5D3-5D4 cross-relaxation of Tb3+ in α-GdOF

The ⁵D₃-⁵D₄ cross-relaxation of Tb³⁺ in α-GdOF has been investigated by analysis of the ⁵D₃ decay curve. It is shown that the relaxation is electric dipole-dipole in character. The relaxation rate has been evaluated in the temperature region between 4.2 K and room temperature. The temperature dependence of the relaxation rate points to the thermal population of a higher-lying ⁷F₆ or ⁵D₃ level.     

Vibrational relaxation in COF2 and Xe,C2H6 mixtures following intense laser excitation

The VR,T rate in COF₂ was measured directly by the ultrasonic absorption technique. The laser-induced fluorescence method was used to study VV exchange between the strongly pumped ν₂, ν₁ manifold and other vibrational degrees of freedom. At a few Torr a transition of the μm fluorescence from a single exponential decay to a relaxation spectrum is observed. This relaxation spectrum was analyzed by determining the “differential decay rate” for successive small time intervals. The relaxation spectrum is not changed by a pressure increase from ≈ 4 to 15 Torr. Similarly, relatively large amounts of Xe must be added before a change in the relaxation behavior of COF₂ can be detected. C₂H₆ is a very effective quencher of the 5μm fluorescence.     

The vibrational relaxation of I2 by H2 in a supersonic jet

The vibrational relaxation of I₂ by H₂ has been studied in a supersonic free jet. It was observed that the addition of 5% H₂ to the helium carrier gas greatly reduces the concentration of X ¹Σ⁺_g(ν″ = 1) I₂ in the jet as compared to the concentration in a pure helium carrier. From this observation we have determined that the average vibrational relaxation cross sections of H₂ is 7.1 times as large as that of helium. Since the average vibrational relaxation cross section of deuterium is at least as large as that of hydrogen, the mechanism responsible for this phenomenon appears not to be dominated by mass effects.     

Structural relaxation of PbO–WO3–P2O5 glasses

The structural relaxation of three compositional series of PbO–WO₃–P₂O₅ glasses with composition (0.5 ? x/2)PbO·xWO₃·(0.5 ? x/2)P₂O₅, x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5; 0.5PbO·xWO₃·(0.5 ? x)P₂O₅, x = 0, 0.1, 0.2, and 0.3; and (0.5 ? x)PbO·xWO₃·0.5P₂O₅, x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5 was studied by thermomechanical analysis. The structural relaxation was studied in the transformation region using the Tool–Narayanaswamy–Moynihan’s and Tool–Narayanaswamy–Mazurin’s models. The relaxation function of Kohlrausch Williams and Watts was used. The parameters of both models were calculated by nonlinear regression analysis of thermodilatometric curves measured by thermomechanical analyzer under the constant load. Both models very well describe the experimental data. It was found that the modulus is increasing with increasing amount of WO₃ in all glasses. On the contrary, the width of the spectrum of relaxation times is decreasing with increasing amount of WO₃ in all studied glasses. Both models possess the values of metastable melt thermal expansion coefficient equal to their experimental value.     

Vibrational relaxation of ethylene oxide and ethylene oxide-rare-gas mixtures

The collision-induced vibrational energy relaxation of ethylene oxide (C₂H₄O) was studied by means of laser-induced fluorescence. The time-dependent population of the vibrational modes v₃ and v₅/v₁₂ was measured after excitation of CH-stretching vibrations near 3000 cm^?1. Rate constants for the vibrational energy transfer by collisions with C₂H₄O and the rare gases are deduced, and a simplified model for the vibrational relaxation of C₂H₄O is discussed.     

