Assuming Energy Transfer Research Articles

Stimulated emission at 1.54 μm from erbium/oxygen-doped silicon-based light-emitting diodes

Silicon-based light sources, including light-emitting diodes (LEDs) and laser diodes (LDs) for information transmission, are urgently needed for developing monolithic integrated silicon photonics. Silicon with erbium ions ( Er 3 + ) doped by ion implantation is considered a promising approach, but it suffers from an extremely low quantum efficiency. Here we report an electrically pumped superlinear emission at 1.54 μm from Er/O-doped silicon planar LEDs, which are produced by applying a new deep cooling process. Stimulated emission at room temperature is realized with a low threshold current of ∼ 6 mA ( ∼ 0.8 A / cm 2 ). Time-resolved photoluminescence and photocurrent results have revealed the complex carrier transfer dynamics by relaxing electrons from the Si conduction band to the Er 3 + ion. This picture differs from the frequently assumed energy transfer via electron–hole pair recombination of the silicon host. Moreover, the amplified emission from the LEDs is likely due to a quasi-continuous Er/O-related donor band created by the deep cooling technique. This work paves the way for fabricating superluminescent diodes or efficient LEDs at communication wavelengths based on rare-earth-doped silicon.

Driven cast-in-situ piles installed using hydraulic hammers: Installation energy transfer and driveability assessment

The ability of hydraulic pile driving hammers to overcome energy losses during freefall enables a greater proportion of the impact energy to be transferred to the pile than is possible with diesel hammers. This percentage, termed the energy transfer ratio, is not routinely measured in practice however, and there is an element of uncertainty regarding appropriate energy transfer ratios to adopt in driveability analyses. In light of such uncertainties, the energy transfer ratios of four- and five-tonne hydraulic hammers were assessed during installation of driven cast-in-situ (DCIS) piles at several sites in the United Kingdom. The piling rigs were fitted with instrumentation enabling the hammer velocity (and hence kinetic energy) at impact to be inferred for each blow during installation, with the corresponding energy transferred to the closed-ended steel installation tube ascertained using a Pile Driving Analyzer. The results of the study showed that energy transfer ratios were strongly dependent on the hammer drop height, with transfer ratios of 90% and above only achievable for drop heights in excess of 50% of the maximum value. Driveability analyses using measured energy transfer ratios from the fieldwork, as well as the traditional method using an assumed energy transfer ratio which is independent of drop height, yielded good agreement with measured driving records at a clean sand site. In contrast, poorer predictions were apparent for both methods at another site, where the coarse-grained strata had higher fines contents and significant layering, with the traditional method (assumed energy transfer ratio) over-predicting the maximum transferred energy and concealing the influence of dynamic soil properties on driveability. The findings of the study would suggest that lower energy transfer ratios may need to be considered in driveability predictions for these pile types (i.e. steel or DCIS piles) if lower drop heights are used during driving.

Structure and photoluminescence properties of zinc oxide/ytterbium oxide nanocomposites

Nanocomposites based on zinc oxide and ytterbium oxide nanocrystals with Yb atomic content varied from 0.5 up to 6 % were synthesized from alcohol solutions followed by thermal annealing in air. The prepared samples were characterized by means of the X-ray diffraction, transmission electron microscopy and photoluminescence spectroscopy. A sharp photoluminescence line of Yb3+ ions was detected at 983 nm and it was stronger for the samples thermally annealed at 750 °C. The intensity of Yb3+ photoluminescence was found to be slightly depended on Yb concentration and to exhibit a square root dependence on excitation intensity. The experimental results are explained by assuming energy transfer from the photoexcited zinc oxide nanocrystals to ytterbium oxide nanoparticles on their surface. The obtained results indicate possible applications of Yb-doped zinc oxide nanostructures in optoelectronics devices and as infrared biomarkers.

Correlating mechanical work with energy consumption during gait throughout pregnancy.

