Förster energy transfer from poly(arylene–ethynylene)s to an erbium–porphyrin complex

We study the infrared emission at 1.54 μm of an organolanthanide complex, Er(III)-tetraphenylporphyrin [Er(TPP)acac], both as a result of direct optical excitation and via energy transfer from host π-conjugate polymers of type poly(arylene–ethynylene) [PAE]. In the first case, the emission of the neat complex is characterized in inert transparent materials and a value of the quantum yield at 1.54 μm φ_IR=4×10⁻⁴ is measured. Then, fluorescence resonance transfer is investigated in blends of Er(TPP)acac with PAEs by monitoring the quenching of the polymer fluorescence along with the enhancement of both the visible emission of the ligand and the near-infrared band of Er³⁺. These different procedures allow a detailed analysis of the transfer efficiency within a specific implementation of the Förster model for polymeric donors. The experimental values of the critical radius R₀, ranging from 1.3 to 2.5 nm for the different blends, are in good agreement with theory for a wide interval of the physical and spectroscopic parameters. This suggests that other mechanisms for excitation transfer do not play a significant role in these materials.     

Organometallic complexes in ascidian embryonic development: II. Effects on different stages and larvae

The effects of the organometallic compounds Bu₂Sn-D -(?;)sorbitol, Bu₂Sn-D -(+)glucose, Bu₂Sn-D -(?)fructose and Bu₂Sn-D -(+)glyceraldehyde were tested in vivo on different stages of Ascidian development, larval movement and metamorphosis. Organotin(IV) complexes are organometallic compounds widely used as industrial biocides, antifouling agents and agricultural fungicides and are toxic to a range of organisms. Two-cell stage embryos, if incubated for one hour in the organotin (IV) solutions, stopped the cleavage, which was restored when they were transferred into normal sea water. The gastrula stage was seriously affected in 10^?4mol dm^?3 solutions of the above-mentioned complexes: 85% of the embryos were anomalous neurulae with open neural folds, 5% were twisted larvae. The gastrulae, when incubated for 1 h in 10^?5mol dm^?3 solutions, developed twisted larvae in ovular envelopes and immobile larvae with twisted tails. Larvae treated with 10^?4mol dm^?3 and 10^?5 mol dm^?3 Bu₂Sn-D -(?)sorbitol, Bu₂Sn-D -(+)glucose and Bu₂Sn-D -(+)glyceraldehyde solutions stopped swimming, did not metamorphose and afterwards underwent cytolysis. An initial hyperactivity of circular movements, followed by immobility, was observed in the larvae incubated in Bu₂Sn-D -(?;)fructose.     

Numerical modeling of a XeCl laser excited by microwave discharge

12 , 169 (1987). Reasonably good agreements in the peak output power and laser efficiency have been achieved. Model calculations also predict that an efficiency as high as 2.7% can be obtained once the conditions of the above-mentioned experiments have been optimized. From the consideration that the skin depth effectively limits the absorption length of the microwave pumping and hence the excitable volume, it is concluded that high input power densities (>2 MW/cm³) and higher gas pressures (between 3 and 10 atm) are the preferable conditions to achieve higher efficiency. Preliminary calculations on CCl₄ containing XeCl gas mixtures show that improvement in laser efficiency by several folds may be achieved as a result of the higher intrinsic efficiency of excimer formation. Received: 23 September 1996 / Revised version: 25 March 1997     

The Maximal Closed Classes of Unary Functions in p-Valued Logic

In many-valued logic the decision of functional completeness is a basic and important problem, and the thorough solution to this problem depends on determining all maximal closed sets in the set of many-valued logic functions. It includes three famous problems, i.e., to determine all maximal closed sets in the set of the total, of the partial and of the unary many-valued logic functions, respectively. The first two problems have been completely solved ([1], [2], [8]), and the solution to the third problem boils down to determining all maximal subgroups in the k-degree symmetric group S_k, which is an open problem in the finite group theory. In this paper, all maximal closed sets in the set of unary p-valued logic functions are determined, where p is a prime. Mathematics Subject Classification: 03B50, 20B35.     

