Polarization Decay Research Articles

Optical properties of Dy 3+ ions in sodium gadolinium tungstates crystal

Dy 3+-doped NaGd(WO 4) 2 crystal with sizes of about Φ20×40 mm 2 was grown by the Czochralski technique along the (0 0 1) orientation. Polarized absorption spectra, fluorescence spectra, and fluorescence decay curve of Dy 3+-doped NaGd(WO 4) 2 have been recorded at room temperature. Based on the Judd–Ofelt (J–O) theory, the intensity parameters from the measured line strengths were evaluated. The J–O parameters were used to predict radiative transition probabilities, radiative lifetimes and branching ratios for various excited levels of Dy 3+-doped NaGd(WO 4) 2 crystal. The luminescent quantum efficiency of the 4F 9/2 level was determined to be approximately 63% for this material. The emission cross-section of the 4F 9/2→ 6H 13/2 transition was estimated by using the Füchtbauer–Ladengurg method.

Methods for High Resistivity Measurements Related to Spacecraft-Charging

A key parameter in modeling differential spacecraft-charging is the resistivity of insulating materials. This parameter determines how charge will accumulate and redistribute across the spacecraft, as well as the timescale for charge transport and dissipation. American Society for Testing and Materials constant-voltage methods are shown to provide inaccurate resistivity measurements for materials with resistivities greater than ~1017 Omegamiddotcm or with long polarization decay times such as are found in many polymers. These data have been shown to often be inappropriate for spacecraft-charging applications and have been found to underestimate charging effects by one to four orders of magnitude for many materials. The charge storage decay method is shown to be the preferred method to determine the resistivities of such highly insulating materials. A review is presented of methods to measure the resistivity of highly insulating materials-including the electrometer in resistance method, the electrometer in constant-voltage method, and the charge storage method. The different methods are found to be appropriate for different resistivity ranges and for different charging circumstances. A simple macroscopic physics-based model of these methods allows separation of the polarization current and dark current components from long-duration measurements of resistivity over day- to month-long timescales. Model parameters are directly related to the magnitude of charge transfer and storage and the rate of charge transport. The model largely explains the observed differences in resistivity found using the different methods and provides a framework for recommendations for the appropriate test method for spacecraft materials with different resistivities and applications

Thermal and optical properties of Tm3+:NaLa(WO4)2 crystal

A Tm3+-doped NaLa(WO4)2 single crystal with a dimension of Φ20 mm×40 mm was grown by the Czochralski method. Anisotropic thermal expansion coefficients of this crystal were investigated. Polarized absorption spectra, emission spectra and decay curve were recorded at room temperature. The absorption and emission cross-section were presented. Based on the Judd–Ofelt analysis, we obtained the three intensity parameters: Ω2=10.21×10-20, Ω4=2.66×10-20, and Ω6=1.46×10-20 cm2. The radiative probabilities, radiative lifetimes, and branch ratios of Tm3+:NaLa(WO4)2 were calculated, too. Luminescence lifetime of the 3H4 level was measured to be 220 μs. The stimulated emission cross-section for the 3F4→3H6 transition were determined using the reciprocity method, potential laser gain for this transition were also investigated, the gain curves implied that the tunable range is up to 200 nm.

Carrier spin dynamics in modulation-doped InAs∕GaAs quantum dots

Photoexcited electron and hole spin relaxation was studied in modulation-doped and undoped InAs∕GaAs quantum dots by means of time-resolved photoluminescence. After excitation into the barriers or the wetting layer, the electron spin polarization is preserved during the capture and relaxation in the dots, especially in the p-doped structures, and decays with a characteristic time of about 100ps. Spin state admixture in combination with electron interaction with acoustic phonons is suggested as the spin relaxation mechanism. Rapid spin polarization decay during carrier relaxation in undoped quantum dots is attributed to electron-optical phonon interaction. For carrier excitation directly into the dots, no significant spin polarization was observed, which points to the mixed nature of hole levels in quantum dots. The hole spin polarization randomizes on a much shorter time scale and is not detected in the experiment.

