Dynamics of Tm-Ho energy transfer and deactivation of the F-3(4) low level of thulium in fluorozirconate glasses

The mechanism involved in the Tm3+ (F-3(4))-->Ho3+ (I-5(7)) energy transfer and Tm3+ (H-3(4), H-3(6))-->Tm3+ (F-3(4), F-3(4)) cross relaxation as a function of the donor and acceptor concentrations was investigated in Tm-Ho-codoped fluorozirconate glasses. The experimental transfer rates were determined for the Tm-->Ho energy transfer from the best fit of the acceptor luminescence decay using an expression which takes into account the Inokuti-Hirayama model and localized donor-to-acceptor interaction solution. The original acceptor solution derived from the Inokuti-Hirayama model fits well the acceptor luminescence transient only for low-concentrated systems. The results showed that a fast excitation diffusion that occurs in a very short time (t<acceptor (A). A localized donor-to-acceptor interaction takes place, leading to an exponential decay of donors as an average of the microscopic rate equation solution of each D-*-A pair separated by distance R that contributes in addition to the Inokuti-Hirayama solution. The observation that the experimental transfer rates were always much bigger than the one predicted by the diffusion model, in which the energy transfer process is assisted by excitation migration among donors state, reinforces the existence of a fast excitation diffusion among donor ions before the energy transfer to acceptor already observed in Yb:Er:ZBLAN. The fast excitation diffusion effect was observed to dominate both the Tm-->Tm cross relaxation and Tm-->Ho energy transfer ions from H-3(4) and F-3(4) thulium states, respectively. (C) 2004 American Institute of Physics.

Poole-Frenkel effect in Er doped SnO2 thin films deposited by sol-gel-dip-coating

Electrical properties of Er-doped SnO2 thin films obtained by sol-gel-dip-coating technique were measured. When compared to undoped tin dioxide, rare-earth doped films present much higher resistivity, indicating that Er3+ presents an acceptor-like character into the matrix, which leads to a high degree of electric charge compensation. Current-voltage characteristics, measured above room temperature for Er-doped films, lead to non-linear behavior and two conduction regimes. In the lower electric field range the conduction is dominated by Schottky emission over the grain boundary potential barrier, which presents an average value of 0.85 eV. Increasing the applied bias, a second regime of conduction is observed, since the Poole-Frenkel coulombic barrier lowering becomes a significant effect. The obtained activation energy for ionization is 0.67 eV. (C) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Two thermal methods to measure the energy fluence of a brief exposure of diagnostic x rays

This paper describes two simple thermal methods for measuring the energy fluence in J/cm 2 from a diagnostic x-ray exposure. Both detectors absorb essentially 100% of the radiation and give a signal that is directly proportional to the energy fluence of the x-ray beam. One detector measures the thermal effect when a pulse of x rays is totally absorbed in the pyroelectric detector of lead-zirconium-titanate (PZT). The other detector measures the expansion of a gas surrounding a lead disk detector in a photoacoustic chamber. The increased pressure of the gas is transmitted through a 1-mm duct to a sensitive microphone. Both detectors have previously been used to measure the energy fluence rate of continuous x-ray beams in the same energy region using a chopped beam and a lock-in amplifier. Measurement of the energy fluence of a pulse of radiation eliminates the need for the beam chopper and lock-in amplifier and results in a simple, rugged, and inexpensive dosimeter. Either method can be combined with the area of the beam to give an estimate of the imparted energy to the patient from a diagnostic x-ray exposure.

Molecular design of new P3HT derivatives: Adjusting electronic energy levels for blends with PCBM

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry Characterization of Primary Amine End-Functionalized Polystyrene and Poly(methyl methacrylate) Synthesized by Living Anionic Polymerization Techniques

