the two experimental parameters. These results should be useful to optimize the pulse characteristics for many applications based on ultrafast laser-induced electron emission.     

Zur Messung des Rauschens von Transistoren

Ohne Zusammenfassung     

Rapid and simple determination of aflatoxin M1 in milk in the low parts per 10(12) range

A method for extracting aflatoxin M1 from milk is proposed in which the use of disposable Extrelut clean-up columns simplifies the analysis considerably in comparison with existing methods. The quantitative determination is based on one-dimensional thin-layer chromatography and fluorescence densitometric measurement. The detection limit is 5 ppt (parts per 10(12)) in milk and the recovery is 78 +/- 4% at a level of 50 ppt.     

Rapid online equilibration method to determine the D/H ratios of non-exchangeable hydrogen in cellulose

An improved method for the determination of deuterium-to-hydrogen (D/H) ratios of non-exchangeable hydrogen in cellulose is presented. The method is based on the equilibration reaction of the hydroxyl hydrogen of cellulose and water vapour of known isotopic composition. The equilibrated cellulose is pyrolysed and the total D/H ratio determined by subsequent online isotope ratio mass spectrometry (IRMS). With a mass balance system the D/H ratio of non-exchangeable hydrogen is recalculated after an empirical calibration has been performed, yielding a mean exchangeability of 0.239 and an equilibrium fractionation factor of 1.082 between the hydroxyl hydrogen of cellulose and water hydrogen at 110 degrees C. Equilibration takes 10 min per sample. Results obtained by this online equilibration method agree very well with values obtained by the nitration technique (R2 = 0.941). The uncertainty of the equilibration method is +/-4 per thousand resulting from a single standard deviation of +/-2.8 per thousand for the equilibration determined by standard cellulose and 2.8 per thousand from the variable exchangeability of the hydroxyl hydrogen in cellulose due to crystalline areas. The latter uncertainty may be lowered by minimising the crystallinity of the cellulose. Advantages of this new technique are (i) the considerably reduced sample amount required (as low as 0.2 mg, ideally 0.5 mg compared with 20 mg for the conventional nitration technique); (ii) an approximately 100-fold reduced process time; and (iii) no need for the hazardous chemicals used in the nitration technique.     

Stoffwechselprodukte von Mikroorganismen. 233. Mitteilung. Danoxamin,der eisenbindende Teil des Sideromycin-Antibioticums Danomycin

Danoxamine, the Iron-Binding Moiety of the Sideromycin Antibiotic Danomycin The antibiotic danomycin was separated into its components, danomycin A and B ( 1 and 2 , resp.), by various methods. Danomycin A is a carbamate of danomycin B and can be transformed to the latter by boiling with H₂O. Both danomycins can be hydrolyzed by alkali to the Fe-containing moiety, danoxamine ( 3 ). The structure of the latter was determined by spectroscopy and chemical degradation as the Fe(III) complex of 5,16,27,32-tetrahydroxy-4,12,15,23,26-pentaoxo-5,11,16,22,27-pentaaza-dotriacontanoic acid. It belongs thus to the ferrioxamine family of siderophores. In the danomycins, this moiety is linked by an ester bond to a disaccharide moiety of unknown structure. One of the sugars is believed to be an aminohexane methyl ether. Chromatographically homogenous danomycins A and B are still mixtures of isomers, a neutral sugar being glucose or mannose.     

Triaziridine. II. Erste Beispiele: 2, 3-Dialkyl-triaziridin-1-carbonsäurealkylester