Synthesis and morphological investigation of pulsed current formed nano-structured lead dioxide

An electro-forming method was used for the preparation of nano-structured lead dioxide on lead substrate in 4.8 M H₂SO₄. A two electrode system composed of two similarly prepared lead electrodes was employed. A constant current was applied between two electrodes for a short time, followed by a relaxation period. A dark oxidation film of suitable stability and homogeneity was formed on the anode electrode. Different parameters including pulse height, pulse time (t_on) and relaxation time (t_off) were optimized. A change in morphology of lead dioxide from globular to nanofibriliar was observed when the pulse time and relaxation time varied from 0.01 to 10 and 0.1 to 5 s, respectively, at a pulse height of 25 mA cm⁻². A variation in pulse height caused a variation in diameter and length of nanofibers formed at a pulsed time of 0.1 s and relaxation time of 5 s. The scanning electron microscopy (SEM) showed that lead dioxide particles obtained under these conditions have nanofibriliar morphology with an average diameter in the range between 20 and 50 nm and length of 500–1000 nm.     

Absorption and fluorescence of Ho3+ IN La2S3·3Ga2S3

Absorption bands of Ho³⁺ in vitreous La₂S₃·3Ga₂S₃ in the range 500 to 2000 nm were assigned. Excitation spectra reveal additional levels ⁵G₆ and ⁵F₃ obscured by the intrinsic absorption of the glass. The Ho³⁺ emission in chalcogenide glasses is more intense than in oxide glasses due to smaller non radiative relaxation as predicted by the theory of multiphonon relaxation.     

A carbon-13 study of relaxation in the mesophases of 5O·7 and the 5CB homologous series

Abstract

Carbon-13 spin-lattice relaxation times of the protonated ring carbons have been measured at 22·6 MHz in the nematic and all four smectic phases of 5O·7 (4-n-pentyloxybenzylidene-4′-n-heptylaniline). Dong has obtained the deuterium spectral densities J ₁ and J ₂ at 15·4MHz for the deuterated aniline ring of 5O·7-d ₄, and has presented and applied a theory in which the spectral densities are expressed in terms of the diffusion constants D∥ and D?. His results are used to calculate ¹³C relaxation times from the spectral densities J ₀, J ₁ and J ₂. The calculated ¹³C spin-lattice relaxation times are then compared with our experimental values to test the theory. The ¹³C spin-lattice relaxation times of all the resolved resonances in the various phases of the first four members of the 5CB homologous series have been published previously. Dong has also published an analysis of 5CB deuterium data, and we use his results for the diffusion constants D∥ and D? to calculate ¹³C relaxation times of the protonated aromatic carbons of 5CB, 6CB, 7CB and 8CB. The ¹³C relaxation times of the unprotonated aromatic carbons of the 5CB series are calculated in the manner of Wittebort et al., but using the spectral density expressions developed by Dong. The calculated ¹³C spin-lattice relaxation times of the 5CB homologous series are then compared with our experimental values to test the theory for the protonated and unprotonated ring carbons.     

Relaxation of vibrationally excited gas-phase Cr(CO) 5 −

Radiative relaxation of Cr(CO)₅⁻ was investigated by two techniques: a standard two-pulse photodissociation experiment and by using the branching ratio of its reaction with oxygen as an ion thermometric probe. Photoexcitation at 1064 nm was used to prepare highly vibrationally excited Cr(CO)₅⁻. Although the overall oxidation rate changes only slightly upon excitation (actually decreasing by a factor of 1.2 ± 0.1), the primary product distribution shifts dramatically, from Cr(CO)₃O⁻ (the thermodynamic product) to Cr(CO)₃O₂⁻ (the kinetic product). The two-pulse photodissociation measurement gave a radiative relaxation rate constant (k _rad) of 15 ± 2 s⁻¹, whereas the branching ratio experiments gave a k _rad value of 3. 3 ± 0.7 s⁻¹. The large difference between these two values is due to the difference in Cr(CO)₅⁻ internal energy ranges probed by the two techniques. In the high internal energy regime interrogated by the two-pulse measurements (about 12,000 to 6000 cm⁻¹), the strongly emitting C-O stretching modes are populated and contribute to fast relaxation. In contrast, the branching ratio measurements remain sensitive to internal energy changes all the way down to thermal energies, where the C-O stretches are depopulated and thus unavailable for radiative relaxation.     