The reaction of living anionic polymers with 2,2,5,5-tetramethyl-1-(3-bromopropyl)-1-aza-2,5- disilacyclopentane (1) was investigated using coupled thin layer chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Structures of byproducts as well as the major product were determined. The anionic initiator having a protected primary amine functional group, 2,2,5,5-tetramethyl- 1-(3-lithiopropyl)-1-aza-2,5-disilacyclopentane (2), was synthesized using all-glass high-vacuum techniques, which allows the long-term stability of this initiator to be maintained. The use of 2 in the preparation of well-defined aliphatic primary amine R-end-functionalized polystyrene and poly(methyl methacrylate) was investigated. Primary amino R-end-functionalized poly(methyl methacrylate) can be obtained near-quantitatively by reacting 2 with 1,1-diphenylethylene in tetrahydrofuran at room temperature prior to polymerizing methyl methacrylate at -78 °C. When 2 is used to initiate styrene at room temperature in benzene, an additive such as N,N,N',N'- tetramethylethylenediamine is necessary to activate the polymerization. However, although the resulting polymers have narrow molecular weight distributions and well-controlled molecular weights, our mass spectra data suggest that the yield of primary amine α-end-functionalized polystyrene from these syntheses is very low. The majority of the products are methyl α-end-functionalized polystyrene.

Perturbative analysis of the triply differential cross section and circular dichroism in photo-double-ionization of He

We extend application of our lowest-order perturbative approach (in electron-electron correlation) for analysis of photo-double-ionization (PDI) of He [A.Y. Istomin et al., J. Phys. B 35, L543 (2002)] to excess energies up to 450 eV and to analysis of circular dichroism. We find that account of electron correlation in the final state to first order provides predictions for the triply differential cross section and circular dichroism that are in reasonable agreement with absolute data for excess energies up to 80 eV. For an excess energy of 450 eV, account of electron correlation in both initial and final states is necessary and the predicted triply differential cross sections are in agreement with absolute data only for large mutual ejection angles. We find that at excess energies of a few tens of eV, the PDI is dominated by the "virtual" knock-out mechanism, while the "direct" (on-shell) knock-out process gives only small contributions for large mutual ejection angles. As a result, we conclude that the circular dichroism effect at these energies originates from the nonzero electron Coulomb phase shifts.

Characterization of an extrapolation chamber for low-energy X-rays: Experimental and Monte Carlo preliminary results

The extrapolation chamber is a parallel-plate ionization chamber that allows variation of its air-cavity volume. In this work, an experimental study and MCNP-4C Monte Carlo code simulations of an ionization chamber designed and constructed at the Calibration Laboratory at IFEN to be used as a secondary dosimetry standard for low-energy X-rays are reported. The results obtained were within the international recommendations, and the simulations showed that the components of the extrapolation chamber may influence its response up to 11.0%. (C) 2011 Elsevier Ltd. All rights reserved.

The incomplete ionization of substitutional dopants in Silicon Carbide

This thesis analyzes theoretically and computationally the phenomenon of partial ionization of the substitutional dopants in Silicon Carbide at thermal equilibrium. It is based on the solution of the charge neutrality equation and takes into account the following phenomena: several energy levels in the bandgap; Fermi-Dirac statistics for free carriers; screening effects on the dopant ionization energies; the formation of impurity bands. A self-consistent model and a corresponding simulation software have been realized. A preliminary comparison of our calculations with existing experimental results is carried out.

