Peak effect studies in single crystals CeRu2 and 2H-NbS2

We have studied the peak effect (PE) phenomenon in single crystals of weakly pinned superconductors CeRu₂ and 2H-NbS₂. 2H-NbS₂ is iso-structural and iso-electronic to 2H-NbSe₂, whose similarity with CeRu₂ as regards the PE representing the order-to-disorder transformation of the flux line lattice was claimed some time ago. We report on the step change in equilibrium magnetization across the peak effect in CeRu₂. We also present the vortex phase diagram of 2H-NbS₂ obtained from the magnetization data, and compare the PE phenomenon in 2H-NbS₂ and 2H-NbSe₂.     

Electron release by a tertiary carbon atom

Summary Heterocyclic iminoxyl free radicals display the effect of electron release through a tertiary carbon atom via the sigma bond system of a molecule.     

A model of macroradical recombination in dilute solutions

The dependence of exchange rate constants and diffusion coefficients of stable macroradicals (modified polydimethylsiloxane ($\text{P}\text{= 15, 29, 97}$) and polyethylenoxide ($\text{P}\text{= 25, 47, 140}$) on the length of polymer chain in various solvents is investigated. The results are compared with the rate constants for diffusive bimolecular recombination of macroradicals of various lengths. The dependence of rate constants of exchange and combination for macroradicals of different lengths obey the same law.The rate of combination for macroradical with P = 15-10⁵ is shown to depend mainly on the mobility of an active centre, limited by diffusion of the whole macromolecule. The diffusion rate constant of combination can be described by the Smoluchowski equation in which the radius of an active centre is used as the effective radius of interaction.A model of bimolecular termination of macroradicals, a model of “freely penetrating coils” (FPC), is proposed. It implies that k_ter ～ P^{?0.9 ± 0.1} (for “good” solvents). The reactivity of an active centre in recombination is independent of the length of a macroradical chain for P > 15.The exchange interaction in a radical pair broadens the ESR lines of radicals and recombination with equal effectiveness.     

The Al+ -H(2) cation complex: rotationally resolved infrared spectrum, potential energy surface, and rovibrational calculations

The infrared spectrum of the Al(+)-H(2) complex is recorded in the H-H stretch region (4075-4110 cm(-1)) by monitoring Al(+) photofragments. The H-H stretch band is centered at 4095.2 cm(-1), a shift of -66.0 cm(-1) from the Q(1)(0) transition of the free H(2) molecule. Altogether, 47 rovibrational transitions belonging to the parallel K(a)=0-0 and 1-1 subbands were identified and fitted using a Watson A-reduced Hamiltonian, yielding effective spectroscopic constants. The results suggest that Al(+)-H(2) has a T-shaped equilibrium configuration with the Al(+) ion attached to a slightly perturbed H(2) molecule, but that large-amplitude intermolecular vibrational motions significantly influence the rotational constants derived from an asymmetric rotor analysis. The vibrationally averaged intermolecular separation in the ground vibrational state is estimated as 3.03 A, decreasing by 0.03 A when the H(2) subunit is vibrationally excited. A three-dimensional potential energy surface for Al(+)-H(2) is calculated ab initio using the coupled cluster CCSD(T) method and employed for variational calculations of the rovibrational energy levels and wave functions. Effective dissociation energies for Al(+)-H(2)(para) and Al(+)-H(2)(ortho) are predicted, respectively, to be 469.4 and 506.4 cm(-1), in good agreement with previous measurements. The calculations reproduce the experimental H-H stretch frequency to within 3.75 cm(-1), and the calculated B and C rotational constants to within approximately 2%. Agreement between experiment and theory supports both the accuracy of the ab initio potential energy surface and the interpretation of the measured spectrum.     

On the role of scattering resonances in the F+HD reaction dynamics

We study scattering resonances in the F+HD-->HF+D reaction using a new method for direct evaluation of the lifetime Q-matrix [Aquilanti et al., J. Chem. Phys. 2005, 123, 054314]. We show that most of the resonances are due to van der Waals states in the entrance and exit reaction channels. The metastable states observed in the product reaction channel are assigned by calculating the energy levels and wave functions of the HF...D van der Waals complex. The behavior of resonance energies, widths, and decay branching ratios as functions of total angular momentum is analyzed. The effect of isotopic substitution on resonance energies and lifetimes is elucidated by comparison with previous results for the F+H2 reaction. It is demonstrated that HF(v'=3) products near threshold are formed by decay of the narrow resonances supported by van der Waals wells in the exit channel. State-to-state differential cross sections in the HF(v'=3) channel exhibit characteristic forward-backward peaks due to the formation of a long-lived metastable complex. The role of the exit-channel resonances in the interpretation of molecular beam experiments is discussed.     

Closed model of oxygen recombination on an Al2O3 surface

On the basis of cluster-approximation quantum-chemical calculations of the interaction of an α-Al₂O₃ surface with oxygen, the rate coefficients for the elementary steps of the heterogeneous recombination of atomic oxygen are determined in the framework of the Eley-Rideal and Langmuir-Hinshelwood mechanisms. For the diffusion layer near the studied surface, these coefficients are used to calculate the probabilities of heterogeneous catalytic recombination, surface coverage, and heat flux to the surface at temperatures of 200–2000 K and pressures of 1000–7000 Pa. The results are compared to the results a solid-state periodic model, low-temperature plasma etching studies, and empirical models of recombination of atomic oxygen on a SiO₂ surface.     

Numerical method of quantum capture probability determination for molecular collisions at ultralow temperatures

The numerical method suggested by Truhlar and Kuppermann (J. Am. Chem. Soc., 1971, vol. 93, p. 1840) to determine tunneling probabilities is adapted for quantum capture calculations in barrierless molecular processes by means of absorbing boundary conditions imposed in the range of strong interactions. It is shown that the phase uncertainty of the singular scattering problem, which arises during the extrapolation of the long-range interaction potential to short distances, is revealed as the oscillatory dependence of the transmission coefficient on the point at which the boundary conditions were imposed. The mean transmission coefficient computation makes it possible to decrease the uncertainty of the results. The method is evaluated to calculate the KRb + KRb reaction rates and K₂ + K vibrational relaxation at ultralow temperatures using model dispersion and adiabatic channel potentials derived from ab initio calculations. The results are in good agreement with the data of analytic models based on the solution of the singular scattering problem close to Bethe-Wigner energy threshold and, within the capture approximation accuracy, with the data of a rigorous quantum scattering theory.