Magnetization Slow Dynamics in Ferrocenium Complexes

The single-molecule magnet (SMM) properties of a series of ferrocenium complexes, [Fe(η⁵-C₅R₅)₂]⁺ (R=Me, Bn), are reported. In the presence of an applied dc field, the slow dynamics of the magnetization in [Fe(η⁵-C₅Me₅)₂]BAr_F are revealed. Multireference quantum mechanical calculations show a large energy difference between the ground and first excited states, excluding the commonly invoked, thermally activated (Orbach-like) mechanism of relaxation. In contrast, a detailed analysis of the relaxation time highlights that both direct and Raman processes are responsible for the SMM properties. Similarly, the bulky ferrocenium complexes, [Fe(η⁵-C₅Bn₅)₂]BF₄ and [Fe(η⁵-C₅Bn₅)₂]PF₆, also exhibit magnetization slow dynamics, however an additional relaxation process is clearly detected for these analogous systems.     

Pulsed NMR study of molecular motion in the uncured diglycidyl ether of bisphenol-A

The diglycidyl ether of bisphenol-A, an uncured epoxy resin, has been studied by pulsed NMR. Values of the proton relaxation times T₁, T_1p, and T₂ have been measured over the temperature range from ?160 to 200°C. The resin was studied in its monomeric form and in two mixtures containing higher oligomers. The relaxation times are interpreted in terms of the molecular motion in the resins. The motion responsible for relaxation in the solid monomer form is thought to be methyl group reorientation at low temperatures and general molecular motion at high temperatures. The motions are characterized by activation energies of 5 kcal/mole and 33 kcal/mole, respectively. The solid mixtures exhibit similar effects to the monomer, but an additional relaxation mechanism is observed which is attributed to segmental motion. This motion is characterized by an activation energy of 12–15 kcal/mole. The self-diffusion coefficient was measured in the liquid monomer, and the activation energy for self-diffusion is found to be 11 kcal/mole.     

Structural relaxation of scintillating Ce-doped NaGd(PO<Subscript>3</Subscript>)<Subscript>4</Subscript> glass

Structural relaxation of scintillating Ce-doped Na–Gd phosphate glass with a nominal composition of Ce:NaGd(PO₃)₄ was experimentally studied using non-isothermal thermo-mechanical analysis, and the relaxation process was described by the Tool–Narayanaswamy–Mazurin model. The distribution of relaxation times was expressed by the empirical Kohlrausch–Williams–Watts relaxation function with relaxation time directly proportional to dynamic viscosity. The model parameters and material constants were obtained by the nonlinear regression analysis of thermo-mechanical data. It has been concluded that the model used of structural relaxation correctly describes relaxation processes in studied Ce-doped NaGd(PO₃)₄ glass.     

Dielectric relaxation and conductivity studies on (PEO:LiClO4) polymer electrolyte with added ionic liquid [BMIM][PF6]: Evidence of ion–ion interaction

Polymer electrolytes, (PEO:LiClO₄)+x IL (1‐Buty‐3‐methylimidazolium hexafluorophosphate) with varying concentration of IL; x = 0,5,10,15,20 wt % have been prepared by solution cast technique and characterized by X‐Ray diffraction, differential scanning calorimetery, FTIR, conductivity and dielectric relaxation measurements in the frequency range of 100 Hz–5 MHz. Temperature dependence of relaxation frequency and conductivity were found to be typical of thermally activated process both at T > T_m and T < T_m. Composition dependence of conductivity, dielectric relaxation, and degree of crystallinity has also been studied. On addition of IL, the degree of crystallinity after a decrease at 5 wt % IL increases slightly at 10 wt % and then finally decreasing. Variation of conductivity and relaxation frequency with composition could only be partly explained on the basis of variation of degree of crystallinity. An additional feature of ion–ion interaction (contact ion pair formation between IL or salt cations and their associated anions) has been invoked which was supported by FTIR studies. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Uranocenium: Synthesis,Structure, and Chemical Bonding

Abstraction of iodide from [(η⁵‐C₅ⁱPr₅)₂UI] ( 1 ) produced the cationic uranium(III) metallocene [(η⁵‐C₅ⁱPr₅)₂U]⁺ ( 2 ) as a salt of [B(C₆F₅)₄]^?. The structure of 2 consists of unsymmetrically bonded cyclopentadienyl ligands and a bending angle of 167.82° at uranium. Analysis of the bonding in 2 showed that the uranium 5f orbitals are strongly split and mixed with the ligand orbitals, thus leading to non‐negligible covalent contributions to the bonding. Investigation of the dynamic magnetic properties of 2 revealed that the 5f covalency leads to partially quenched anisotropy and fast magnetic relaxation in zero applied magnetic field. Application of a magnetic field leads to dominant relaxation by a Raman process.     

