Forward Energy Transfer Research Articles

Subharmonic Energy Transfer from the Semidiurnal Internal Tide to Near-Diurnal Motions over Kaena Ridge, Hawaii

Abstract Nonlinear energy transfers from the semidiurnal internal tide to high-mode, near-diurnal motions are documented near Kaena Ridge, Hawaii, an energetic generation site for the baroclinic tide. Data were collected aboard the Research Floating Instrument Platform (FLIP) over a 35-day period during the fall of 2002, as part of the Hawaii Ocean Mixing Experiment (HOME) Nearfield program. Energy transfer terms for a PSI resonant interaction at midlatitude are identified and compared to those for near-inertial PSI close to the M2 critical latitude. Bispectral techniques are used to demonstrate significant energy transfers in the Nearfield, between the low-mode M2 internal tide and subharmonic waves with frequencies near M2/2 and vertical wavelengths of O(120 m). A novel prefilter is used to test the PSI wavenumber resonance condition, which requires the subharmonic waves to propagate in opposite vertical directions. Depth–time maps of the interactions, formed by directly estimating the energy transfer terms, show that energy is transferred predominantly from the tide to subharmonic waves, but numerous reverse energy transfers are also found. A net forward energy transfer rate of 2 × 10−9 W kg−1 is found below 400 m. The suggestion is that the HOME observations of energy transfer from the tide to subharmonic waves represent a first step in the open-ocean energy cascade. Observed PSI transfer rates could account for a small but significant fraction of the turbulent dissipation of the tide within 60 km of Kaena Ridge. Further extrapolation suggests that integrated PSI energy transfers equatorward of the M2 critical latitude may be comparable to PSI energy transfers previously observed near 28.8°N.

Tracing of backward energy transfer from LH1 to LH2 in photosynthetic membranes grown under high and low irradiation.

By introducing derivative transient absorption spectroscopy, we obtain rate constants for backward and forward energy transfer between LH1 and LH2 complexes in purple bacterial membranes. We find that backward energy transfer is strongly reduced in membranes grown under low irradiation conditions, compared to high light grown ones. We conclude that backward energy transfer is managed actively by the bacteria to avoid LH1 exciton deactivation under high irradiation conditions. The analytical method is generally applicable to excitonically coupled systems.

Photoluminescence quantum efficiency and energy transfer of ErRE silicate (RE = Y, Yb) thin films

Photoluminescence quantum efficiency and energy transfer were studied in ErRE (RE = Y, Yb) silicate thin films. For the first time, we experimentally determined the radiative/non-radiative decay rates of ErYb and ErY silicates and obtained photoluminescence quantum efficiencies of 0.28 ± 0.08 and 0.39 ± 0.13, respectively. Energy transfer between Yb and Er in ErYb silicate was investigated by comparing the effective excitation cross sections of ErYb and ErY silicate. At different pumping wavelengths, forward and backward energy transfer caused by the presence of Yb was experimentally demonstrated.

Tracking energy transfer between light harvesting complex 2 and 1 in photosynthetic membranes grown under high and low illumination

Energy transfer (ET) between B850 and B875 molecules in light harvesting complexes LH2 and LH1/RC (reaction center) complexes has been investigated in membranes of Rhodopseudomonas palustris grown under high- and low-light conditions. In these bacteria, illumination intensity during growth strongly affects the type of LH2 complexes synthesized, their optical spectra, and their amount of energetic disorder. We used a specially built femtosecond spectrometer, combining tunable narrowband pump with broadband white-light probe pulses, together with an analytical method based on derivative spectroscopy for disentangling the congested transient absorption spectra of LH1 and LH2 complexes. This procedure allows real-time tracking of the forward (LH2→LH1) and backward (LH2←LH1) ET processes and unambiguous determination of the corresponding rate constants. In low-light grown samples, we measured lower ET rates in both directions with respect to high-light ones, which is explained by reduced spectral overlap between B850 and B875 due to partial redistribution of oscillator strength into a higher energetic exciton transition. We find that the low-light adaptation in R. palustris leads to a reduced elementary backward ET rate, in accordance with the low probability of two simultaneous excitations reaching the same LH1/RC complex under weak illumination. Our study suggests that backward ET is not just an inevitable consequence of vectorial ET with small energetic offsets, but is in fact actively managed by photosynthetic bacteria.