Quantum dot-dye hybrid systems for energy transfer applications

In this thesis, we focus on the preparation of energy transfer-based quantum dot (QD)-dye hybrid systems. Two kinds of QD-dye hybrid systems have been successfully synthesized: QD-silica-dye and QD-dye hybrid systems.rn rnIn the QD-silica-dye hybrid system, multishell CdSe/CdS/ZnS QDs were adsorbed onto monodisperse Stöber silica particles with an outer silica shell of thickness 2 - 24 nm containing organic dye molecules (Texas Red). The thickness of this dye layer has a strong effect on the total sensitized acceptor emission, which is explained by the increase in the number of dye molecules homogeneously distributed within the silica shell, in combination with an enhanced surface adsorption of QDs with increasing dye amount. Our conclusions were underlined by comparison of the experimental results with Monte-Carlo simulations, and by control experiments confirming attractive interactions between QDs and Texas Red freely dissolved in solution. rnrnNew QD-dye hybrid system consisting of multishell QDs and organic perylene dyes have been synthesized. We developed a versatile approach to assemble extraordinarily stable QD-dye hybrids, which uses dicarboxylate anchors to bind rylene dyes to QD. This system yields a good basis to study the energy transfer between QD and dye because of its simple and compact design: there is no third kind of molecule linking QD and dye; no spacer; and the affinity of the functional group to the QD surface is strong. The FRET signal was measured for these complexes as a function of both dye to QD ratio and center-to-center distance between QD and dye by controlling number of covered ZnS layers. Data showed that fluorescence resonance energy transfer (FRET) was the dominant mechanism of the energy transfer in our QD-dye hybrid system. FRET efficiency can be controlled by not only adjusting the number of dyes on the QD surface or the QD to dye distance, but also properly choosing different dye and QD components. Due to the strong stability, our QD-dye complexes can also be easily transferred into water. Our approach can apply to not only dye molecules but also other organic molecules. As an example, the QDs have been complexed with calixarene molecules and the QD-calixarene complexes also have potential for QD-based energy transfer study. rn

Einzelmolekülspektroskopie an einer homologen Reihe von Rylendiimiden und an einem bichromophoren Modellsystem für elektronischen Energietransfer

In dieser Arbeit wurden Untersuchungen zur Fluoreszenzdynamik und zum Mechanismus des Photobleichens einzelner Farbstoffmoleküle einer homologen Reihe von Rylentetracarbonsäurediimiden durchgeführt. Mit der Erweiterung des elektronischen π-Systems verringert sich die HOMO/LUMO-Energiedifferenz, so dass strahlungslose Relaxationsprozesse gemäß des Energielückengesetzes zunehmen. Die konfokale Einzelmolekülspektroskopie in Kombination mit zeitkorrelierter Einzelphotonenzählung ermöglicht es, Fluktuationen der inneren Konversionsrate zu detektieren. Der limitierende Faktor jedes Einzelmolekülexperiments ist die Photostabilität der Moleküle. Für die homologe Reihe konnten mindestens zwei Photobleichmechanismen identifiziert werden. Wenn Singulett-Sauerstoff durch Selbstsensibilisierung erzeugt werden kann, ist unter Luft die Photooxidation der wahrscheinlichste Mechanismus. Unter Ausschluss von Luftsauerstoff spielt die Bildung langlebiger Dunkelzustände eine entscheidende Rolle, die bevorzugt über höher angeregte Triplett- und Singulett-Zustände abläuft. Es wird angenommen, dass es sich hierbei um einen reversiblen Ionisierungsprozess handelt, bei dem das Radikal-Kation der Rylendiimide gebildet wird. Es konnte gezeigt werden, dass durch eine geeignete Wahl der Anregungsbedingungen die Dunkelzustandspopulierung verhindert und zugleich die Photostabilität der Fluorophore deutlich erhöht wird. Durch die Verknüpfung der beiden niedrigsten Homologen erhält man ein Donor-Akzeptor-Modellsystem, bei dem die Anregungsenergie mit hoher Effizienz vom Donor- auf den Akzeptor-Chromophor übertragen wird. In der Fluoreszenz einzelner Bichromophore wurden bei selektiver Anregung des Donors kollektive Auszeiten beobachtet, die durch effiziente Singulett-Triplett-Annihilation verursacht werden.

Novel porphyrin amino acids as building blocks for artificial photosynthetic reaction centers : photoinduced energy and electron transfer