BackgroundMeasures of mechanical work may be useful in evaluating efficiency of walking during pregnancy. Various adaptations in the body during pregnancy lead to altered gait, consequently contributing to the total energy cost of walking. Measures of metabolic energy expenditure may not be reliable for measuring energetic cost of gait during pregnancy as pregnancy results in numerous metabolic changes resulting from foetal development. Therefore, the aim of this study is to determine if mechanical work prediction equations correlate with the metabolic energy cost of gait during pregnancy.MethodsThirty-five (35) women (27.5 ± 6.1 years) gave informed consent for participation in the study at different weeks of gestation pregnancy. Gas exchange and gait data were recorded while walking at a fixed self-selected walking speed. External (Wext) work was estimated assuming no energy transfer between segments, while internal work (Wint) assumed energy transfer between segments. Hence total energy of the body (Wtot) was calculated based on the segmental changes relative to the surrounding, and relative to the centre of mass of the whole body. Equations for mechanical work were correlated with net and gross O2 rate, and O2 cost.ResultsExternal, internal and total mechanical energy showed significant positive relationship with gross O2 rate (r = 0.48, r = 0.35; and r = 0.49 respectively), and gross O2 cost (r = 0.42; r = 0.70, and r = 0.62, respectively). In contrast, external, internal and total mechanical energy had no significant relationship with net O2 rate (r = 0.19, r = 0.24, and r = 0.24, respectively). Net O2 cost was significant related Wext (r = 0.49) Wint (r = 0.66) and Wtot (r = 0.62). Energy recovery improved with increase in gait speed.ConclusionsMeasures of mechanical work, when adjusted for resting energy expenditure, and walking speed may be useful in comparing metabolic energy consumption between women during pregnancy, or assessment or gait changes of the same individual throughout pregnancy.

Evolutionary scenario for W UMa-type stars

AbstractAn alternative to TRO model of a W UMa-type star is presented in which the binary is past mass exchange with mass ratio reversal. The secondary is hydrogen depleted and both components are in thermal equilibrium. Evolution in contact is driven by orbital angular momentum loss and mass transfer from the secondary to primary component, similarly as it is observed in Algols. Temperature equalization of both components results from an assumed energy transfer by a large scale flow encircling the whole system in the common envelope.

Scanning Tunneling Spectroscopy and Luminescent Properties of ZnS:Mn,Cu,Cl Thin Films

Electrical and luminescent properties of ZnS:Mn,Cu,Cl thin films were investigated. Combined both studies: scanning tunneling microscopy and scanning tunneling spectroscopy were made. The current and differential conductance versus applied voltage were measured for the ZnS:Mn,Cu,Cl thin films. Additionally, the spectral and kinetic properties of the electroluminescent cells based on the ZnS:Mn,Cu,Cl thin films were measured. The maximum of the electroluminescence lies at 586 nm. The electroluminescence was excited by rectangular wave voltage pulses with pulse length from 1 μs to 1 ms. It was shown that time dependence of the electroluminescence is well explained assuming energy transfer between monomolecular centres.

Modeling the Organic Nitrate Yields in the Reaction of Alkyl Peroxy Radicals with Nitric Oxide. 2. Reaction Simulations

Master equation calculations are used to model gas-phase literature experimental data for alkyl nitrate formation via the following reaction system of reversible reactions: (1) RO2 + NO ↔ ROONO, (2) ROONO ↔ RO + NO2, (3) ROONO ↔ RONO2, and (4) RONO2 ↔ RO + NO2 for R = CH3, i-C3H7, and 2-C5H11. The structures and thermochemistry of the stable species are based on electronic structure calculations described in the preceding companion paper in this issue (Lohr et al. J. Phys. Chem. A 2003, 107, xxx−xxx). Literature data for recombination rate constants are used to constrain the model calculations. Several transition state models and a range of energy transfer parameters are investigated. The results for R = CH3 show that a wide variety of plausible transition state models for k-4 gives good agreement with experiment for reaction (−4), because changes in assumed energy transfer parameters can compensate for differences between the transition state models. It is concluded that recombination reactions are good sources of absolute energy transfer parameters only when transition state properties are known with great accuracy. Although satisfactory models are obtained for the individual systems, the parameters cannot be transferred reliably from one system to another. Master equation models can be made to reproduce the experimental 2-pentyl nitrate yields from the title reaction as long as 〈ΔE〉down, the average energy transferred in deactivating collisions, is assumed to be surprisingly, and perhaps unphysically, small (∼25 cm-1), regardless of assumptions about the barrier to isomerization reaction 3. Several critical assumptions in the master equation models are examined, but none of them accounts for the small value of 〈ΔE〉down. It is concluded that new experiments should be carried out to verify or possibly revise the pressure-dependent alkyl nitrate yield data currently available in the literature.