A New Basic Motif in Cyanometallate Coordination Polymers: Structure and Magnetic Behavior of M(μ‐OH2)2[Au(CN)2]2(M=Cu, Ni)

The structures of two cyanoaurate-based coordination polymers, M(mu-OH(2))(2)[Au(CN)(2)](2) (M=Cu, Ni), were determined by using a combination of powder and single-crystal X-ray diffraction techniques. The basic structural motif for both polymers contains rarely observed M(mu-OH(2))(2)M double aqua-bridges, which generate an infinite chain; two trans [Au(CN)(2)](-) units also dangle from each metal center. The chains form ribbons that interact three dimensionally through CNH hydrogen bonding. The magnetic properties of both compounds and of the dehydrated analogue Cu[Au(CN)(2)](2) were investigated by direct current (dc) and alternating current (ac) magnetometry; muon spin-relaxation data was also obtained to probe their magnetic properties in zero-field. In M(mu-OH(2))(2)[Au(CN)(2)](2), ferromagnetic chains of M(mu-OH(2))(2)M are present below 20 K. Interchain magnetic interactions mediated through hydrogen bonding, involving water and cyanoaurate units, yield a long-range magnetically ordered system in Cu(mu-OH(2))(2)[Au(CN)(2)](2) below 0.20 K, as indicated by precession in the muon spin polarization decay. Ni(mu-OH(2))(2)[Au(CN)(2)](2) undergoes a transition to a spin-glass state in zero-field at 3.6 K, as indicated by a combination of muon spin-relaxation and ac-susceptibility data. This transition is probably due to competing interactions that lead to spin frustration. A phase transition to a paramagnetic state is possible for Ni(mu-OH(2))(2)[Au(CN)(2)](2) upon application of an external field; the critical field was determined to be 700 Oe at 1.8 K. The dehydrated compound Cu[Au(CN)(2)](2) shows weak antiferromagnetic interactions at low temperatures.

Light-Induced Electron Spin Polarization in Vanadyl Octaethylporphyrin: II. Dynamics of the Excited States

The dynamics of the low-lying excited states of vanadyl octaethylporphyrin (OEPVO) in frozen solution is investigated by transient electron paramagnetic resonance (TREPR). The observation of spin-polarized TREPR spectra from the lowest excited trip-quartet state of OEPVO, reported in the preceding paper, opens a new avenue for investigation of the excited states of such molecules. Here, a model based on the back-and-forth transitions between the trip-quartet and trip-doublet states is developed and used to explain the time dependence of the low-temperature laser flash-induced electron spin polarization of OEPVO. At early times, the TREPR spectra show predominantly multiplet polarization, whereas strong net polarization develops at later times. An analysis of the time dependence reveals two well-separated processes: (i) fast evolution of the polarization from the multiplet pattern to the net absorptive pattern and (ii) very slow decay of the net polarization. Both processes are temperature dependent and are faster at higher temperature. All of these observed features can be reproduced, and the experimental data can be simulated within the framework of the model. For simplicity, only the two nearly degenerate orbital states resulting from the a(1) --> e triplet excitation of the porphyrin are considered. Each of these is split into a trip-doublet and trip-quartet giving a total of four low-lying excited states. Transitions between the trip-doublet and trip-quartet states are assumed to be governed by spin-orbit coupling, which mixes the four low-lying states. It is known that following light excitation, the molecule initially decays to the lowest trip-doublet state and then to the trip-quartet state. In agreement with the observed TREPR spectra, the model predicts that this decay results in predominantly multiplet polarization of the trip-quartet. However, a small amount of net polarization is also predicted due to the spin selectivity associated with the Zeeman interaction. Because the energy gap between the trip-doublet and trip-quartet states is small, back-and-forth electronic transitions between the trip-doublet and trip-quartet are expected to occur as thermal equilibrium is established. The model predicts that it is these transitions that lead to the observed evolution of the initial multiplet polarization to net absorptive polarization.

Shear Viscosities of Methycyclohexane, Perfluoromethylcyclohexane, and Their Mixtures in the Vicinity of the Upper Critical Mixing Temperature. 1. Critical Isopleth and Coexistence Curve

