Simple and efficient H2 Raman conversion of a XeCl laser with a variable numerical aperture coupling geometry

A simple combination of a lens-based unstable resonator and an inverted telescope coupling geometry working in a XeCl laser pumped H₂ Raman cell are presented. Raman spectrum optimization is achieved by varying the pump beam numerical aperture. 70% pump depletion and energy conversion efficiencies as high as 66% and 7.8% have been obtained for S₁, S₂ and S₃ respectively.     

Kinetics of the C2H2 dissociation on Ni(111): A HREELS study

The C₂H₂ isothermal dissociation rate on Ni(111) was measured by following, with HREEL spectroscopy, the intensity of the CH stretching peak as a function of time. By repeating the experiment at several different temperatures we have obtained an activation energy of 1.2 ± 0.1 eV/molecule and a pre-exponential factor of 10^14±1 s⁻¹ for the first step of the C₂H₂ dissociation reaction.     

The orbit space of the action of gauge transformation group on connections

The behaviour of orbits of the action of the group of smooth gauge transformations on connections for a principal bundle P(M, G) is discussed with and without compactness assumption on M and G. In the case of compact M and with suitable conditions on G a stratification structure for the space of orbits is established. A natural tame weak Riemannian metric is given on each stratum.     

Redox properties of NN′-X-phenylenebis(Y-salicyli-deneiminato)iron(II) complexes and reactivity of the corresponding iron(III) complexes towards catecholates

The electrochemical behaviour of a series of iron(II) complexes with the tetradentate ligand NN′-1,2-phenylenebis(salicylideneimine), [Fe(II)L], was studied in non-aqueous solvents. The redox properties of the complexes were related to the nature of the substituents in the aromatic rings. Attention was devoted to dioxygen reactivity of the complexes. The electrode activity of the catechol—[NN′-1,2-phenylenebis(salicylidene-iminato) iron(III)] system, [Fe(III)L(catH)], was also studied; the results gave evidence that both the electrochemical oxidation and the chemical oxidation by dioxygen of [Fe(II)L] in the presence of catechol lead to the complex [Fe(III)L(catH)].     

Modelling hydrological data with and without long memory

The paper focuses on the problem of long-range dependence when analysing time series of hydrological data. Three time series are analysed: the monthly rainfall in the town of Florence, Italy; the daily minimum temperatures in the same town; and, finally, the daily water inflow to Lake Maggiore, Italy. Heuristic methods and maximum likelihood estimation of a parametric model are used to investigate the Hurst phenomena and to detect whether long-range dependence is present in any of the time series. We found that long-range dependence is not present in the first series but it is present in the last two. The daily water inflow to Lake Maggiore was modelled by a fractionally differenced arima model (farima) which contains a long-range dependence component. It is shown that the fit is much better than the one provided by more traditional arima models that do not have such a component.

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Sommario Lo studio considera il problema dell'identificazione dei fenomeni di dipendenza a lungo termine (long range dependence) nelle serie temporali di dati idrologici. Allo scopo sono state analizzate tre serie temporali, rispettivamente quella dei totali mensili di precipitazione rilevati alla stazione dell'Osservatorio Ximeniano de Firenze, quella delle temperature minime giornaliere per la stessa stazione e quella degli afflussi giornalieri al Lago Maggiore. Per identificare la presenza di fenomeni di dipendenza long range, attraverso la valutazione della consistenza del fenomeno di Hurst, sono stati utilizzati sia metodi euristici, sia la stima a massima verosimiglianza di un modello parametrico. Due delle tre serie analizzate sono risultate caratterizzate da tale dipendenza. Per la serie degli afflussi al Lago Maggiore, si è inoltre proceduto alla simulazione attraverso un modello arima a differenziazione frazionaria (farima), la cui struttura contiene una componente long-range. I risultati ottenuti, mostrano che tale modello fornisce risultati significativamente migliori dei tradizionali modelli arima, privi di tale componente.

     

Counting labels in binary trees

We investigate properties of binary (search) trees related to the inorder labelling of the nodes. Both permutation and uniform trees are considered. We give explicit formulas to count the number of interior, middle and final nodes (leaves) containing a specific label. Possible applications are discussed.This work was supported by the Italian Ministry for Public Education.     

Self-assembled monolayers and electrostatically self-assembled multilayers of polyalkylviologens on sulfonate-modified gold and indium–tin–oxide electrodes

Self-assembly (SA) of polyviologens with linear alkyl spacers PVn (n = 3–10) on sulfonate-primed gold and ITO electrode surfaces has been investigated by CV, QCM, XPS and AFM. The polymers form stable and relatively dense monolayers even from 10⁻⁴ M PVn concentration. The viologen surface density is decreased as n is increased but for n = 10 and on alkylsulfonate-primed surfaces a strong adsorption takes place, with decreased redox potential and very narrow cyclic voltammogram of the absorbed layer due to organized structures on the surface.Stable and regular electrostatically self-assembled (ESA) multilayers are built on sulfonate-primed surfaces with PV3 and polysulfonates. From XPS analysis, the composition of the PVn/polysulfonate multilayers corresponds to a 1:1 ratio of polyanion and polycation charges, i.e., no extra non-polymeric ion is present, independently from the alkane chain length. The rate of electron transfer within the multilayers (diffusion coefficient = ca. 10⁻⁹ cm² s⁻¹) is higher than for analogous bulk materials.     

New synthesis of optically active 5-isoxazolidinones and beta-amino acids

A new simple and stereoselective synthesis of 5-isoxazolidinones based on the reaction of lithiated 2-isopropyl-2-oxazolines with nitrones is described. A chiral version of such a methodology allows the preparation of highly enantioenriched 5-isoxazolidinones which are useful precursors for the synthesis of beta-amino acids