The 1.2 and 2.0 μm Emission from Ho3+ in Glass Ceramics Containing BaF2 Nanocrystals

Ho31 and/or Yb31 doped glass ceramics containing spherical BaF2 nanocrystals sized 10–20 nm were prepared by melt quenching and subsequent thermal treatment. Radiative properties such as transition probabilities (A), radiative lifetimes (τ), branching ratios (β) along with absorption and emission cross sections have been calculated from the absorption spectra. Upon 637 nm excitation, Ho31‐doped glass ceramics yield an intense infrared emission at 1191 nm with a full‐width at half‐maximum of 80 nm and weaker emissions at 980 and 1473 nm. Efficient 1.2 and 2.0 μm infrared emissions have been achieved in Ho31/Yb31 co‐doped glass ceramics under 980 nm excitation, demonstrating the occurrence of energy transfer (ET) from Yb31 to Ho31. The microscopic parameters of the ET processes between Yb31 and Ho31 have been evaluated, revealing that the forward ET Yb31 → Ho31 is more efficient than the backward one and both the processes are mainly assisted by one‐phonon emission (annihilation).

Transfer and backtransfer processes in Yb 3+–Er 3+ codoped Strontium Barium Niobate glass-ceramics

The forward and backward energy transfer processes in Strontium Barium Niobate glass-ceramics double doped with Yb 3+ and Er 3+ ions have been studied. In these samples the rare earth ions are incorporated into the nanocrystals with an average size of 50 nm. Using laser excitation at 950 nm is possible to excite selectively the Yb 3+ ions and detect emission due to these ions (at 1040 nm) or combined with the Er 3+ ions (at 980 nm). In previous works, the energy transfer processes between these ions in different matrices have been analyzed in order to improve the emission at 1550 nm, but these analyses are restricted to fast migration processes among ions. In this fast migration regimen the results are valid only for larger concentrations. However, in this work the dynamics of these transfer processes has been carried out using a general method called “transfer function model”. The parameters which characterize these processes have been obtained and it has been possible to explain the important increase of the emission at 1550 nm due to the co-doping with Yb 3+ ions. This analysis is valid for any range of doping concentrations.

Upconversion luminescence properties of Y2SiO5: Er3+ ,Yb3+ ,Tm3+ synthesized by co-precipitation method

A series of up-conversion luminescence materials of Y1.98-2xYb2xEr0.02SiO5(0.00≤x≤0.15) and Y1.736Yb0.24Er0.02Tm0.004SiO5 were synthesized by co-precipitation method.The absorption spectra from 400 to 1600 nm of Y1.98-2xYb2x Er0.02SiO5 (x=0.00,0.08) were compared, and the up-conversion emission spectra of all samples were investigated when excited by 976 nm opo laser. The decay curves of Er3+4S3/2(4F9/2)→ 4I15/2,Tm3+1G4→3H6,as well as the relationship between the blue up-conversion emission intensity and pumping power were measured,consequently, the possible up-conversion luminescence mechanisms of Er3+,Tm3+ in Y2SiO5 were discussed. The results show that X2-type monoclinic Ln3+(Ln3+=Er3+,Yb3+,Tm3+) co-doped Y2SiO5 phosphors have been synthesised under relatively low temperature of 1250 ℃, and the absorption intensity and bandwidth are remarkably increased by Yb3+ sensitizing around 976 nm. Up-conversion emission spectra has been analyzed and the results suggest that both the green and red lights increase first and then decrease with the increase of the Yb3+ doping concentration. However,the concentration quenching of the red light emission is higher, due to the forward (ETU3) and back (ETU4) energy transfer between Yb3+ and Er3+ ions; the blue emission occurs mainly through phonon-assisted energy transfer between Yb3+ and Tm3+ ions, which is a three photon absorption process.

Nonlocal Interaction of Inverse Magnetic Energy Transfer in Hall Magnetohydrodynamic Turbulence

A detailed analysis of forward and inverse energy transfer processes due to the Hall term effect in freely decaying, homogeneous, isotropic Hall magnetohydrodynamics (HMHD) turbulence is performed through Fourier and wavelet analyses. We analyzed three snapshot datasets that were taken from such a period to allow the turbulence to develop sufficiently with a nearly constant magnetic Reynolds number. Because the Fourier energy spectra in these snapshots show remarkable agreement after the normalization in terms of the dissipation rates and the diffusion coefficients, they are considered as a universal equilibrium state. By analyzing the numerical solutions that are generated without any external forcing, it is confirmed that the inverse energy transfer due to the Hall term effect is intrinsic to HMHD dynamics. Orthonormal divergence-free wavelet analysis reveals that nonlinear mode interactions contributing to the inverse energy transfer exhibit a nonlocal feature, while those for the forward transfer are dominated by a local feature.

