Search results for " Transition"
showing 10 items of 2751 documents
Some fundamentals of the vapor and solution growth of ZnSe and ZnO
1999
Abstract Some fundamentals of ZnSe and ZnO vapor and solution growth are investigated. Residual water present in gases or gaseous mixtures such as H 2 , Ar or H 2 +H 2 O is shown to act as a sublimation activator in the vapor-phase transport of both compounds. The processes involved in the growth by chemically activated sublimation with such gases and gas mixtures have been studied by close-spaced vapor transport (CSVT). The ZnSe growth rate is found to be constant, while in the ZnO case a high initial growth rate is followed by slower growth subsequently. Using a theoretical model, the thermodynamic constants of the transport – energies of activation, sublimation and condensation and entha…
Synthesis, structure and magnetic properties of iron (II), cobalt (II) and nickel (II) complexes of 2,6-bis(pyrazol-3-yl)pyridine and paramagnetic co…
2003
Abstract Iron (II), cobalt (II) and nickel (II) complexes of 2,6-bis(pyrazol-3-yl)pyridine (bpp) with [Cr(C2O4)3]3− have been prepared. They were characterised by single-crystal X-ray diffraction, magnetic susceptibility measurements and thermal gravimetric analyses. All three compounds are isostructural and they are formed by isolated [MII(bpp)2]2+ and [Cr(C2O4)3]3− complexes and free ClO4 −. As expected, only the salt [Fe(bpp)2]2[Cr(C2O4)3]ClO4·5H2O shows a thermal spin transition with transition temperature (T1/2) around 375 K that is correlated to the loss of water molecules.
Glass Transition and Food Technology: A Critical Appraisal
2002
ABSTRACT: Most low water content or frozen food products are partly or fully amorphous. This review will discuss the extent to which it is possible to understand and predict their behavior during processing and storage, on the basis of glass transition temperature values (Tg) and phenomena related to glass transition. Two main conclusions are provisionally proposed. Firstly, glass transition cannot be considered as an absolute threshold for molecular mobility. Transport of water and other small molecules takes place even in the glassy state at a significant rate, resulting in effective exchange of water in multi-domains foods or sensitivity to oxidation of encapsulated materials. Texture pr…
Non-exponential relaxation in disordered materials: Phenomenological correlations and spectrally selective experiments
1998
Abstract In most glass-forming materials external perturbations are relaxed in a non-exponential fashion. It is shown that the degree of non-exponentiality is phenomenologically correlated with the departure from simple thermally activated behavior as measured by the fragility index m. In model glass formers such as the Ge-As-Se ternary alloy, and to some degree for amorphous materials in general, the correlations with these properties are observed also for other characteristic features. These include the specific heat step and the aging kinetics in the glass transformation range. While phenomenological correlations have proven very useful for rationalizing the properties of many glass form…
Extension of Conjugation Leading to Bathochromic or Hypsochromic Effects in OPV Series
2004
Four OPV series 1-4 (a-d) with a terminal dialkylamino group as electron donor were prepared by Wittig-Horner reactions. To study the influence of the push-pull effect on the long-wavelength absorption, three of the four series contained terminal acceptor groups (CN, CHO, NO(2)). The length of the chromophores strongly affects the intramolecular charge transfer (ICT)-an effect which superimposes upon the extension of the conjugation. Increasing numbers n of repeat units cause an overall bathochromic shift for the purely donor-substituted series 1 a-4 a and the series 1 b-4 b with CN as weak acceptor. The two effects annihilate each other in the series 1 c-4 c with terminal CHO groups, so th…
Synthesis and Characterisation of a New Series of Bistable Iron(II) Spin-Crossover 2D Metal-Organic Frameworks
2009
Twelve coordination polymers with formula {Fe(3-Xpy)(2)[M(II)(CN)(4)]} (M(II): Ni, Pd, Pt; X: F, Cl, Br, I; py: pyridine) have been synthesised, and their crystal structures have been determined by single-crystal or powder X-ray analysis. All of the fluoro and iodo compounds, as well as the chloro derivative in which M(II) is Pt, crystallise in the monoclinic C2/m space group, whereas the rest of the chloro and all of the bromo derivatives crystallise in the orthorhombic Pnc2 space group. In all cases, the iron(II) atom resides in a pseudo-octahedral [FeN(6)] coordination core, with similar bond lengths and angles in the various derivatives. The major difference between the two kinds of str…
Electronic, vibrational, and structural properties of a spin-crossover catecholato-iron system in the solid state: theoretical study of the electroni…
2005
As a functional model of the catechol dioxygenases, [(TPA)Fe(Cat)]BPh4 (TPA = tris(2-pyridylmethyl)amine and Cat = catecholate dianion) exhibits the purple-blue coloration indicative of some charge transfer within the ground state. In contrast to a number of high-spin bioinspired systems, it was previously shown that, in the solid state, [(TPA)Fe(Cat)]BPh4 undergoes a two-step S = 1/2 = S = 5/2 spin-crossover. Therefore, the electronic and vibrational characteristics of this compound were investigated in the solid state by UV/Vis absorption and resonance Raman spectroscopies over the temperature range of the transition. This allowed the charge-transfer transitions of the low-spin (LS) form …
Breaking the semi-quinoid structure: spin-switching from strongly coupled singlet to polarized triplet state.
2014
2,7-TMPNO (4,5,9,10-tetramethoxypyrene-2,7-bis(tert-butylnitroxide)) was found to exist in semi-quinoid form with unprecedented strong intramolecular magnetic exchange interaction of 2 J/kB = 1185 K operating over a distance of 10 A. Structural transformations with the activation energy of ΔEeq = 949 K were observed by varying the temperature, from more quinoid structure at low temperature to more biradicaloid structure at higher temperature. Moreover, this molecule undergoes a transient spin transition from singlet to polarized triplet state upon photoexcitation revealed by TREPR spectroscopy. The spin Hamiltonian parameters were determined to be S = 1, g = 2.0065, D = -0.0112 cm(-1), and …
Translational and rotational molecular motion in supercooled liquids studied by NMR and forced Rayleigh scattering
1994
It has been shown that translational diffusion coefficients, Dt, in the supercooled van der Waals liquids, orthoterphenyl, phenolphthaleindimethylether, and salol, have a weaker temperature dependence than the shear viscosity, η, at T ≲ 1.2Tg and can be described by Dt ∼ η−χ with χ < 1 whereas Dr ∼ η−1 applies for the mean rotational diffusion coefficients, Dr, down to the glass transition temperature, Tg. This apparent decoupling of translational and rotational motion has been discussed in relation with possible anomalous short time diffusion, spatial heterogeneity, and cooperative molecular motions close to Tg.
State-to-state vibrational and rotational energy transfer in CO2 gas from time-resolved raman-infrared double resonance experiments
1998
A time-resolved Raman–infrared double resonance technique was used to study collisional relaxation rates of vibrational and rotational energy levels in CO2 gas at 295 K. A pulsed Raman excitation populated a selected rovibrational initial state. Measurements of the rates of transfer from the pumped initial state into specific final states were carried out using time-resolved laser absorption spectroscopy. First, the transfer rates were determined for the five lower vibrational energy levels. In particular, it was confirmed that the rate of transfer between the two Fermi levels (1000) and (0200) is very small [(5.3±0.2)×104 Torr-1 s-1]. The rotational structure inside the (0200) vibrational …