Shear viscosities η are reported for pure liquid methylcyclohexane (MCH) from 298.610 K to 333.694 K, for perfluoromethylcyclohexane (PFMCH) from 319.196 K to 333.114 K and for a MCH + PFMCH mixture of overall PFMCH near-critical mole fraction, xc = 0.3640, from (TUCS/K − 7) in the region of biphase liquid coexistence to (TUCS/K + 20) in the uniphase region, where TUCS = 320.13 K is the air-saturated upper liquid−liquid critical solution temperature. The measurements were made using a capillary rheometer that permits the measurement of the viscosity of thermally equilibrated coexisting-liquid phases. The results confirm that the near-critical viscosity exhibits a weak enhancement that strictly speaking becomes a divergence when account is taken of the finite shear gradients in the capillary during measurements. The viscosity of the uniphase mixture of critical composition is well-described after shear gradient correction by a multiplicative combination of an Arrhenius background and a critical power expression with an index close to the now-accepted universal value y = 0.0435. The chief objective of the work, in addition to contributing to knowledge of this aspect of near-critical rheology, is the development for the biphase of a simple expression for the temperature dependence of the viscosities of the coexisting phases, η+ and η-, that combines (a) an expression for the viscosity diameter <η> = 1/2(η+ + η-) similar to that for the viscosity of the critical mixture in the uniphase region, with a similar best critical index y‘ between 0.041 and 0.0435, and (b) an expression for Δη = (η+ − η-) that behaves like an order parameter, with an index β very close to the normal value β = 0.325 and as many Wegner correction terms as the data require. The best two-phase fit emerges from a freely fitted exponent y‘ ≈ 0.037 with one Wegner-extended scaling term, but we believe that were shear gradient correction to be applied, the best y‘ would be the consensus value y‘ = 0.0435. The magnitude of y‘ notwithstanding, we believe that our primary objective has been satisfied, namely, the formulation of an expression that affords a good description of the shear and background viscosities of near-critical mixtures in the one- and two-liquid phases in relation to our estimates of the nano- or molecular-viscosity derived from measurements of fluorescence polarization decay rates.

Microscopic Polarization Retention Properties of Ferroelectric Pb(Zr,Ti)O3 Thin Films

Microscopic retention properties have been studied in detail. Domain growth and stability of ferroelectric Pb(Zr,Ti)O 3 thin films were investigated with electrostatic force microscope to understand microscopic polarization retention loss mechanism in polycrystalline and epitaxial Pb(Zr,Ti)O 3 thin films. Growth of domains and their relaxation behaviors were investigated and compared. The larger domains were the more stable than the smaller ones. The decay behavior of domain size could be fitted to a simple exponential function. The stretched exponential decay of the averaged polarization was attributed to the superposition of relaxation processes of all domains relaxing independently with their characteristic relaxation times.

Optical properties of Ho3+-doped NaGd(WO4)2 crystal for laser materials

Holmium doped sodium gadolinium tungstate crystals with sizes of about Ø20mm×40mm were grown successfully by the Czochralski technique along the (001) orientation. Polarized absorption spectra, fluorescence spectra and fluorescence decay curve of Ho3+-doped NaGd(WO4)2 have been recorded at room temperature. Based on the Judd–Ofelt theory, three intensity parameters were obtained. The spectroscopic parameters of this crystal such as the oscillator strengths, radiative transition probabilities, radiative lifetimes as well as the branching ratios were calculated. The fluorescence lifetime τf of the 5S2 level was measured to be 5μs.

Polarization relaxation in (CH3NH3)5Bi2Br11 ferroelectric crystals

The time-dependent relaxation of retained polarization in unipolar methylammonium bromobismuthate, (CH3NH3)5Bi2Br11, single crystal was examined by optical observation of the domain structure. The polarization relaxation is attributed to the presence of a built-in internal bias field caused by structured disorder in the crystal. The internal field accounts for the broad spectrum of potential barriers for the domain walls. The slow polarization decay, in the long-time regime (t > 1 s), follows a stretched exponential and/or power-law function. On the assumption that relaxation centers (domains) are noninteracting, a distribution function of relaxation times is reconstructed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of internal bias field on domain switching in (CH3NH3)5Bi2Br11 crystal

In this report we investigate the microscopic features of 180° domain wall dynamics and relate its dynamics to the defect structure in unipolar methylammonium bromobismuthate, (CH3NH3)5Bi2Br11, single crystal. The switching and spontaneous backswitching processes have been studied by observation of domain structure by using nematic liquid crystal method and by switching current registration. We found a two-step relaxation decay of polarization induced by Eb; the fast one decays on the time scale of milliseconds, while the slow one lasts over seconds. In the late-time regime, the stretched-exponential function was used as an empirical fit of relaxation data.

