Time resolved spectroscopy of carbenepyridene ylides: Distinguishing carbenes from diazirine excited states

The photochemistry of diazirines and diazo compounds is not as simple as nitrogen extrusion and carbene formation. The C-H bonds adjacent to the diazo and diazirine moieties can migrate in the excited state and produce stable products without the benefit of a relaxed carbene intermediate. Additionally, cyclobutyl substituted systems exhibit carbon migration. It is unfortunate that the products of photochemical rearrangement of precursor excited states are identical to the products of thermal rearrangement of carbenes. This has prevented accurate measurement of the yield and absolute reactivity of alkylcarbenes. That pyridine reacts selectively with carbenes and not with the excited states of their nitrogenous precursors has allowed the separation of these two pathways and an appreciation of their relative importance with structural variation.     

Characterization of glucoamylase immobilized on celite

Immobilization of glucoamylase (EC 3.2.1.3) on Celite R649 bio-catalyst carrier for hydrolysis of maltose and maltodextrin has been investigated in both packed bed and recirculated batch reactors. The kinetics parameters on the hydrolysis of maltose were estimated from the packed bed reactor. It is found that this immobilized enzyme is as efficient as the soluble enzyme in catalyzing hydrolysis of maltose. However, it is less efficient than the soluble enzyme in hydrolyzing 30% (w/v) maltodextrin, giving a maximum dextrose equivalent (DE) value of 96.0% instead of 98.2%.     

Temperature independent length optimization of L-band EDFAs providing flat gain

In this study, single stage (SS), double-stage (DS), and gain flattened (GF) DS L-band erbium-doped fiber amplifier (EDFA) configurations are designed in order to obtain a flat gain amplifier. Temperature dependence of the mentioned configurations is also analyzed. Maximum spectral dependence of EDFA gain with respect to temperature is obtained for SS EDFA design while smaller spectral dependence of gains is obtained for both DS and GF DS L-EDFA configurations. It is observed that the maximum temperature dependence is in the range of 1570-1580 nm band for all configurations. It has also been found that for all configurations, reducing the temperature has greater effect than raising the temperature on EDFA gain. The overall results show that a temperature independent L-band configuration has not been possible. However, for some signal wavelengths, the erbium-doped fiber (EDF) lengths at which the gain is temperature independent are observed.     

Muonium dynamics in transparent conducting oxides

The motional and electrical properties of positively charged muonium (Mu⁺)$({\mathrm{Mu}}^{+})$ centers in single crystal β-Ga₂O₃$\mathrm{\beta }\mathrm{-}{\mathrm{Ga}}_2{\mathrm{O}}_3$ are investigated via zero field muon spin relaxation (ZF-MuSR). Below room temperature we find two distinct shallow muonium centers with ionization energies of 7 and 16 meV. Above room temperature, at least three different Mu⁺ signals are resolved; two of these are metastable while the third shows characteristics of a stable ground state. As the temperature is elevated, metastable centers undergo several transitions. We obtain the relevant barrier energies associated with these site-change transitions. By 700 K, most muons occupy the mobile ground state, and an activation energy of about 1.65 eV is inferred for Mu⁺ diffusion from the hop rates obtained for this state.     

Motion of positively charged muonium in ZnO

We report on a study of the motional characteristics of positively charged muonium defect centers in ZnO as an analog for H⁺ behavior. Muon spin depolarization measurements at zero applied magnetic field were completed from 20 K to 400 K, with preliminary results to 750 K. Results at the lower temperatures imply that Mu⁺ occupied two sites, and indicate local motion as thermally assisted tunneling with a characteristic energy of ∼60 meV, as well as a site change transition above 200 K with barrier energy ∼440 meV. Based on theoretical results, we have tentatively assigned these features to tunneling among three equivalent oxygen anti-bonding sites (AB_⊥) and a transition to a lower-energy bond-centered site (BC_‖) oriented along the c-axis. Preliminary fits suggest that global diffusion of muonium occurs above 400 K, with a diffusion barrier energy of ∼0.7 eV.     

A different approach to gain computation in laser diodes with respect to different number of quantum-wells

This study presents a different approach for the modelling of optical gain in laser diodes as a function of quantum-well (QW) number based on Artificial Neural Networks (ANNs). Different learning algorithms with different network configurations are tried and tested in order to minimize the rms errors in terms of the ANN structure, number of layers, and number of neurons in each layer. The optical gain results obtained by using this method are in very good agreement with the experimental results reported elsewhere.     

Synthesis and characterization of poly(acrylic acid) stabilized cadmium sulfide quantum dots

Cadmium sulfide (CdS) nanoparticles (NPs) capped with poly(acrylic acid) (PAA) were prepared in aqueous solutions from Cd(NO3)2 and Na2S. Influence of the COOH/Cd ratio (0.8-12.5), reaction pH (5.5 and 7.5), and PAA molecular weight (2100 and 5100 g/mol) on the particle size, colloidal stability, and photoluminescence were investigated. A Cd/S ratio of <1 causes ineffective passivization of the surface with the carboxylate and therefore results in a red shift of the absorption band and a significant drop in photoluminescence. Therefore, the Cd/S ratio was fixed at 1.1 for all experiments studying the mentioned variables. PAA coating provided excellent colloidal stability at a COOH/Cd ratio above 1. Absorption edges of PAA-coated CdS NPs are in the range of 460-508 nm. The size of the NPs increases slightly with increasing PAA molecular weight and COOH/Cd ratio at pH 7.5. It is demonstrated that there is a critical COOH/Cd ratio (1.5-2) that maximizes the photoluminescence intensity and quantum yield (QY, 17%). Above this critical ratio, which corresponds to smaller crystal sizes (3.7-4.1 nm) for each reaction set, the quantum yield decreases and the crystal size increases. Moreover, CdS NPs prepared at pH 7.5 have significantly higher QY and absorb at lower wavelengths in comparison with those prepared at pH 5.5. Luminescence quenching has not been observed over 8 months.