Steroids as Molecular Photonic Wires.Z→EOlefin Photoisomerization by Intramolecular Triplet−Triplet Energy Transfer with and without Intervening Olefinic Gates1

The steroids 3β-((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-5α-androstane (1), 3β-((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-5-androstene (2), 3β-((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-6-methylene-5α-androstane (3a), and 3α-((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-6-methylene-5α-androstane (3b) have been prepared. The triplet−triplet excited-state energy transfer (TTET) that occurs from the C3 aryl “donor” group to the C17 ethylidene “acceptor” has been studied in detail at 10 mM steroid concentration. Irradiation with 266 nm light results in Z → E olefin isomerization of the C17 ethylidene group, a consequence of both intra- and interTTET. ΦZ→E = 0.037, 0.018, 0.028, and 0.004 for 1, 2, 3a, and 3b, respectively. Detailed kinetic analyses of these compounds and appropriate models, with and without added olefin quenchers, provide a complete set of rate constants which are determined relative to an assumed energy transfer rate constant to piperylene of 7.0 × 109 M-1 s-1. In particular, kintraTTET for 1 = [1.7 (±0.7)] × 106 s-1. Isomerization at C17 in 2, due to intraTTET, is reduced (83% vs 1) but not completely eliminated by the endocyclic alkene in ring B, which functions as a “triplet gate”. The exocyclic methylene group in 3b is more efficient in gating the intraTTET than it is in 3a (ΦTTET = 0.08 vs 0.71, respectively). This higher level of gating that occurs in 3b is attributed to a much shorter lifetime of the axial DPSO triplet, caused by an efficient through-space intraTTET from the axial DPSO group to the C6 exocyclic olefin.

Theoretical description of luminescence during exciting pulse

Two types of time dependence of luminescence excited by rectangular pulse were discussed. Direct excitation and energy transfer were taken into account to explain the dependence during pulse excitation. A model, which assumed energy transfer from one monomolecular centre to another was used. A theoretical relation, describing the time dependence of luminescence, was obtained. The relation is qualitatively in good agreement with observations cited in the literature.

UV-Empfindlichkeit von λ-Phagen mit variablem Bromuracil-Gehalt / UV-Sensitivity of λ-Phages with Various BU-Contents

Heat induction of E. coli CR 34 (λcI857)/λ growing in media with variing bromouracil (BU)-contents yielded A-phages with different amounts of BU in their DNA-strands. The percentage substitution of thymine was determined by buoyant density measurements in a CsCl-density gradient. After irradiation of the phages with long wavelength UV (302/313 nm) and short wavelength UV (254 nm) the sensitivity (loss of infectivity) was determined as a function of the degree of BU-substitution. It was found that after irradiation with long wavelength UV the cross section is proportional to the percentage BU-substitution, whereas after irradiation with short wavelength UV it is not. The results can be explained by assuming energy transfer from the four DNA-bases to the BU.

Reaction between primary and secondary electron acceptors of Photosystem II of photosynthesis

We studied some properties and interactions of intermediate ‘Q’, the fluorescence-quenching trapping center of the O 2-evolving photosystem, and its associated electron acceptor, ‘A’. 1. 1. Direct observation of the oxidation in darkness of photoreduced Q by the fully oxidized A pool revealed a half-time of the reaction of approx. 0.6 msec at room temperature. 2. 2. A study of the time course of the reduction of pool A in the light, using the chloroplast's capacity to reduce 2,6-dichlorophenolindophenol in the subsequent darkness as a measure of the pool, showed pool A to be unhomogeneous, 1 3 of it reacting more rapidly with Q - than the remainder. For this rapidly reacting fraction we estimated a bimolecular rate constant k 1 = 165 (pool units) −·sec −, with one pool unit corresponding to [Q] tot. 3. 3. From fluorescence-rise curves, effected by a series of brief flashes (converting Q only), we computed 18 for the ratio of the concentrations of A and Q. 4. 4. The shape of the rise curve in weak light was analyzed in terms of a bimolecular reaction between Q and A, varying the parameters: ratio [A]/[Q] and equilibrium constant K. Good fit with the experiment was obtained by assuming energy transfer between photosynthetic units, an [A]/[Q] ratio of approx. 20, and K < 10. 5. 5. It was concluded that the small initial rise of the total rise curve did not reflect the photoreduction of Q, but rather the activation of this trapping center.

Energy Transfer and Quenching in Plastic Scintillators

Abstract Luminescence quenching in polystyrene + 9,10-diphenylanthracene + quencher and polystyrene + quencher systems has been investigated. In the latter system, the residual styrene monomer is regarded as a fluorescent solute. Result show that solvent quenching observed under uv excitation is due mainly to the competition between dipole-dipole energy transfer from polystyrene to the fluor and that from polystyrene to the quencher, and that such a transfer occurs directly from an excited polystyrene segment before it forms an excimer with an unexcited segment. Solvent quenching under beta-ray excitation was generally greater than that under uv excitation. It is suggested that this phenomenon may result from the occurrence of energy transfer at higher excited state of polystyrene. Solute quenching in some series of polystyrene + 9,10-diphenylanthracene + quencher is not accounted for by usual mechanisms of quenching. Such quenching can be explained by assuming energy transfer from the singlet excited state of 9,10-diphenylanthracene to the triplet state of the quencher.