Efficient non-resonant energy transfer in Nd^3+-Yb^3+ codoped Ba-Al-metaphosphate glasses

An efficient Nd3+→Yb3+ energy transfer in a new series of alkali-free barium-alumino-metaphosphate glasses with a transfer efficiency reaching up to 95% has been reported here. It is due to the effective phonon assistance arising with the excellent matching of the present host phonon energy with the energy mismatch between Nd3+(4F3/2) and Yb3+(2F5/2) excited levels. The energy transfer microparameters for Nd3+→Yb3+ forward and back energy transfers are estimated from the spectral data analysis. A parameter, ΦET(=CDANd-Yb/CDANd-Nd), is proposed as a quantitative measure of sensitization ability per unit loss of the donor. The parameter is found to be highest for the presently reported barium-alumino-metaphosphate glasses.

2.0 μ m emission properties and energy transfer processes of Yb3+/Ho3+ codoped germanate glass

Ho 3 + doped germanate glass sensitized by Yb3+ has been investigated for potential application as a near-infrared laser material. The characteristic temperatures are determined by differential scanning calorimeter, and the absorption and emission spectra are measured. According to absorption spectra, Judd–Ofelt parameters and radiative transition probabilities are calculated and analyzed by Judd–Ofelt theory. The intensity of emission spectra shows a strong dependence upon the Yb3+ concentration. The result also indicates that Yb3+ plays an important role in the emission of 2.0 μm by transferring the 980 nm excitation energy to Ho3+. The energy transfer process from Yb3+ to Ho3+ is analyzed and the result shows that coefficient of the forward energy transfer Yb3+→Ho3+ is 19 times of magnitude larger than that of the backward energy transfer Yb3+←Ho3+. It is found that the highest gain in 2.0 μm region could be achieved from the germanate glass with 5.0 mol % Yb2O3 and 1.0 mol % Ho2O3. The present Yb3+/Ho3+ codoped germanate glass with high emission cross section of Ho3+:I57→I58 at 2.0 μm and efficient energy transfer from Yb3+ to Ho3+ will be a suitable material for developing solid state lasers around 2.0 μm.

Using EMGs and kinematics data to study the take-off technique of experts and novices for a pole vaulting short run-up educational exercise

This study attempts to characterise the electromyographic activity and kinematics exhibited during the performance of take-off for a pole vaulting short run-up educational exercise, for different expertise levels. Two groups (experts and novices) participated in this study. Both groups were asked to execute their take-off technique for that specific exercise. Among the kinematics variables studied, the knee, hip and ankle angles and the hip and knee angular velocities were significantly different. There were also significant differences in the EMG variables, especially in terms of (i) biceps femoris and gastrocnemius lateralis activity at touchdown and (ii) vastus lateralis and gastrocnemius lateralis activity during take-off. During touchdown, the experts tended to increase the stiffness of the take-off leg to decrease braking. Novices exhibited less stiffness in the take-off leg due to their tendency to maintain a tighter knee angle. Novices also transferred less energy forward during take-off due to lack of contraction in the vastus lateralis, which is known to contribute to forward energy transfers. This study highlights the differences in both groups in terms of muscular and angular control according to the studied variables. Such studies of pole vaulting could be useful to help novices to learn expert's technique.

Energy transfer between Yb3+ and Er3+ in barium gallogermanate glass

Spectroscopic properties of Er3+/Yb3+ codoped barium gallogermanate glass demonstrate that an efficient energy transfer from Yb3+ to Er3+ ions occurs while the back energy transfer from Er3+ to Yb3+ ions cannot be ignored. Based on the rate equations of electron transitions, the forward energy transfer Yb3+(F25/2)+Er3+(I415/2)→Yb3+(F27/2)+Er3+(I411/2) and the back transfer Er3+(I411/2)+Yb3+(F27/2)→Er3+(I415/2)+Yb3+(F25/2) coefficients were calculated to be 9.2×10−17 cm3 s−1 and 1.1×10−17 cm3 s−1, respectively. Also the energy transfer upconversion coefficient Yb3+(F25/2)+Er3+(I411/2)→Yb3+(F27/2)+Er3+(F27/2) was determined to be 1.1×10−16 cm3 s−1. The calculated results show that a population inversion between the I413/2 and I415/2 levels can be attained at a low pumping flux of around 1.7×1022 cm−2 s−1, while the inversion threshold between the S43/2 and I415/2 levels is about 30 times higher when pumped by 980 nm laser diode. With increasing pump flux the back energy transfer probability becomes larger and when the pump flux is more than 2.0×1022 cm−2 s−1, the back energy transfer probability dominates the forward energy transfer probability.

