One-neutron knockout from individual single-particle states of 11Be

The structure of the halo nucleus 11Be has been studied using the reaction 9Be(11Be,10Be+gamma)X at 60 MeV/nucleon. The ground state structure of 11Be is determined by comparing the experimental cross sections to a calculation combining spectroscopic factors from the shell model with l-dependent single-particle cross sections obtained in an eikonal model. This experiment shows the dominant 1s single-particle character of the 11Be ground state and indicates a small contribution of 0d admixture in the wave function. After correction for the approximately 22% intensity to excited levels, a clean and precise distribution of parallel momentum for knockout from the 1s halo wave function is obtained for the first time.     

Identification of the Li ground state

The proton-stripping reaction from a ¹¹Be radioactive beam incident on a beryllium target demonstrates that only (7±3)% of the ⁹Li residues in the reaction are in coincidence with the 2.7 MeV γ-ray corresponding to the ⁹Li first excited state. This implies that the previously observed low-energy neutrons from the decay of the unbound nucleus ¹⁰Li represent a direct l=0 transition to the ⁹Li ground state. Consequently, neutron-unbound ¹⁰Li is proven to have the same parity inversion as occurs in the case of ¹¹Be with a  intruder state below the natural parity state.     

Dicyanoperylene-diimide thin-film growth: a combined optical and morphological study

N,N′-bis (n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI8-CN₂) molecules represent an important example of novel n-type organic materials for realization of air-stable n-channel organic thin film transistors. In this work, the growth by evaporation of PDI8-CN₂ thin films deposited on silicon/silicon dioxide substrates has been investigated as a function of the film thickness through the combination of optical and morphological analyses. A continuous transition from two-dimensional to three-dimensional growth is observed at increasing thickness, accompanied by a modification of the photoluminescence spectrum of the films. Correlations between morphology and optical emission (photoluminescence) of the films are evidenced: namely, an emission band, red shifted with respect to the excitonic transition, emerges at increasing thickness. Time-resolved photoluminescence analysis demonstrates that the decay kinetics of such a band is different from the one associated with exciton recombination. Such a feature may thus represent a useful signature of defect-related trap states.     

Degradation Products from Naturally Aged Paper Leaves of a 16th‐Century‐Printed Book: A Spectrochemical Study

In this work, we present a wide‐range spectrochemical analysis of the degradation products from naturally aged paper. The samples obtained from wash waters used during the de‐acidification treatment of leaves from a 16th‐century‐printed book were analysed through NMR, IR, Raman UV/Vis, EPR and X‐ray fluorescence (XRF) spectroscopy and HPLC‐MS and inductively coupled plasma (ICP) analysis. By these methods we also studied some of the previous samples treated by acidification (sample AP) and catalytic hydrogenation (sample HP). Crossing all the data, we obtained precise indications about the main functional groups occurring on the degraded, water‐soluble cellulose oligomers. These results point out that the chromophores responsible for browning are conjugated carbonyl and carboxyl compounds. As a whole, we show that the analysis of wash waters, used in the usual conservation treatments of paper de‐acidification, gives much valuable information about both the conservation state of the book and the degradation reactions occurring on the leaves, due to the huge amount of cellulose by‐products contained in the samples. We propose therefore this procedure as a new very convenient general method to obtain precious and normally unavailable information on the cellulose degradation by‐products from naturally aged paper.     

Optical Freedericksz transitions in twisted planar nematics

Summary We study the propagation of an elliptically polarized light beam normally incident onto a twisted nematic liquid-crystal film. A set of nonlinear differential equations for the molecular director and the polarization of the light beam is derived; it is solved analytically in the limit of small perturbations around the steady-state solutions that represent the nonreoriented sample. We prove that optical Freedericksz transitions are possible only for a particular polarization state of the incident light. The threshold behaviour is studied as a function of the light intensity and of the twist angle between the molecular directors at the sample walls. The results are characteristic of the optical interaction and have no analogue in the molecular reorientation induced on such a system by d.c. fields.

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Riassunto Si è studiata la propagazione di un fascio di luce polarizzata ellitticamente che incide normalmente su un campione di cristallo liquido nematico planare ?twisted?, cioè nel quale l'angolo α fra la direzione delle molecole nel piano di ingresso della luce e quella nel piano di uscita è≠O. è stato derivato un sistema non lineare di equazioni differenziali che governa l'evoluzione del direttore molecolare e dello stato di polarizzazione della luce. Questo sistema è stato risolto analiticamente nel limite di piccole variazioni intorno a soluzioni corrispondenti allo stato imperturbato. Si è dimostrato che transizioni di Freedericksz ottiche sono possibili solo se la luce incidente si trova in un particolare stato di polarizzazione. I risultati ottenuti sono peculiari dell'interazione ottica e non sono riconducibili al fenomeno della riorientazione molecolare indotta su campioni di questo tipo da campi statici, elettrici o magnetici.