This thesis reports on the synthesis and characterisation of trans-(M)AB2C meso-substituted porphyrin amino acid esters (PAr) (M = 2H or Zn) with tunable electron donating and electron withdrawing Ar substituents at B positions (Ar = 4-C6H4OnBu, 4-C6H4OMe, 2,4,6-C6H2Me3, 4-C6H4Me, C6H5, 4-C6H4F, 4-C6H4CF3, C6F5). These porphyrins were used as key building blocks for photosynthetic LHC (LHC = light-harvesting antenna complex) and RC (RC = reaction center) model compounds.rnBased on free-base or zinc(II) porphyrin amino acid esters and porphyrin acids several amide linked free-base bis(porphyrins) PAr1-PAr2 (Ar1 = 2,4,6-C6H2Me3, C6F5 and Ar2 = 2,4,6-C6H2Me3, 4-C6H4F, 4-C6H4CF3, C6F5), mono metallated bis(porphyrin) PAr1-(Zn)PAr2 (Ar1 = 2,4,6-C6H2Me3 and Ar2 =4-C6H4F) and its doubly zincated complexes (Zn)PAr1-(Zn)PAr2 were prepared. In the fluorescence spectra of free-base bis(porphyrins) the porphyrin with the strongest electron donating power of Ar substituents at B positions is the light emitting unity. The emission of mono metallated bis(porphyrin) occurs only from the free-base porphyrin building block. This phenomenon is caused by an efficient energy transfer likely via the Dexter through-bond mechanism.rnLinking of anthraquinone (Q) as electron acceptor (A) to the N-terminus of porphyrin amino acid esters ((M)PAr) and aminoferrocene (Fc) as electron donor (D) to the C-terminus of the porphyrin resulting in Q-(M)PAr-Fc triads (M = 2H or Zn, Ar = 4-C6H4OnBu, 4-C6H4OMe, 2,4,6-C6H2Me3, 4-C6H4Me, C6H5, 4-C6H4F, 4-C6H4CF3, C6F5) with tunable electron density at the porphyrin chromophore. In these triads initial oxidative PET (Q←(M)PAr) and reductive PET ((M)PAr→Fc) (PET = photoinduced electron transfer) are possible. Both processes leads to an emission quenching of (M)PAr. The efficiency of the PET pathways occurring in the Marcus normal region is controlled by the specific porphyrin electron density.rnAmide-linked conjugates PAr-Fc (Ar = 2,4,6-C6H2Me3, C6F5) and Fmoc-Fc-PAr1 (N-Fmoc-Fc = N-Fmoc protected 1,1’-ferrocene amino acid; Ar1 = C6H5, 4-C6H4F, 4-C6H4CF3, C6F5) as well as hinges PAr2-Fc-PAr1 (Ar1 = C6H5, 4-C6H4F and Ar2 = 2,4,6-C6H2Me3) were studied with respect to the reductive PET. The PET driving force (−GET) in dyads increases with the increasing electron withdrawing character of Ar substituents. Additionally, intramolecular energy transfer between porphyrins PAr1 and PAr2 is feasible in the hinges via the Förster mechanism.rn

Investigation of acceptor dopants in ZrNiSn half-Heusler materials

Thermoelectric generators (TEG) are solid state devices and are able to convert thermal energy directly into electricity and thus could play an important role in waste heat recovery in the near future. Half-Heusler (HH) compounds with the general formula MNiSn (M = Ti, Zr, Hf) built a promising class of materials for these applications because of their high Seebeck coefficients, their environmentally friendliness and their cost advantage over conventional thermoelectric materials.rnrnMuch of the existing literature on HH deals with thermoelectric characterization of n-type MNiSn and p-type MCoSb compounds. Studies on p-type MNiSn-based HHs are far fewer in number. To fabricate high efficient thermoelectric modules based on HH compounds, high performance p-type MNiSn systems need to be developed that are compatible with the existing n-type HH compounds. This thesis explores synthesis strategies for p-type MNiSn based compounds. In particular, the efficacy of transition metals (Sc, La) and main group elements (Al, Ga, In) as acceptor dopants on the Sn-site in ZrNiSn, was investigated by evaluating their thermoelectric performance. The most promising p-type materials could be achieved with transition metal dopants, where the introduction of Sc on the Zr side, yielded the highest Seebeck coefficient in a ternary NiSn-based HH compound up to this date. Hall effect and band gap measurements of this system showed, that the high mobility of minority carrier electrons dominate the transport properties at temperatures above 500 K. It could be shown that this is the reason, why n-type HH are successful TE materials for high temperature applications, and that p-types are subjected to bipolar effects which will lead to diminished thermoelectric efficiencies at high temperatures.rnrnTo complement the experimental investigations on different metal dopants and their influence on the TE properties of HH compounds, numerical solutions to the Boltzmann transport equation were used to predict the optimum carrier concentration where the maximum TE efficiency occurs for p-type HH compounds. The results for p-type samples showed that can not be treated within a simple parabolic band model approach, due to bipolar and multi-band effects.rnrnThe parabolic band model is commonly used for bulk TE materials. It is most accurate when the transport properties are dominated by one single carrier type. Since the transport properties of n-type HH are dominated by only one carrier type (high mobility electrons), it could be shown, that the use of a simple parabolic band model lead to a successful prediction of the optimized carrier concentration and thermoelectric efficiency in n-type HH compounds. rn