Uranocenium: Synthesis,Structure, and Chemical Bonding

Abstraction of iodide from [(η⁵‐C₅ⁱPr₅)₂UI] ( 1 ) produced the cationic uranium(III) metallocene [(η⁵‐C₅ⁱPr₅)₂U]⁺ ( 2 ) as a salt of [B(C₆F₅)₄]^?. The structure of 2 consists of unsymmetrically bonded cyclopentadienyl ligands and a bending angle of 167.82° at uranium. Analysis of the bonding in 2 showed that the uranium 5f orbitals are strongly split and mixed with the ligand orbitals, thus leading to non‐negligible covalent contributions to the bonding. Investigation of the dynamic magnetic properties of 2 revealed that the 5f covalency leads to partially quenched anisotropy and fast magnetic relaxation in zero applied magnetic field. Application of a magnetic field leads to dominant relaxation by a Raman process.     

Molecular dynamics simulation of local motion of polystyrene chain end—comparison with the fluorescence depolarization study

Molecular dynamics (MD) simulation of the local motion of a polystyrene (PS) chain with anthryl group at the chain end surrounded by benzene molecules was performed and the results were compared with those obtained experimentally by the fluorescence depolarization method. The molecular weight dependence of the relaxation time of the probe obtained by the MD simulation was qualitatively in agreement with the results obtained by the fluorescence depolarization method. We also estimated the molecular weight dependence of the relaxation time for the end-to-end vector. Below the degree of polymerization (DP)≤3, the mean relaxation time T_m for the end-to-end vector was similar to that for the vector corresponding to the transition moment of the probe. With the increase of DP, the T_m for the probe tended to reach an asymptotic value unlike that for the end-to-end vector, which monotonically increased with DP. This indicates that the entire motion of a polymer coil contributes to the local motion to a lesser extent as the molecular weight increases. The MD simulations using artificial restraints showed that the rotational relaxation of the probe at the chain end for a dynamically stiff PS chain is realized by the cooperative rotation of the main chain bonds. The internal modes which takes place below 5 monomer units mainly led to the rotational relaxation of the probe at the PS chain end. Finally, the change of T_m with the position along the PS main chain was examined.     

Porous Mn2+ Magnet with a Pt−Cl Framework: Correlation between Water Vapor Adsorption/Desorption and Slow Magnetic Relaxation

We report the water adsorption/desorption behavior and dynamic magnetic properties of the Pt−Cl chain complex [{[Pt(en)₂][PtCl₂(en)₂]}₃][{(MnCl₅)Cl₃}₂] ⋅ 12H₂O ( 1 ). Upon heating 1 in a vacuum, we obtained the dehydrated form [{[Pt(en)₂][PtCl₂(en)₂]}₃][{(MnCl₅)Cl₃}₂] ( 1DH ). The framework structures of 1 and 1DH are identical, and both complexes underwent slow magnetic relaxation. However, the magnetic relaxation times for 1DH were shorter than those for 1 , meaning that the dynamic magnetic properties were controlled upon water vapor adsorption/desorption. From detailed analyses of the dynamic magnetic behavior, a phonon-bottleneck effect contributes to the magnetic relaxation processes. We discuss the mechanism for the changes in the magnetic relaxation processes upon dehydration in terms of the heat capacity and thermal conductivity.     