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Резюме Мы исследуем распространение эллипчески поляриэованного светового пучка, падаюшего номально на пленку эакрученного нематического жидкого кристалла. Выводится система нелинейных дифференциальиых уравнений для молекулярного директора и поляриэации светового пучка. Полуенная система решается аналитчески в пределе малЫх воэмущений относительно решений для стционарнЫх состояний, которЫе соответствуют не-переориентированному обраэцы. МЫ докаэЫваем, что оптические перехоые фредерикса воэможиы только для частично поляриэованного состояния падаюшего света. Исследуется пороповое поведение как функция интенсвности света и упла эакручивания между молекуляриыми директорами на стенках обраэца. Полученные реэультаты являются характерными для оптического вэаимодействия и не имеют аналога при молекулярной переориентации, индуцрованной в такой системе d.c. полями.

     

New copper(II)/cyclic tetrapeptide system that easily oxidizes to copper(III) under atmospheric oxygen

A 13-membered ring cyclic tetrapeptide was synthesized by the solid-phase peptide synthesis method, and its copper(II) coordination properties were analyzed by optical spectroscopy, mass spectrometry, and electrochemistry. All collected data strongly support the presence, at alkaline pH, of a stable peptide/copper(III) complex that is formed in solution by atmospheric dioxygen oxidation. On the basis of previous studies on cyclic peptide/copper systems, we suggest that the copper(III) ion is at the center of the ligand's cavity being coordinated to four deprotonated amide nitrogen atoms. This donor set would greatly lower the redox potential for the CuIII/CuII couple, thus allowing easy oxidation of the coordinated copper(II) by atmospheric oxygen.     

Combinatorics of Thin Posets: Application to Monotone Covering Properties

A class of posets, called thin posets, is introduced, and it is shown that every thin poset can be covered by a finite family of trees. This fact is used to show that (within ZFC) every separable monotonically Menger space is first countable. This contrasts with the previously known fact that under CH there are countable monotonically Lindelöf spaces which are not first countable.     

Mixed-ligand Pt(II) dithione-dithiolato complexes: influence of the dicyanobenzodithiolato ligand on the second-order NLO properties

The mixed-ligand dithiolene complex [Pt(Bz(2)pipdt)(dcbdt)] (1) bearing the two ligands Bz(2)pipdt = 1,4-dibenzyl-piperazine-3,2-dithione and dcbdt = dicyanobenzodithiolato, has been synthesized, characterized and studied to evaluate its second-order optical nonlinearity. The dithione/dithiolato character of the two ligands gives rise to an asymmetric distribution of the charge in the molecule. This is reflected by structural data showing that in the C(2)S(2)PtS(2)C(2) dithiolene core the four sulfur atoms define a square-planar coordination environment of the metal where the Pt-S bond distances involving the two ligands are similar, while the C-S bond distances in the C(2)S(2) units exhibit a significant difference in Bz(2)pipdt (dithione) and dcbdt (dithiolato). 1 shows a moderately strong absorption peak in the visible region, which can be related to a HOMO-LUMO transition, where the dcbdt ligand (dithiolato) contributes mostly to the HOMO, and the Bz(2)pipdt one (dithione) mostly to the LUMO. Thus this transition has ligand-to-ligand charge transfer (CT) character with some contribution of the metal and undergoes negative solvatochromism and molecular quadratic optical nonlinearity (μβ(0) = -1296 × 10(-48) esu), which was determined by the EFISH (electric-field-induced second-harmonic generation) technique and compared with the values of similar complexes on varying the dithiolato ligand (mnt = maleonitriledithiolato, dmit = 2-thioxo-1,3-dithiole-4,5-dithiolato). Theoretical calculations help to elucidate the role of the dithiolato ligands in affecting the molecular quadratic optical nonlinearity of these complexes.     

Kinetics of phase transitions in two dimensional Ising models studied with the string method

The kinetics of phase transitions in the two dimensional Ising model under different conditions is studied using the string method. The key idea is to work in collective variables, consisting of block of spins, which allow for a continuous approximation of the collective variable state-space. The string method computes the minimum free energy path (MFEP) in this collective variable space, which is shown to explain the mechanism of the phase transformation (in particular, an approximation of its committor function, its free energy and its transition state). In this paper the theoretical background of the technique as well as its computational aspects are discussed in details. The string method is then used to analyze phase transition in the Ising model with imposed boundary conditions and in a periodic system under an external field of increasing magnitude. In each case, the mechanism of the phase transformation is elucidated.