Triaziridines. II. First Examples: Alkyl 2,3-Dialkyl-triaziridine-1-carboxylates The first examples of substituted triaziridines 2 are described; they carry an alkoxycarbonyl and two alkyl groups (as in 4 ). The preparation of these novel threemembered nitrogen homocycles was achieved by photolysis of 1-alkoxycarbonylazimines 3c. In this way, methyl 2, 3-(cis-1, 3-cyclopentylene)triaziridine-1-carboxylate ( 6a ), methyl trans-2, 3-diisopropyl-triaziridine- 1 -carboxylate ( 8a ) and their ethyl ester analogues 6b and 8b were obtained in 50, 18, 65 and 21 % yield, respectively. The structure of the triaziridines 6 and 8 was deduced from their spectroscopic properties which reveal several interesting features: 1) N (2) and N (3), carrying alkyl groups, are pyramidal and invert slowly; 2) the isopropyl groups of 8 are situated trans to each other on the three-membered ring, whereas the two alkyl groups of 6 are cis as forced by the C-ring system; 3) N (1) is also pyramidal, despite its «amidic» nature; it inverts with an activation energy of 62 (± 4) kJ/mol; 4) the alkoxycarbonyl, group does not conjugate with N (1) and rotates rapidly. The triaziridines 6 and 8 are thermally labile, isomerizing slowly at room temperature into the corresponding azimines 5 and 7 by cleavage of one of the bonds to N (1 ). The velocity of this ring opening reaction is almost the same for 6 and 8 , so that a dependence on the relative configuration at N (2) and N (3) is not evident. The Arrhenius activation energy for the isornerization of 6a to the corresponding azimine 5a and the enthalpy difference between 6a and 5a were both determined as 100 (± 4) kJ/mol. The photolysis of 1-alkoxycarbonyl-2, 3-diisopropyl-azimines ( 7 ) in diethyl ether was accompanied by a side reaction leading to methyl and ethyl N-(1-ethoxyethyl)carbamate ( 9a and 9b , resp.), presumably by insertion of the alkoxycarbonylnitrene, generated by photofragmentation of the azimines, into the ethereal solvent.     

13C and 18O fractionation effects on open splits and on the ion source in continuous flow isotope ratio mass spectrometry

Measurements of carbon and oxygen isotopes of CO₂ by continuous flow isotope ratio mass spectrometry are widely used in environmental studies and climate change research. Yet, there are remaining problems associated with the reproducibility of measurements, in particular when high precision is required and/or the amount of sample material is limited. Isotopic fractionations in open splits and nonlinear effects occurring in the mass spectrometer due to different sample amounts alter the results. In this study, we discuss the influence and the origin of these two effects and propose procedures for preventing their impact. Fractionation in the open split can be related to diffusion of CO₂ and can lead to shifted δ‐values when measuring a sample gas against a reference gas injected via different open splits. We present a method, where such fractionations can be minimized by adjusting either the position of the capillaries or the flow rates involved or both. The nonlinear peak area dependence of δ¹³C measurements for small sample sizes can be explained by adsorption/desorption processes in the ionization chamber or its vicinity. For constant amplitudes, the magnitude of the nonlinearity only depends on the amount of CO₂ entering the ion source. This nonlinearity can be eliminated by a small additional flux of a conditioning gas fed to the mass spectrometer. The best results were obtained when using carbon monoxide. For the adsorption process in the mass spectrometer we found a fractionation factor of 0.982 ± 0.005 for δ¹³C and 1.002 ± 0.004 for δ¹⁸O. Copyright © 2010 John Wiley & Sons, Ltd.     

Production of (+)-(S)-Ethyl 3-Hydroxybutyrate and (-)-(R)-Ethyl 3-Hydroxybutyrate by Microbial Reduction of Ethyl Acetoacetate

The production of (S)-ethyl 3-hydroxybutyrate (2) from ethyl acetoacetate (1) in higher yields and with higher optical purity than with the usual procedures was achieved by continuous addition of the substrate 1 and sucrose to an aerated suspension of bakers' yeast. The microbial reduction of 1 by the fungus Geotrichum candidum LINK yields - for the first time - the antipode 3 .     

Quantum Spin Dynamics in Molecular Magnets

Summary.  The detailed theoretical understanding of quantum spin dynamics in various molecular magnets is an important step on the roadway to technological applications of these systems. Quantum effects in both ferromagnetic and antiferromagnetic molecular clusters are, by now, theoretically well understood. Ferromagnetic molecular clusters allow one to study the interplay of incoherent quantum tunneling and thermally activated transitions between states with different spin orientation. The Berry phase oscillations found in Fe₈ are signatures of the quantum mechanical interference of different tunneling paths. Antiferromagnetic molecular clusters are promising candidates for the observation of coherent quantum tunneling on the mesoscopic scale. Although challenging, application of molecular magnetic clusters for data storage and quantum data processing are within experimental reach already with present day technology. Corresponding author. E-mail: Daniel.Loss@unibas.ch Received May 7, 2002; accepted May 22, 2002