Effects of Bose-Einstein condensation of exciton polaritons in microcavities on the polarization of emitted light

It is shown theoretically that Bose condensation of spin-degenerated exciton-polaritons results in spontaneous buildup of the linear polarization in emission spectra of semiconductor microcavities. The linear polarization degree is a good order parameter for the polariton Bose condensation. If spin-degeneracy is lifted, an elliptically polarized light is emitted by the polariton condensate. The main axis of the ellipse rotates in time due to self-induced Larmor precession of the polariton condensate pseudospin. The polarization decay time is governed by the dephasing induced by the polariton-polariton interaction and strongly depends on the statistics of the condensed state.

Non-uniform superconductivity in S/F nanostructures

A strong exchange field acting on the Cooper pairs in the ferromagnetic (F) metal leads to the damping oscillatory behaviour of the superconducting order parameter inside the F layer. In consequence the electron density of state and spin polarization decay non-monotonously in S/F proximity structures. We review the influence of these effects on the properties of S-F multilayers where the critical temperature and Josephson current depend in an oscillatory manner on the exchange field and thickness of ferromagnetic metal. These oscillations are related with the transition into the state where the phase of superconducting order parameter is opposite on the adjacent superconducting layers. We briefly discuss the properties of the Josephson junction at this transition.

Kinetics of ferroelectric switching in unipolar (CH3NH3)5Bi2Br11 crystal

Ferroelectric switching in unipolar (CH3NH3)5Bi2Br11 crystal has been studied by optical observation of the domain structure in low electric fields and switching current registration in high fields. We discuss investigations on the temporal behaviour of the electric polarization in static electric fields as well as polarization decay after removal of the electric field. The backswitching process appears to have a correlation to a defect-induced internal field in the crystal. For the interpretation of experimental data we utilize the classical nucleation and growth model of phase transformation (e.g. Avrami–Kolmogorov theory). An analysis assuming a statistical distribution of the characteristic domain growth times is proposed to explain and to find the functional form of the experimental data.

Dynamics of spin proximity polarization in a hybrid ferromagnet/semiconductor structure

We study theoretically dynamics of spin proximity polarization of semiconductor electrons in a hybrid ferromagnet/semiconductor (F/S) structure. When the hybrid structure comes into equilibrium after a photoexcitation a spin current flowing through the interface produces a spin polarization of semiconductor electrons. In turn, a spin backflow arising in the thin semiconductor layer affects the interface spin current. We analyze the interplay of these processes by using the spin diffusion equation with appropriate boundary conditions at the F/S interface and show that the decay of the proximity polarization with time depends on the temporal behavior of the spin current emitted by the interface.

Frans De Schryver: Forty Years of Photochemistry and Photophysics

Frans De Schryver (born 1939), to whom this special issue of ChemPhysChem is dedicated on the occasion of his 65th birthday, started his career in 1961 as a polymer chemist with a master thesis entitled “Reaction of Phenyl Isocyanate with Aryl Amines” in the laboratory of J. C. Jungers at the K.U.Leuven. He then moved to the group of the famous polymer chemist G. Smets, where he received his Ph.D. for a thesis on the “Influence of the Viscosity on the Photopolymerization of Styrene” in 1964. In this thesis we already find the major foci of his scientific achievements: polymers and photochemistry. In 1964 he joined the group of C. S. Marvel at the University of Arizona in Tucson as a Fullbright Fellow engaged in polymer synthesis. After returning to the K.U.Leuven, where he studied new systems leading to photopolymerization and photocycloisomerization, he continued his postdoctoral research, focused on the emerging domain of photochemistry and photophysics, in the groups of Th. Fcrster (Stuttgart University) and A. Weller (Max Planck Institute for Biophysical Chemistry, Gcttingen). In this period he married Suzy Bosteels and had two daughters, Dominique (born 1964) and Catherine (born 1971). In 1969 he became assistant professor at the K.U.Leuven, where he built up a research group which evolved into the Laboratory for Photochemistry and Spectroscopy (LPS). In the 1970s, when the curricula at K.U.Leuven were reformed, he started teaching general chemistry and introduced new courses on spectrosco11/2005

Application of Spin Counting to the Solid‐State 31P NMR Analysis of Pasture Soils with Varying Phosphorus Content