Effects of thermal history,crystallinity, and solvent on the transitions and relaxations in poly(bisphenol-A carbonate)

The following system of nomenclature for the transitions and relaxations in polycarbonate has been proposed: α = T_g = 150, β = 70, γ = ?100, and δ = ?220°C (frequency range of 10–50 Hz). The three component peaks of the γ relaxation are denoted by γ₁, γ₂, and γ₃ relaxations correspond to phenylene, coupled phenylene-carbonate, and carbonate motions, respectively. Dynamic mechanical analysis of poly(bisphenol-A carbonate) using the DuPont 981–990 DMA system shows that the magnitude of the β relaxation depends upon the thermal history of the polycarbonate; annealing greatly reduces the intensity of the β relaxation. A relaxation map constructed for the β relaxation gives an activation energy of 46 kcal/mol. Exposure of polycarbonate to methylene chloride vapor for various times shows that after an induction period of about 5 min the intensity of the γ₃ relaxation at ?78°C decreases whereas the intensity of the γ₁ relaxation of ?30°C is unaffected and the ratio E″(γ₁)/E″(γ₃) increases linearly with the square root of time. This has been ascribed to the interaction of methylene chloride on the carbonate group in polycarbonate. Thermal crystallization of polycarbonate does not affect the positions of the γ relaxation and the glass transition peaks, but merely reduces their intensity. The glass transition peak intensity falls off sharply in comparison to the γ relaxation intensity. Both the γ₃ and γ₁ peaks in polycarbonate have been observed simultaneously for the first time by dynamic mechanical analysis. Impact strength measurements show that methylene chloride treatment of polycarbonate results in a change in mode of failure from ductile to brittle with a resultant 40-fold reduction in impact energy for fracture. Thermally crystallized polycarbonate exhibits brittle fracture with very low force and energy at break.     

Deuteron NMR spin–lattice relaxation of NH3D ions in partially deuterated (NH4)2SnCl6 and NH4ClO4

Deuteron spin–lattice relaxation is studied in 5% and 100% deuterated ammonium hexachlorostannate and perchlorate. The relaxation rate is observed to be independent of deuteration down to temperatures slightly lower than that of the maximum. At lower temperatures the rate of the 5% deuterated sample exceeds that of the 100% deuterated sample by four and two orders of magnitude in ammonium hexachlorostannate and perchlorate, respectively. The angular dependence of the deuteron relaxation rate in 5% deuterated ammonium hexachlorostannate at 6 K is explained in terms of existing models on quadrupolar relaxation. In 5% ammonium perchlorate one hydrogen equilibrium position, which lies on the preferred axis for 120° rotations, has a larger probability to be occupied by the deuteron of NH₃D⁺ ions. The deuterons at the other positions are still performing rotational jumps about the preferred C₃ axis and also about the other threefold axes, although at a slower rate. Such observations require a reconsideration of the relaxation process. A somewhat more general expression is derived for the relaxation rate, which agrees with the experimentally observed angular dependence for 5% deuterated ammonium perchlorate at 60 K. At lower temperatures the quadrupole coupling of the deuterons at the preferred axis may become practically time-independent. Then a significant contribution to the relaxation rate can be provided by the deuteron–proton magnetic dipolar interaction, which is still fluctuating fast via the rotation of the three protons about the axis through the stationary deuteron.     

Study of cooperative effects of silanols on modified silica by dielectric relaxation method

Two types of chemically modified silica gels with covalently bonded hydrophobic (alkyl, SiO₂-C₈) and hydrophilic (aminoalkyl, SiO₂-NH₂) groups were studied by dielectric relaxation method. For SiO₂-C₈ two relaxation maxima were detected at 140 and 238K. From comparison with bulk silica the lower temperature maximum was assigned to cooperative re-orientation of residual silanols and higher one to melting of physically adsorbed water. Both maxima are much weaker for SiO₂-C₈ then for bulk silica. In Arrhenius coordinates relaxation at 238K exhibits similar behavior as OH groups in polyvinyl alcohol and so it was assigned to mobility of orientation defects in mono-dimensional chain of physically adsorbed water. For SiO₂-NH₂ no low temperature relaxation effect was observed. Intensive relaxation was measured at 248 K only. Strong interaction between supramolecules of silanol groups, bonded aminogroups and adsorbed water was assumed.