Spray method: Dependence of deposition efficiency on substrate temperature

Application of the spray method for the deposition of thin films has shown that the deposition efficiency is a decreasing function of the substrate temperature. The forces which determine the path and the time the droplets take to go from the nozzle to the substrate are analyzed. A theoretical curve of the deposition efficiency as a function of the substrate temperature has been obtained by a successive-approximation method. Experimental values of the efficiency obtained from the growth rate of the films are always a function of the substrate temperature. The good agreement of the theoretical curve with the experimental data confirms the validity of the method applied for our analysis.Work supported by Ministero della Universitá e della Ricerca Scientifica and Centro Regionale Ricerche Nucleari e Struttura della Materia     

A Brownian Particle in a Microscopic Periodic Potential

We study a model for a massive test particle in a microscopic periodic potential and interacting with a reservoir of light particles. In the regime considered, the fluctuations in the test particle’s momentum resulting from collisions typically outweigh the shifts in momentum generated by the periodic force, so the force is effectively a perturbative contribution. The mathematical starting point is an idealized reduced dynamics for the test particle given by a linear Boltzmann equation. In the limit that the mass ratio of a single reservoir particle to the test particle tends to zero, we show that there is convergence to the Ornstein–Uhlenbeck process under the standard normalizations for the test particle variables. Our analysis is primarily directed towards bounding the perturbative effect of the periodic potential on the particle’s momentum.     

Theoretical study of diluted Co/Cu alloys

This study employs the self-consistent Green's function method to study the magnetic properties of diluted _CoxCu1-x${\mathrm{Co}}_x{\mathrm{Cu}}_{1-x}$ alloys from a consideration of their spin dynamics characteristics. The numerical results show that in dilute cobalt concentrations (i.e. x?0.4$x?0.4$), the critical temperatures vary linearly with x for different itinerant carrier concentration conditions. Interestingly, the carrier concentration does not affect the degree of dependency of the temperature on the cobalt concentration when the carrier concentration is less than the atomic number concentration of the alloy.     

Growth and characterization of A-plane ZnO and ZnCoO based heterostructures

Non polar ZnO and (Zn, Co)O layers were successfully grown on (11̄02) sapphire (R-plane sapphire). The growth process was shown to directly influence the surface morphology as well as the strain state in (112̄0) ZnO (A-plane ZnO). The dominant defect lines seen in photoluminescence were due to basal stacking faults as demonstrated by means of selective photoluminescence and transmission electron microscopy. We present a novel method for growing high quality A-plane ZnO by inserting a (Zn, Co)O thin buffer layer, which strongly reduced the surface roughness. Finally (Zn, Mg)O/ZnO quantum well structures were grown on such a buffer layer. These quantum wells exhibited no intrinsic quantum confined Stark effect. PACS 81.05.Dz; 81.15.Hi; 78.67.Hc; 68.65.Fg     

Noise effects in two different biological systems

We investigate the role of the colored noise in two biological systems: (i) adults of Nezara viridula (L.) (Heteroptera: Pentatomidae), and (ii) polymer translocation. In the first system we analyze, by directionality tests, the response of N. viridula individuals to subthreshold signals plus noise in their mating behaviour. The percentage of insects that react to the subthreshold signal shows a nonmonotonic behaviour, characterized by the presence of a maximum, as a function of the noise intensity. This is the signature of the non-dynamical stochastic resonance phenomenon. By using a “soft” threshold model we find that the maximum of the input-output cross correlation occurs in the same range of noise intensity values for which the behavioural activation of the insects has a maximum. Moreover this maximum value is lowered and shifted towards higher noise intensities, compared to the case of white noise. In the second biological system the noise driven translocation of short polymers in crowded solutions is analyzed. An improved version of the Rouse model for a flexible polymer is adopted to mimic the molecular dynamics by taking into account both the interactions between adjacent monomers and the effects of a Lennard-Jones potential between all beads. The polymer dynamics is simulated in a two-dimensional domain by numerically solving the Langevin equations of motion in the presence of thermal fluctuations and a colored noise source. At low temperatures or for strong colored noise intensities the translocation process of the polymer chain is delayed. At low noise intensity, as the polymer length increases, we find a nonmonotonic behaviour for the mean first translocation time of the polymer centre of inertia. We show how colored noise influences the motion of short polymers, by inducing two different regimes of translocation in the dynamics of molecule transport.