Solid‐state 31P NMR spectroscopy has the potential to identify forms of soil P without the need for extractions or pretreatment. We used both cross polarization (CP) and direct polarization or Bloch Decay (DP) solid‐state 31P NMR to examine the forms of P in a set of soil samples that vary widely in their total P contents and proportions of organic and inorganic P. Using the technique of spin counting, we found that the 31P NMR observability (Pobs) of P in our soils was poor. Average Pobs was 9% by CP and 22% by DP. We attributed the poor observability to paramagnetic iron in close association with both organic and inorganic P. Using a series of selective extractions, we assigned the broad resonances of whole soil 31P NMR spectra to organic P and prominent, sharp resonances to inorganic P. Pretreatment of soils with HF, as commonly used in 13C and 15N NMR analyses, resulted in Pobs of &gt;70% by both CP and DP. However, organic P recovery in this fraction was poor. Our findings highlight the risks of trying to quantify different P types by integrating NMR spectra without taking into the account possible differences in their NMR sensitivity. Furthermore, we believe that significant improvements in the information garnered from solid‐state 31P NMR analysis of soil will come not from improving resolution—there are fundamental limitations here—but in using information contained in nonfrequency parameters, such as observability, chemical shift anisotropy, and relaxation rates.

Ultrashort pulse propagation in multilevel systems

Here, I discuss the propagation of an ultrashort pulse through a collection of harmonic multilevel systems. In the limit of weak excitation and a large number of excited states, I show that the amplitude of the input driving pulse decays exponentially with propagation distance. The absorption coefficient associated with this decay is determined by the characteristic time of the manifold of excited states, instead of the polarization decay time as in the conventional absorption coefficient of a two-level atom. The input ultrashort pulse creates in the excited states a wave packet, which oscillates emitting secondary pulses in the process. Analytic solutions are obtained that describe the propagation of individual wave-packet re-emission pulses, and it is shown that their phase depends on the detuning of the input pulse.

Spin-Lattice Relaxation and Rotational Motion of Aromatic Triplet-State Molecules in Supercooled Alkane Solvents (Part 1)

The phosphorescence of phenazine (PZ) and quinoxaline (QX) was investigated after pulsed laser excitation in the glass-transition range of several alkane solvents. Three relaxation processes of PZ and QX in the metastable triplet state, T1, were studied as a function of temperature: (1) the decay of the selective population of the strongly phosphorescent triplet substate T1x due to spin-lattice relaxation (SLR), (2) the time-dependent red shift of the phosphorescence spectrum due to the solvation of triplet-state molecules, and (3) the decay of the phosphorescence polarization due to orientational relaxation (OR). Various aspects and connections of the mechanisms governing the three relaxation phenomena are discussed. The relaxation dynamics were characterized at temperatures above the glass-transition temperature of the respective solvent, where the fundamental processes involved are strongly dependent upon the solvent viscosities. For the systems treated here, OR and solvation were satisfactorily described by a Vogel-Fulcher-Tammann temperature behavior. SLR also depends on properties of the alkane solvent above the glass transition. Upon cooling, SLR becomes independent of the specific solvent properties and is based on mechanisms that are typical for amorphous glasses or solids. (This particular aspect will be the subject of a subsequent publication, part 2).

Chemically Induced Dynamic Electron Polarization Generated through the Interaction between Singlet Molecular Oxygen and Nitroxide Radicals

An absorptive chemically induced dynamic electron polarization (CIDEP) was generated by the quenching of singlet oxygen by nitroxide radicals (TEMPO derivatives). The spin polarization decay time of the nitroxide (measured by time-resolved EPR) correlates with the lifetime of singlet oxygen (measured by singlet oxygen phosphorescence spectroscopy). In addition, a deuterium isotope effect on the spin polarization decay time was observed, a signature of singlet oxygen involvement. With use of isotope labeled nitroxides (15N, 14N), the relative spin polarization efficiencies of TEMPO, 4-oxo-TEMPO, and 4-hydroxy-TEMPO by singlet oxygen were determined. The relative spin polarization efficiencies (per quenching event) decrease in the order 4-hydroxy-TEMPO > TEMPO > 4-oxo-TEMPO, whereas an opposite trend was observed for the total quenching rate constants of singlet oxygen by the nitroxides where the order is 4-hydroxy-TEMPO < TEMPO < 4-oxo-TEMPO.