Analysis of various optical pumping schemes for liquid oxygen lasers

Singlet oxygen generated by optical pumping in liquid oxygen/air medium has recently been reported as a potential gain medium for high power lasers at 1580 nm, by USA’s defense agency DARPA. However, the details with reference to the underlying physics of this laser and the potential pumping techniques for achieving lasing are still unclear. The present paper investigates numerically the population kinetics of both upper and lower lasing levels viz. O₂ and O₂ states in liquid oxygen and discusses the criticality involved in obtaining lasing with this medium. Isotopic liquid oxygen and liquid air medium where improved conditions for lasing are anticipated, as compared to that in natural liquid oxygen, have also been considered in the present study. The studies have been carried out for optical pumping by both continuous and pulsed mode lasers at 1064 nm and 634 nm wavelengths. The temporal variation of small signal gain in each case has been investigated along with limitations of the same from point of view of lasing. The available and extractable power from all three medium for an optimum pumping case has also been discussed. The studies reveal that liquid air has high potential for very high power lasers as compared to both natural and isotopic liquid oxygen. PACS 31.15.ag     

Improved thermal energy storage behavior of polyethylene glycol-based NEOPCM containing aluminum oxide nanoparticles for solar thermal applications

Journal of Thermal Analysis and Calorimetry - In the present investigation, a novel composite of Polyethylene glycol (PEG) with molecular weight 10,000 (10 k) and aluminum oxide...     

Dynamics of small-molecule glass formers confined in nanopores

We report a comparative neutron scattering study of the molecular mobility and nonexponential relaxation of three structurally similar glass-forming liquids, isopropanol, propylene glycol, and glycerol, both in bulk and confined in porous Vycor glass. Confinement reduces molecular mobility in all three liquids, and suppresses crystallization in isopropanol. High-resolution quasielastic neutron scattering spectra were fit to Fourier transformed Kohlrausch functions exp[-(t∕τ)(β)], describing the α-relaxation processes in these liquids. The stretching parameter β is roughly constant with wavevector Q and over the temperature range explored in bulk glycerol and propylene glycol, but varies both with Q and temperature in confinement. Average relaxation times <τ(Q)> are longer at lower temperatures and in confinement. They obey a power law <τ(Q)> ∝ Q(-γ), where the exponent γ is modified by confinement. Comparison of the bulk and confined liquids lends support to the idea that structural and∕or dynamical heterogeneity underlies the nonexponential relaxation of glass formers, as widely hypothesized in the literature.     

Absorption and emission spectroscopic characterization of lumichrome in aqueous solutions

The spectroscopic behavior of lumichrome (7,8-dimethyl-alloxazine, LC) in aqueous solutions in a pH range from -1.08 to 14.6 is studied. Absorption spectra, fluorescence quantum distributions, quantum yields, and lifetimes are determined. The ionization stage of ground-state LC changes with rising pH from the cationic form (LCH(2)(+)) to the neutral form (LCH) with a mid-point pH of pK(c) ≈ -0.53, and to the anionic form (LC(-)) with a mid-point pH of pK(a) ≈ 12.5. Above pH 7 a partial ground-state tautomerization of LCH to 7,8-dimethyl-isoalloxazine (IAH) occurs by N1-N10 intra-molecular proton transfer. For pH > pK(a) ≈ 12.5 LCH and IAH change to the anionic forms LC(-) and IA(-), and above pH 14 LC(-) tautomerizes completely to IA(-). In the excited state some neutral lumichrome (LCH*) converts to cationic lumichrome (LCH(2)(+)) at low pH by proton transfer from H(3)O(+) to LCH*. No photoinduced excited-state tautomerization of lumichrome was observed. LCH for pH > 3 and IAH are reasonably fluorescent. The fluorescence efficiencies of LC(-) and IA(-) are lower than those of LCH and IAH. The fluorescence of LCH(2)(+) is strongly quenched likely by intra-molecular diabatic charge transfer and excited-state relaxation by potential surface touching with the ground state.     

Electrospray as a tool for drug micro- and nanoparticle patterning

Carbamazepine (CBZ) microparticles of different sizes and shapes, including spheres, q-tips, elongated spheres, and tear-shaped particles, were formed by electrospraying solutions of different CBZ concentrations. The particle characteristics were determined by the interplay between jet formation, droplet breakup, solvent evaporation, and eventual particle solidification. The average particle size increased with increasing CBZ concentration, with particles of different shapes being observed for different CBZ concentrations. The cascade of sizes and shapes observed was interpreted in terms of Rayleigh instability theory as applied to charged jets and droplets, with the final sizes depending upon the time needed to evaporate the solvent sufficiently for CBZ to solidify; the lower the initial concentration of CBZ, the smaller the final droplets/particles that are formed.     

