Endor frequencies of randomly oriented triplets by electron spin echo spectroscopy

ENDOR frequencies of randomly oriented triplets have been measured from the fast Fourier transform of electron spin echo envelope modulation. Important experimental conditions are low temperatures, high microwave power and long phase memory time in a proper form of solvent. The technique is exemplified by using diphenylmethylenes in rigid glasses, polycrystalline forms and single crystals of benzophenones.     

Vibrational lineshapes of adsorbates: the role of substrate electronic excitations

The vibrational spectrum of an atom or molecule adsorbed on a metallic surface is influenced by the dynamic interaction between the localized oscillator and the continuum of electron-hole pair excitations in the solid. Features of an exactly soluble model for this interacting system are presented, the local spectral function is obtained in terms of calculable level shift and broadening functions, and some prototypical numerical results are displayed and discussed.     

Conduction electron spin resonance in acceptor-type graphite intercalation compounds

An initial survey of the conduction electron spin resonance is presented for a series of graphite compounds intercalated with acceptor molecules: stages 1–3 AsF₅, stages 2–5 HNO₃, and stage 2 Br₂ and ICl. The g-values and lineshapes were studied as functions of temperature and concentration. The results suggest metallic behavior but with very small density of states at the Fermi energy: N(E_F) ～10²⁰/cm³ eV. The temperature dependence of the linewidth is dominated by an order-disorder transition of the intercalant layers, implying that the conduction electrons are not entirely confined to the graphite portion of the crystal. The decrease in g-value anisotropy upon intercalation can be understood in terms of Elliott's theory.     

Oxidopyrylium [5+2] cycloaddition chemistry: Historical perspective and recent advances (2008–2018)

The following review article provides an overview of oxidopyrylium [5+2] cycloaddition chemistry, with a particular emphasis placed on seminal historical developments and advancements made over the last decade. It is our hope this review serves as a valuable resource to those interested in the oxidopyrylium cycloaddition chemistry, and helps inspire future advancements.     

Liquid phase microextraction applications in food analysis

Over the last years, liquid-phase microextraction (LPME) in its different application modes (single drop microextraction, dispersive liquid-liquid microextraction and hollow fiber-LPME) has been increasingly applied for the extraction of both inorganic and organic analytes from different matrices. Its advantages over conventional extraction procedures (simplicity, effectiveness, rapidity and low consumption of organic solvents) has also attracted its application in the complex food analysis field, in which it has clearly provided good and challenging results. A comprehensive review dealing with those articles published since its introduction till the end of March 2011 is presented, offering also a critical vision of the analytical potential of LPME for the analysis of foods.     

Influence of surface relaxation on the IR spectrum of microcrystalline MgO

Relaxation effects on the infrared spectra of microcrystalline MgO prepared under controlled conditions have been studied both by varying the average particle size and by chemisorption of pyndine. The results are compared with the phonon density of states distribution calculated by other authors.     

Electronic and magnetic properties of rare-earth-transition-metal glasses

We have studied the magnetic and electrical properties of several amorphous rare-earth—transition-metal alloys obtained by splat cooling. Low-field susceptibility, high-field magnetization and electrical resistivity measurements were made between 1.6 and 300 K. Mössbauer results are reported for (Gd₆₅Fe₃₅)₉₀B₁₀ at the temperatures of 80 and 300 K. Resistivity measurements show both positive and negative coefficients and in some cases maxima or minima that are associated with magnetic ordering or Kondo-type scattering, respectively.     

Effect of collision and magnetic field on quantum beat in biacetyl

Molecular quantum beats are shown for the case of biacetyl, to vary strongly under the influence of weak magnetic fields. Values for dephasing constants for a gas-phase molecular system were determined. Information on sub-Doppler level splittings in the intersystem-crossing process is obtained.     

Picosecond rotational diffusion by differential single-photon fluorescence spectroscopy

The rotational diffusion contants of rhodamine-6G, coumarin-1 and N-acetyltryptophanamide in various solvents have been determined by an experimental method where the vertically and horizontally polarized fluorescence spectra are measured simultaneously by the same detector. The results indicate that the Debye—Stokes—Einstein model does not hold for coumarin-1 and N-acetyltryptophanamide in ethylene glycol.     

Nonlinear dynamics of the heartbeat: I. The AV junction: Passive conduit or active oscillator?

Under physiologic conditions, the AV junction is traditionally regarded as a passive conduit for the conduction of impulses from the atria to the ventricles. An alternative view, namely that subsidiary pacemakers play an active role in normal electrophysiologic dynamics during sinus rhythm, has been suggested based on nonlinear models of cardiac oscillators. A central problem has been the development of a simple but explicit mathematical model for coupled nonlinear oscillators relevant both to stable and perturbed cardiac dynamics. We use equations describing an analog electrical circuit with an external d.c. voltage source (V₀) and two nonlinear oscillators with intrinsic frequencies in the ratio of 3:2, comparable to the SA node and AV junction rates. The oscillators are coupled by means of a resistor. 1:1 (SA:AV) phase-locking of the oscillators occurs over a critical range of V₀. Externally driving the SA oscillator at increasing rates results in 3:2 AV Wenckebach periodicity and a 2:1 AV block. These findings appear with no assumptions about conduction time or refractoriness. This dynamical model is consistent with the new interpretation that normal sinus rhythm may represent 1:1 coupling of two or more active nonlinear oscillators and also accounts for the appearance of an AV block with critical changes in a single parameter such as the pacing rate.