On ultra-thin oxide/Si and very-thin oxide/Si structures prepared by wet chemical process

The properties of ultra-thin oxide/Si and very-thin oxide/Si structures prepared by wet chemical oxidation in nitric acid aqueous solutions (NAOS) and passivated in HCN aqueous solutions were investigated by electrical, optical and structural methods. n- and p-doped (1 0 0) crystalline Si substrates were used. There were identified more types of interface defect states in dependence on both post-oxidation treatment and passivation procedure. On samples prepared on n-type Si, continuous spectrum of defect states of 0.05-0.2 eV range and discrete defect traps, ∼E_CB − 0.26 eV and ∼E_CB − 0.39 eV, were found. All mentioned defects are related with various types of Si dangling bonds and/or with SiO_x precipitates. Post-metallization annealing of investigated MOS structures reduced the interface defect density and suppressed the leakage currents. It did not change spectral profile of interface defect states in the Si band gap. In addition, there are presented following two optical phenomena: relation between amplitude of photoluminescence signal of NAOS samples and parameters of chemical oxidation process and quantum confinement effect observed on samples containing Si grains of size less as ∼2 nm.     

Glycine phases formed from frozen aqueous solutions: Revisited

Glycine phases formed when aqueous solutions were frozen and subsequently heated under different conditions were studied by Raman scattering, x-ray diffraction, and differential scanning calorimetry (DSC) techniques. Crystallization of ice I(h) was observed in all the cases. On cooling at the rates of 0.5 K∕min and 5 K∕min, glassy glycine was formed as an intermediate phase which lived about 1 min or less only, and then transformed into β-polymorph of glycine. Quench cooling of glycine solutions (15% w∕w) in liquid nitrogen resulted in the formation of a mixture of crystalline water ice I(h) and a glassy glycine, which could be preserved at cryogenic temperatures (80 K) for an indefinitely long time. This mixture remained also quite stable for some time after heating above the cryogenic temperature. Subsequent heating under various conditions resulted in the transformation of the glycine glass into an unknown crystalline phase (glycine "X-phase") at 209-216 K, which at 218-226 K transformed into β-polymorph of glycine. The "X-phase" was characterized by Raman spectroscopy; it could be obtained in noticeable amounts using a special preparation technique and tentatively characterized by x-ray powder diffraction (P2, a = 6.648 A?, b = 25.867 A?, c = 5.610 A?, β = 113.12[ordinal indicator, masculine]); the formation of "X-phase" from the glycine glassy phase and its transformation into β-polymorph were followed by DSC. Raman scattering technique with its power for unambiguous identification of the crystalline and glassy polymorphs without limitation on the crystallite size helped us to follow the phase transformations during quenching, heating, and annealing. The experimental findings are considered in relation to the problem of control of glycine polymorphism on crystallization.     

Optical control of the singlet-triplet transition in Rb2

By controlling nonresonant dynamic Stark shifts it is possible to effectively decouple the intramolecular couplings of simple molecules. We have illustrated this effect in the 1 (1)Sigma(u)-->1 (3)Pi(u) transition in Rb(2). The laser scheme implies two important control knobs: the laser frequency, which must be chosen to avoid both single and multiphoton resonances and to select different electronic environments for the singlet and triplet states, and the pulse intensity, which must amplify the asymmetry in the dynamic polarizabilities that allows the decoupling, avoiding undesired strong-pulse effects. The mechanism of the scheme implies not only brute-force energy shifts but also light-induced reshaping of the potentials to avoid the undesired crossings. Quantitative aspects of the method are analyzed by using molecular models of increasing complexity for Rb(2).     

Implementing quantum gates on oriented optical isomers

Optical enantiomers are proposed to encode molecular two-qubit information processing. Using sequences of pairs of nonresonant optimally polarized pulses, different schemes to implement quantum gates, and to prepare entangled states, are described. We discuss the role of the entanglement phase and the robustness of the pulse sequences which depend on the area theorem. Finally, possible scenarios to generalize the schemes to n-qubit systems are suggested.     

Extremely slow intramolecular vibrational redistribution: Direct observation by time-resolved raman spectroscopy in trifluoropropyne

We have studied the dynamics of intramolecular vibrational redistribution (IVR) from the initially excited mode v₁ ≈ 3330 cm⁻¹ (acetylene-type H-C bond) in H-C≡C-CF₃ molecules in the gaseous phase by means of anti-Stokes spontaneous Raman scattering. The time constant of this process is estimated as 2.3 ns—this is the slowest IVR time reported so far for the room-temperature gases. It is suggested that so long IVR time with respect to the other propyne derivatives can be explained by a larger defect, in this case, of the Fermi resonance of v₁ with v₂ + 2v₇—the most probable doorway state leading to IVR from v₁ to the bath of all vibrational-rotational states with the close energies. In addition, it is shown that the observed dynamics is in agreement with a theoretical model assuming strong vibrational-rotational mixing.     

Mass-independent isotope fractionation of heavy elements measured by MC-ICPMS: a unique probe in environmental sciences

This article overviews recent developments in the use of multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) in studies of mass-independent isotope chemistry of heavy elements. Origins of mass-independent isotope effects and their relevance to isotope ratio measurements by MC-ICPMS are briefly described. The extent to which these effects can affect instrumental mass bias in MC-ICPMS is critically discussed on the basis of the experimental observations. Furthermore, key findings reported in studies of mass-independent isotope fractionation (MIF) of mercury in the field of environmental sciences are reviewed. MIF of heavy elements is not only of interest from a fundamental point of view, but also provides scientists with a new and effective means of studying the biogeochemistry of these elements.     

Low-frequency Raman scattering study of the ferroelectric phase transition in the DKDP crystal

The low-frequency Raman scattering spectra of the DKDP ferroelectric crystal are studied in the temperature range 30–393 K. At temperatures above 150 K, the Raman spectra exhibit a central peak which reflects the lattice relaxation susceptibility. The width and integrated intensity of the central peak are derived from the experimental spectra. The critical slowing down of the relaxation response predicted by the Ginzburg-Landau-Devonshire theory is observed throughout the temperature range in which the central peak persists. Its integrated intensity does not, however, follow the predictions of the theory and reveals a strong temperature dependence in the ferroelectric phase and a weaker dependence in the paraphase. It is shown that the thermal activation law describes well the temperature dependence of the intensity of the central peak. An interpretation is proposed according to which the intensity of order parameter fluctuations is related to the activation barrier whose height is proportional to the deviation from the phase transition temperature.     

Studying the Nonlinear Optical Response from Local Polar Inhomogeneities in Strontium Barium Niobate Crystals of Different Chemical Composition

The generation of the second optical harmonic in a series of SBN-x crystals (x = 0.75, 0.61, 0.50, 0.33) is studied in the 200–900 K temperature range. It is shown that the spectral width of nonlinear response in all samples is less than 1 cm^?1 and does not depend on the temperature. Its integral intensity follows the Arrhenius law. The parameters of this Arrhenius presentation are determined and compared with the corresponding parameters for a number of ferroelectrics and relaxors.