Gas-phase photooxidation of alkenes by V-doped TiO2-MCM-41: mechanistic insights of ethylene photooxidation and understanding the structure-activity correlation

Nanoparticles of Ti(0.95)V(0.05)O(2) were found to be impregnated in the hexagonal channels of the MCM-41 host, with a distribution of some particles on the surface, thus leading to an effective variation in the particle size as a function of loading host MCM-41 matrix. These catalysts were subjected to the photocatalytic degradation of alkenes under the ambient conditions in which the photocatalytic activity varied as a function of the loading percentage of Ti(0.95)V(0.05)O(2) in the host MCM-41.This is explained in light of the structure-activity correlation, and the better catalytic activity can be attributed to an electronic interaction between the host and guest molecules, as established from X-ray photoelectron spectroscopy. To understand the mechanistic aspect of the photooxidation of ethylene on the vanadium-doped titania dispersed in the MCM-41 matrix, extensive in situ FTIR experiments were undertaken. The intermediate species produced on bare Ti(0.95)V(0.05)O(2) are different from that produced on the Ti(0.95)V(0.05)O(2)/MCM-41 surface. Moreover, different intermediates were produced during ethylene oxidation under UV and visible irradiation, thus leading to different rates. The ethylene decomposition over bare Ti(0.95)V(0.05)O(2) occurs by means of formation of ethoxy groups, transformed to acetaldehyde or enolates, subsequently to acetates, and then to CO(2) under both UV and visible irradiation. However, in the case of Ti(0.95)V(0.05)O(2)/MCM-41 catalyst with UV irradiation, the adsorbed acetaldehyde thus formed undergoes aldol condensation over the Lewis acid sites to lead to the formation of crotonaldehyde, which is subsequently oxidized to acetate and consequently to CO(2). It was observed that during visible irradiation labile ethyl acetate is produced either by the Tischenko reaction or by the reaction between the labile acetic acid and the unreacted ethoxy groups. The ethyl acetate produces acetic acid monomer, which is oxidized to CO(2). Furthermore, in this work the effects of particle size on the intermediate species were also studied.     

Kinetics of Alkali-Catalyzed 2,5-Dimethylphenol-Forrnaldehyde Reaction

A kinetic study of the reaction between 2,5-dimethylphenol (2,5-DMP) and formaldehyde has been carried out at 65, 70, 75, and 80 ± 0.05° C by using sodium hydroxide as the catalyst. The solvent mixture used in the kinetic experiments was 50% (v/v) methanol-water. The various alkali concentrations used were 0.003, 0.006, 0.010, 0.018 and 0.025 N. The reaction was found to obey second-order rate law. The rate of reaction was observed to increase with an increase in the alkali concentration. The effect of changing the reactant concentrations and the nature of the solvent was also studied. The overall rate constant has been resolved into stepwise rate constants. The entropy of activation and Arrhenius parameters for the overall reaction have also been calculated.     

Synthesis and Characterization of Photoactive Amphiphilic Polymers

We have synthesized light sensitive nano micelles for their applications as drug delivery agents. Various azo compounds having different dipole moment values have been covalently attached to an amphiphilic pegylated co‐polymer and their light sensitive behavior has been studied in both aqueous and organic media with ultra violet light to understand the light dependent supramolecular organization.     

Design and Lipase Catalyzed Synthesis of 4-Methylcoumarin-siloxane Hybrid Copolymers

Poly dimethylsiloxanes with amino end groups were copolymerized with diesters of 4-methylcoumarins enzymatically using a lipase (Candida antarctica lipase) as a biocatalyst. In a separate synthesis, 4-methylcoumarin was also incorporated into the poly siloxanes-isophthalate copolymers by functionalization of hydroxyl groups in the isophthalate moiety. The synthesis and characterization of two sets of novel copolymers are presented. The thermal and flammability properties of these polymers have also been studied using TGA and microcalorimetry, respectively.     

Ascorbic Acid/Hydrogen Peroxide–Initiated Graft Copolymerization of MMA onto Bombyx mori Silk Fibroin

Graft copolymerization of methyl methacrylate (MMA) onto Bombyx mori silk fibroin was investigated using ascorbic acid and hydrogen peroxide as redox system. The effect of different reaction parameters such as reaction time, temperature, and initiator and monomer concentrations on graft yield were determined. The optimum grafting conditions were found to be temperature = 65°C, time = 120 min, monomer concentration = 2.82 × 10^?1 mol/L, ascorbic acid concentration = 2.83 × 10^?2 mol/L, and H₂O₂ concentration = 1.58 × 10^?1 mol/L. The structural properties of the fibers were studied with the help of FT-IR, scanning electron microscopy (SEM), X-ray diffraction, and TGA techniques.