Hybrid Monte-Carlo simulations of fluorescence anisotropy decay in disordered two-component systems in the presence of forward and back energy transfer

Hybrid Monte-Carlo simulation algorithm of fluorescence anisotropy decay in the presence of forward and back energy transfer is presented for rigid disordered systems. The results of hybrid Monte-Carlo simulations are compared with the results of SCDM theoretical model and with Monte–Carlo step by step simulations. Very good agreement between theoretical and simulation data as well as Monte–Carlo simulations has been obtained. The hybrid Monte-Carlo simulation of emission anisotropy is characterized by much better accuracy and little time consumption, especially for long times and also for low donor and at the same time high acceptor concentrations.

Fluorescence anisotropy decay in the presence of multistep energy migration and back transfer in disordered two-component systems

Fluorescence anisotropy decay in the presence of forward and back energy transfer is studied for rhodamine 6G (donor)–rhodamine B (acceptor) rigid system. Experimental data are compared with the results of Monte-Carlo simulations. Good agreement between experimental data and Monte-Carlo simulations has been obtained. Monte-Carlo simulation of emission anisotropy is performed using an originally elaborated by us hybrid method, which is characterized by much better accuracy and little time consumption.

Energy-Transfer Mechanism in Photoluminescent Terbium(III) Complexes Causing Their Temperature-Dependence

Abstract Photoluminescence of Terbium(III) complexes was investigated as a function of temperatures in the range of 80–280 K for [Tb(bfa)3(H2O)2] (bfa: 4,4,4-trifluoro-1-phenyl-1,3-butanedionato), [Tb(hfa)3(H2O)3] (hfa: hexafluoroacetylacetonato), [Tb(tfa)3(H2O)2] (tfa: trifluoroacetylacetonato), [Tb(acac)3(H2O)3] (acac: acetylacetonato), and [Tb(hfa)3(tppo)2] (tppo: triphenylphosphine oxide). These complexes were classified into the two groups with different temperature-dependences. The first group consisting of [Tb(bfa)3(H2O)2], [Tb(tfa)3(H2O)2], and [Tb(acac)3(H2O)3] showed a dependence determined by the energy gap between the excited triplet state of the ligand and the emitting level of terbium(III) ion. In contrast, for [Tb(hfa)3(H2O)3] and [Tb(hfa)3(tppo)2] containing hfa as a ligand, not only the energy gap but also the energy barriers of the “Forward energy transfer” from the ligand to terbium(III) ion and “Back energy transfer” from terbium(III) ion to the ligand were taken into account for understanding their dependences. These results are discussed based on the re-orientation of the complexes accompanied by the forward and back energy transfer processes using DFT calculations.

A Non-Linear SGS Model Based On The Spatial Velocity Increment

A new subgrid scale (SGS) modelling concept for large-eddy simulation (LES) of incompressible flow is proposed based on the three-dimensional spatial velocity increment δ u i . The new model is inspired by the structure function formulation developed by Metais and Lesieur [39] and applied in the context of the scale similarity type formulation. First, the similarity between the SGS stress tensor τ ij and the velocity increment tensor Q ij = δ u i δ u j is analyzed analytically and numerically using a priori tests of fully developed pipe flow at Re τ = 180. Both forward and backward energy transfers between resolved and unresolved scales of the flow are well predicted with a SGS model based on Q ij . Secondly, a posteriori tests are performed for two families of turbulent shear flows. LES of fully developed pipe flow up to Re τ = 520 and LES of round turbulent jet at Re D = 25000 carried out with a dynamic version of the model provide promising results that confirm the power of this approach for wall-bounded and free shear flows.

Investigation of the subgrid-scale stress and its production rate in a convective atmospheric boundary layer using measurement data