Intermolecular Clamping by Hydrogen Bonds: 2-Pyridone center dot NH3

A combined spectroscopic and ab initio theoretical study of the doubly hydrogen-bonded complex of 2-pyridone (2PY) with NH3 has been performed. The S-1 <- S-0 spectrum extends up to approximate to 1200 cm(-1) above the 0(0)(0) band, close to twice the range observed for 2PY. The S-1 state nonradiative decay for vibrations above approximate to 300 cm(-1) in the NH3 complex is dramatically slowed down relative to bare 2PY. Also, the Delta v=2,4,... overtone bands of the v(1)' and v(2)' out-of-plane vibrations that dominate the low-energy spectral region of 2PY are much weaker or missing for 2PY center dot NH3, which implies that the bridging (2PY)NH center dot center dot center dot NH3 and H2NH center dot center dot center dot O=C H-bonds clamp the 2PY at a planar geometry in the S-1 state. The mass-resolved UV vibronic spectra of jet-cooled 2PY center dot NH3 and its H/D mixed isotopomers are measured using two-color resonant two-photon ionization spectroscopy. The S-0 and S-1 equilibrium structures and normal-mode frequencies are calculated by density functional (B3LYP) and correlated ab initio methods (MP2 and approximate second-order coupled-cluster, CC2). The S-1 <- S-0 vibronic assignments are based on configuration interaction singles (CIS) and CC2 calculations. A doubly H-bonded bridged structure of C-S symmetry is predicted, in agreement with that of Held and Pratt [J. Am. Chem. Soc. 1993, 115, 9718]. While the B3LYP and MP2 calculated rotational constants are in very good agreement with experiment, the calculated H2NH center dot center dot center dot O=C H-bond distance is approximate to 0.7 angstrom shorter than that derived by Held and Pratt. On the other hand, this underlines their observation that ammonia can act as a strong H-bond donor when built into an H-bonded bridge. The CC2 calculations predict the H2NH center dot center dot center dot O distance to increase by 0.2 angstrom upon S-1 <- S-0 electronic excitation, while the (2PY)NH center dot center dot center dot NH3 H-bond remains nearly unchanged. Thus, the expansion of the doubly H-bonded bridge in the excited state is asymmetric and almost wholly due to the weakening of the interaction of ammonia with the keto acceptor group.

The ArgoNeuT detector in the NuMI low-energy beam line at Fermilab

The ArgoNeuT liquid argon time projection chamber has collected thousands of neutrino and anti-neutrino events during an extended run period in the NuMI beam-line at Fermilab. This paper focuses on the main aspects of the detector layout and related technical features, including the cryogenic equipment, time projection chamber, read-out electronics, and off-line data treatment. The detector commissioning phase, physics run, and first neutrino event displays are also reported. The characterization of the main working parameters of the detector during data-taking, the ionization electron drift velocity and lifetime in liquid argon, as obtained from through-going muon data complete the present report.

Improvement of γ-ray energy resolution of LaBr3:Ce3+ scintillation detectors by Sr2+ and Ca2+ co-doping

Commercially available LaBr3:5% Ce3+ scintillators show with photomultiplier tube readout about 2.7% energy resolution for the detection of 662 keV γ-rays. Here we will show that by co-doping LaBr3:Ce3+ with Sr2+ or Ca2+ the resolution is improved to 2.0%. Such an improvement is attributed to a strong reduction of the scintillation light losses that are due to radiationless recombination of free electrons and holes during the earliest stages (1–10 ps) inside the high free charge carrier density parts of the ionization track.