Low temperature polarization effects in ice

When an initially unpolarized HF doped ice specimen is warmed at a constant rate in an applied electric field two peaks are observed in the current. The low temperature peak occurs near 100°K and the temperature at which this peak occurs is seen to increase as the HF concentration decreases. The second peak appears to occur randomly in the temperature range 125–135°K. These peaks are also observed if the sample is cooled in an applied electric field and then warmed at a constant rate with the field removed. It is suggested that the first peak is due to a dielectric relaxation process which is governed by the L defects released from the HF molecules. This release of L defects is shown to obey the law of mass action with an activation energy for liberation of an L defect of 0.12±0.06 eV and a dissociation constantk _o ^L≈10²⁹ m^?3. A simple theoretical model of ice is also developed which predicts the current reversal phenomenon observed by Dengelet al. [11] suggesting that it is due to dipole relaxation and not to ferroelectric ordering.     

On second-order optical processes

It is shown that if the full significance of the algebra involved in the performance of time-dependent perturbation calculations is appreciated, it is possible to predict the existence of a range of effects. These include magnetic field-dependent effects which can also be Raman shifted at sum, difference and second harmonic frequencies. The advantages of using these effects in conducting media is pointed out as well as the possible application to surface studies. These effects are all advantageously inproved by using resonance effects and in double-rather than single-beam experiments.     

Photoelectric properties of aluminium/copper phthalocyanine/gold photovoltaic cells

Solid-state photovoltaic cells based on the semiconductor copper phthalocyanine depend on the presence of dopants for their operation. Electronegative dopants such as oxygen, iodine and water form trap levels in the semiconductor, which aid the dissociation of photoproduced excited states into charge carriers. The cross section for this reaction has been estimated to be about the same as the molecular cross section of phthalocyanine.     

The reactions of C2F5 radicals with benzene in the vapor phase

C₂F₅ radicals were generated in the presence of benzene vapor by the reaction The radicals react with the benzene by addition and pseudo H abstraction The rate constant k_add for the addition reaction (7) is given by where θ = 2.303RT cal/mole and k_c is the rate constant for combination of C₂F₅ radicals. The addition becomes reversible above 110°C. The reactions of CF₃ and C₂F₅ radicals with benzene vapor are compared.     

Terahertz radiation from a nonlinear slab traversed by an optical pulse

We report on the theoretical calculations considering collinear electromagnetic radiation at the propagation of an optical pulse through a slab of nonlinear material. Calculated waveforms of the radiated field fit well to the experimental dependencies showing the remarkable similarities between the radiation at nonlinear wave interaction and the radiation phenomena of moving external charges, similarly to discussed in the Tamm Problem and transition radiation of moving external charges.     

Negative refraction and the spectral filtering of terahertz radiation by a photonic crystal prism

We demonstrate how micromachined photonic crystals can be used to negatively refract terahertz frequency light. The photonic crystals, which are constructed from conventional dielectric materials, manipulate the incident beam via interaction with their photonic bands. Consequently, we show that different components of a broadband beam incident on the structure may be positively or negatively refracted, depending upon its frequency and that the structure can be used as an effective spectral filter of THz radiation.     

Validation of pH-Metric Technique for Measurement of pKa and log Pow of Ionizable Herbicides

Abstract

Knowing the values of pK_a and partition coefficients between 1-octanol and water (P_ow) of ionizable herbicides helps us to understand the movement of these compounds in plants and soil. However, shake-flask and high-pressure liquid chromatography (HPLC) techniques. though valuable for measurement of log P_ow of herbicide molecules that do not ionize in aqueous solution, are difficult to use for ionizable molecules; this is especially so for molecules that form ion pairs having appreciable log P_ow values, those with multiple ionizations and for bases with high pK_a values. Our aim in this study was to validate the use of the pH-metric technique (based on potentiometric titration) for measurements of pK_a and log P_ow of ionizable standard substances and herbicides. The values obtained show good correlation with results from other techniques, including shake-flask and HPLC. The OECD Guideline for Testing of Chemicals 117, adopted 30th March 1989, describes the use of HPLC for the measurement of log P_ow. It is hoped that these studies and further testing of this technique will permit it to be included in these OECD guidelines.     

Azafullerenes encapsulated within single-walled carbon nanotubes

Methods of insertion of azafullerenes in single-walled carbon nanotubes (SWNTs) at different temperatures were investigated, while the effects of the conditions applied on the structure of azafullerene-based peapods, namely, C59N@SWNTs, were explored. Morphological characteristics of C59N@SWNTs were assessed and evaluated by means of high-resolution transmission electron microscopy (HR-TEM). Pathways and chemical reactions that occur upon encapsulation of C59N within SWNTs were evaluated. Monomeric azafullerenyl radical C59N. as inserted into SWNTs at high temperature, from purified (C59N)2 in the gas phase, can undergo a variety of different transformations forming dimers, oligomers or existing in its monomeric form inside SWNTs due to the stabilization effect by nanotube side walls. However, under milder conditions, that is, at lower temperature, bisazafullerene (C59N)2 can be inserted into SWNTs in its pristine dimeric form.     

Reversible carboxamide-mediated internal activation at C(6) of 2-chloro-4-anilino-1H-pyrrolo[2,3-d]pyrimidines

A synthetic route to bisanilino-1H-pyrrolo[2,3-d]pyrimidines has been discovered, wherein the C(6)-chloride reactivity is necessarily enhanced via reversible acid-catalyzed internal activation of the pyrimidine ring by a C(1')-carboxamide moiety. Subsequent selective nucleophilic displacements at C(6) and C(1') constitute a one-pot tandem protocol for the rapid assembly of bisanilino-1H-pyrrolo[2,3-d]pyrimidines.