Kinetics of the reactions of the IO radical with dimethyl sulfide,methanethiol, ethylene,and propylene

The reactions of IO radicals with CH₃SCH₃, CH₃SH, C₂H₄, and C₃H₆ have been studied using the discharge flow method with direct detection of IO radicals by mass spectrometry. The absolute rate constants obtained at 298 K are the following: IO + CH₃SCH₃ → products (1): k₁ = (1.5 ± 0.2) × 10^?14; IO + CH₃SH → products (2): k₂ = (6.6 ± 1.3) × 10^?16; IO + C₂H₄ →products (3): k₃ < 2 × 10^?16; IO + C₃H₆ → products (4): k₄ < 2 × 10^?16 (units are cm³ molecule^?1 s^?1). CH₃S(O)CH₃ and HOI were found as products of reactions (1) and (2), respectively. The present lower value of k₁ compared to our previous determination is discussed.     

Improved error estimates for mixed finite-element approximations for nonlinear parabolic equations: The discrete-time case

This article treats mixed finite-element methods for second-order quasilinear parabolic equations. A fully discrete scheme is presented and L₂-error estimates are established. The convergence of both the function value and the flux is demonstrated. In a previous paper, Part I the author introduced powerful numerical methods for dealing with parabolic problems. The technique used here is an extension of the continuous-time results for the discrete-time case. © 1994 John Wiley & Sons, Inc.     

A positron annihilation study in some amorphous scintillators

Two organic scintillators — NE104 and a liquid solution of 2,5 diphenyloxazole (PPO) — were investigated through positron annihilation spectroscopy to study the influence of the structural order on the magnetic quenching effect on positron bound systems in solids.Preliminary note in [11]     

Synthesis of 4H-pyrans by O-cyclization of 1,5-diketones

Substituted 4H-pyrans were obtained in high preparative yields by the reaction of 1,5-diketones with acetic anhydride and boron trifluoride etherate in diethyl ether. It is assumed that the heterocyclization of 1,5-diketones includes a step involving the formation and transformations of an acyloxy carbonium ion.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 320–322, March, 1992.     

Superconvergence of recovered gradients of discrete time/piecewise linear Galerkin approximations for linear and nonlinear parabolic problems

Superconvergent error estimates in l₂(H¹) and l∞(H¹) norms are derived for recovered gradients of finite difference in time/piecewise linear Galerkin approximations in space for linear and quasinonlinear parabolic problems in two space dimensions. The analysis extends previous results for elliptic problems to the parabolic context, and covers problems in regions with nonsmooth boundaries under certain assumptions on the regularity of the solutions. © 1994 John Wiley & Sons, Inc.     

Structural Transformations in Fe/Zr Bi- and Multilayers Induced by Ion-Beam Mixing

The Ar-ion-beam mixing of Fe/Zr bi- and multilayers is studied by conversion electron Mössbauer spectroscopy as a function of ion dose ranging from 1×10¹⁴ to 1×10¹⁷ at./cm². The mixing leads to amorphization of the Fe/Zr system. It was shown that the mixing process in bilayers depends strongly on the thickness of the Zr substrate. The amorphization is much more effective and occurs at lower ion doses in multilayers as compared to corresponding bilayers.     

Luminescence of self-trapped excitons in calcium fluoride under pulsed electron irradiation

Time-resolved pulsed spectroscopy was used to measure the luminescence spectra of calcium fluoride. Characteristic features of the luminescence of self-trapped excitons are discussed. It is shown that various configurations of self-trapped excitons incorporating hole nuclei of a more complex structure, may be formed in CaF₂ crystals. Fiz. Tverd. Tela (St. Petersburg) 40, 1226–1227 (July 1998)     

On the optimization of approximate integration by Monte Carlo methods

We solve the problem of optimization of monte Carlo methods for approximate integration over an arbitrary absolutely continuous measure. We propose a convenient model of Monte Carlo methods which uses the notion of transition probability.