Investigation of photoinduced scattering in LiNbO3 crystals

A photoinduced scattering effect in LiNbO₃:Fe crystals was investigated in detail. The scattering depends substantially on illumination geometry, light beam profile, doping degree, and appears under coherent illumination only. It was shown that besides the photoinduced scattering a bleaching effect is possible. It consists in the fact that after the stage of low-scale inhomogeneity formation there occurs their effective erasure. The conditions were found out under which the bleaching effect can be observed.     

General theory of population transfer by adiabatic rapid passage with intense, chirped laser pulses

We present a general theory of adiabatic rapid passage (ARP) with intense, linearly chirped laser pulses. For pulses with a Gaussian profile and a fixed bandwidth, we derive a rigorous formula for the maximum temporal chirp rate that can be sustained by the pulse. A modified Landau-Zener formula displays clearly the relationships among the pulse parameters. This formula is used to derive the optimal conditions for efficient, robust population transfer. As illustrations of the theory, we present results for two- and four-level systems, and selective vibronic excitation in the I₂ molecule. We demonstrate that population transfer with chirped pulses is more robust and more selective than population transfer with transform-limited pulses. Received 6 September 2000 and Received in final form 25 September 2000     

Specific features in the behavior of the central peak in Raman spectra of lithium tantalate

Low-frequency Raman spectra of a LiTaO₃ ferroelectric crystal are studied in the temperature range 300–1273 K. The central peak associated with the relaxation susceptibility of the crystal lattice is recorded throughout the temperature range covered, including the temperatures both substantially lower and considerably higher than the Curie point (T _c = 900 K). The critical slowing down of the relaxation response time predicted by the Ginzburg-Landau-Devonshire theory is observed in the temperature range from 0.9T _c to 1.1T _c . In contrast to the critical slowing down, the width of the central peak γ_R increases below 0.9T _c and decreases above 1.1T _c with increasing temperature.     

Systematic comparison of post-column isotope dilution using LC-CO-IRMS with qNMR for amino acid purity determination

Analytical and Bioanalytical Chemistry - Determination of the purity of a substance traceable to the International System of Units (SI) is important for the production of reference materials...     

On the low-temperature onset of molecular flexibility in lipid bilayers seen by Raman scattering

For dipalmitoylphosphatidylcholine (DPPC) lipid/water bilayers, a detailed temperature dependence of the Raman scattering spectra at the spectral range of the CH 2-stretching modes was investigated. Below 150 K the ratio of intensities of the 2880 cm (-1) antisymmetric vibration line and the 2850 cm (-1) symmetric one was found to be nearly temperature-independent. Between 150 and 230 K it decreases slightly as temperature increases; and above 230 K it decreases remarkably. This decrease is accompanied with broadening of the antisymmetric line, from 4.2 cm (-1) at 100 K to 5.7 cm (-1) at 296 K. According to literature, the decrease of the antisymmetric line may be interpreted in two ways: (i) the appearance of a static conformational disorder (or of a disorder fluctuating at the time scale larger than picoseconds) and (ii) relaxation at the ps time scale, which is induced by coupling with temperature-activated librational-torsional motion of the lipid chain. Both these interpretations imply that obtained data evidence the appearance of molecular flexibility of lipids around approximately 200 K. The observed effect is to be compared with low-temperature dynamical transition found in disordered media with neutron scattering, Mossbauer absorption, molecular dynamics simulations and other techniques. This transition implies that with temperature increase harmonic atomic motions are transformed to large-amplitude anharmonic (or stochastic) ones. The characteristic times of these motions lay at the ps time scale. The closeness of the temperature of the transition and of the time scale of motions with those found in this work by Raman scattering for lipid bilayers supports the dynamic nature of the 2880 cm (-1) antisymmetric vibration line decrease (i.e., that it is induced by coupling with libration-torsion). To prove that the observed onset of flexibility is a property of a disordered state, Langmuir-Blodgett films of behenic acid were studied. These films contain, like lipids, long CH 2-tails, but, in opposite to bilayers, they have a well-ordered crystalline-like structure. The relative intensity of the antisymmetric/symmetric CH 2-stretching lines was found in these films to be temperature-independent in the whole temperature range studied, between 60 and 296 K.     

Structure of the [Co(NioxH)2(PPh 3)2]F complex

Compound [Co(NioxH)₂(PPh ₃)₂]F is synthesized in the CoF₂ · 4H₂O-NioxH₂-PPh ₃ system (where NioxH is the 1,2-cyclohexanedione dioxime monoanion and PPh ₃ is triphenylphosphine), and its structure is determined by X-ray diffraction. It is shown that the cobalt atom has the octahedral environment. Two nioxime residues that are related by the center of symmetry lie in the equatorial plane and are linked by the O-H?O hydrogen bond. The 1,6-positions of the octahedron are occupied by the phosphorus atoms of the triphenylphosphine ligands. The formation of the crystal structure of this compound is discussed.     

Stochastic Precedence and Minima Among Dependent Variables

Methodology and Computing in Applied Probability - The notion of stochastic precedence between two random variables emerges as a relevant concept in several fields of applied probability. When one...     

Raman study of low-frequency modes in three glycine polymorphs

The temperature dependence of selected low-wavenumber (< 200 cm(-1)) Raman bands was studied for the different crystalline phases (α-, β-, γ-) of glycine--the simplest possible "building block" of a biomolecule. The temperature dependence of the frequencies of vibrational modes deviates from the theoretical expectation based on the assumption of cubic anharmonicity. Although relatively small, this deviation was observed above 250 K for all the three polymorphs. This finding was discussed in relation to the "dynamical transition" phenomenon, observed in variety of biomolecules in the range 200-250 K. The similarity of the temperatures suggests, that the origin of the dynamical transition phenomenon can be related to intrinsic conformational states of biomolecules, while water serves rather as a plasticizer or a structure organizer.