The subgrid-scale (SGS) stress in the atmospheric surface layer is studied using measurement data. Field measurements employing a novel array technique were conducted to provide data for obtaining resolvable- and subgrid-scale variables. We analyse the conditional SGS stress and the conditional stress production rate conditional on the resolvable-scale velocity, which must be reproduced by the SGS model for large-eddy simulation (LES) to predict correctly the one-point resolvable-scale velocity statistics. The results show that both buoyancy and shear play important roles in the physics of the SGS stress. Strong buoyancy and vertical shear associated with updrafts and positive streamwise velocity fluctuations cause conditional forward energy transfer and strong anisotropy in the conditional SGS stress. Downward returning flows associated with large convective eddies result in conditional energy backscatter and much less anisotropic SGS stress. Predictions of the conditional SGS stress and the conditional stress production rate predicted using several SGS models are compared with measurements. None of those models are able to predict correctly the trends of both statistics. The Smagorinsky and one nonlinear model under-predict the anisotropy and the variations of the anisotropy, whereas the other nonlinear model and the mixed model over-predict both. The deficiencies of the SGS models that cause inaccurate LES statistics, such as the over-prediction of the mean shear and under-prediction of the vertical velocity skewness, are identified. The present study shows that analyses of conditional SGS stress and conditional SGS stress production provide a systematic approach for studying SGS physics and evaluating SGS models and can potentially be used to target specific aspects of LES that are important for a given application.

Energy fluxes and shell-to-shell transfers in three-dimensional decaying magnetohydrodynamic turbulence

A spectral analysis of the energy cascade in magnetohydrodynamics (MHD) is presented using high-resolution direct numerical simulation of decaying isotropic turbulence. The Fourier representations of both the velocity and the magnetic fields are split into subsets that correspond to shells of wave vectors. A detailed study of the shell-to-shell interactions is performed and a comparison with theoretical prediction based on field-theoretic method is proposed. Two different definitions for the forward and backward energy transfers are suggested. They provide diagnostics that can be used in order to assess subgrid-scale modeling in large eddy simulation for turbulent MHD systems.

Energy-transfer and light-emission mechanism of blue phosphorescent molecules in guest-host systems

We investigated the energy-dissipative processes of blue phosphorescent molecules of bis[(4,6-difluorophenyl)-pyridinato-N,C 2′](picolinate) iridium(III) (FIrpic) doped into two host molecules, 4,4′- N,N′-dicarbazol-biphenyl (CBP) and 4,4′-bis(9-carbazolyl)-2,2′-dimethyl-biphenyl (CDBP), which have different triplet energies. The phosphorescence spectra were measured over a wide range of temperatures from 8 to 295 K. For FIrpic in a CBP host, we observed both forward and backward energy transfer from the FIrpic triplet states to the CBP triplet states, resulting in unique temperature dependence of the phosphorescence intensity. However, the phosphorescence intensity of FIrpic in a CDBP host did not show any temperature dependence, indicating that the triplet energy was efficiently confined on the FIrpic molecules.

Forward (singlet-singlet) and backward (triplet-triplet) energy transfer in a dendrimer with peripheral naphthalene units and a benzophenone core.

The photochemical and photophysical behaviour of two dendrimers consisting of a benzophenone core and branches that contain four (4) and eight (5) naphthalene units at the periphery has been investigated in CH(2)Cl(2) solution (298 K) and in CH(2)Cl(2)/CHCl(3) 1:1 v/v rigid matrix (77 K). For comparison purposes, the photophysical properties of dimethoxybenzophenone (1), 2-methylnaphthalene (2) and of a dendron containing four naphthalene units (3) have also been studied. In both dendrimers 4 and 5, excitation of the peripheral naphthalene units is followed by fast (1.1 x 10(9) s(-1) at 298 K, > 2.5 x 10(9) s(-1) at 77 K for 5; 2.9 x 10(8) s(-1) at 298 K, 7 x 10(5) s(-1) at 77 K for 5) singlet-singlet energy transfer to the benzophenone core. On a longer time scale (>1 x 10(6) s(-1) at 298 K, >6 x 10(3) s(-1) at 77 K for 4; 3.1 x 10(7) s(-1) at 298 K, ca. 3 x 10(2) s(-1) at 77 K for 5) a back energy transfer process takes place from the triplet state of the benzophenone core to the triplet state of the peripheral naphthalene units. Selective excitation of the benzophenone unit is followed by intersystem crossing and triplet-triplet energy transfer to the peripheral naphthalene units. In hydrogen donating solvents, the benzophenone core is protected from degradation by the presence of the naphthalene units. In solutions containing Tb(CF(3)SO(3))(3), sensitization of the green Tb(3+) luminescence is observed on excitation of both the peripheral naphthalene units and the benzophenone core of 5. Upon excitation of the naphthalene absorption band (266 nm) with a laser source, intradendrimer triplet-triplet annihilation of naphthalene excited states leads to delayed naphthalene fluorescence (lambda(max)= 335 nm), that can also be obtained upon excitation at 355 nm (benzophenone absorption band). The results obtained show that preorganization of photoactive units in a dendritic structure can be exploited for a variety of useful functions, including photosensitized emission, protection from undesired photoreactions, and energy up-conversion.