13C CP/MAS NMR studies of vitamin E model compounds

13C cross-polarization magic angle spinning (CP/MAS) NMR data for 2,2,5,7,8-pentamethylchroman-6-ol (2), 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox c) (3) and its acetate (4), 2-methoxy-2,2,5,7,8-pentamethylchroman-6-ol (5), 2-hydroxy-2,2,5,7,8-pentamethylchroman-6-ol (6) and 2,2,5,7,8-pentamethylchroman (7) are reported. A deshielding of 7.7 ppm for the carboxylic carbon was observed in solid Trolox due to formation of intermolecular hydrogen bonds within cyclic dimers. Such crystal packing permits effective cross-polarization and fast relaxation (short T1rho(H)). The impact of the proton concentration on the CP dynamics is reflected by the longer T(CP) and T1rhoH for Trolox-d2 (deuterated at mobile proton sites). The calculated GIAO RHF shielding constants are sensitive to intramolecular effects: rotation around the C-6-O bond (changes of sigma up to 8 ppm) and conformation at C-2.     

Hitting times of Brownian motion and the Matsumoto–Yor property on trees

The Matsumoto–Yor property in the bivariate case was originally defined through properties of functionals of the geometric Brownian motion. A multivariate version of this property was described in the language of directed trees and outside of the framework of stochastic processes in Massam and Weso?owski [H. Massam, J. Weso?owski, The Matsumoto–Yor property on trees, Bernoulli 10 (2004) 685–700]. Here we propose its interpretation through properties of hitting times of Brownian motion, extending the interpretation given in the bivariate case in Matsumoto and Yor [H. Matsumoto, M. Yor, Interpretation via Brownian motion of some independence properties between GIG and gamma variables, Statist. Probab. Lett. 61 (2003) 253–259].     

All-optical determination of initial oxidation of Si(100) and its kinetics

By comparison of measured and ab initio calculated surface optical spectra we demonstrate that two main oxidation processes initially occur after dissociation of oxygen molecules, forming in both cases Si–O–Si entities: (i) breaking of Si dimers by incorporation of oxygen atoms; (ii) incorporation into the silicon backbonds. The kinetics up to half-monolayer coverage is determined, and explained in terms of Langmuir-like adsorption mechanisms with different probabilities.     

Infrared spectra of hydrogen bonded crystals: Uracil

A theory of the infrared spectra of hydrogen bonds in molecular crystals of C$\text{5}\text{2}$h symmetry, containing, like uracil, four molecules and eight hydrogen bonds in the unit cell, is presented. The theoretical spectra are in good agreement in both the frequency and intensity distribution with experiment. The changes in the fine structure introduced by deuteration are quantitatively reproduced.     

Infrared spectra of the hydrogen bonded crystals: 1-methylthymine

A theory for the infrared spectra of hydrogen bonds in molecular crystals like 1-methylthymine, containing four hydrogen bonded molecules in the unit cell is presented. The spectra computed theoretically are in good agreement in both the frequency and intensity distribution with the experimental ones and the changes in the fine structure of the spectra introduced by deuteration are quantitatively reproduced.     

Probing the Si-Si dimer breaking of Si(100)2x1 surfaces upon molecule adsorption by optical spectroscopy

The adsorption of atoms and molecules of several gases of the Si(100)2x1 silicon reconstructed surface is investigated by surface differential reflectance spectroscopy. This UV-visible optical spectroscopy makes possible the discrimination between two adsorption modes, depending on whether or not the adsorption leads to breaking the Si-Si dimers. The observation of two different optical features is assigned to the bonding on dangling bonds or to the breaking of dimers, and gives access to the adsorption mode of hydrogen, water, oxygen, and pyridine. Moreover, the technique being quantitative, we can determine the total amount of dimers involved in the adsorption and monitor the adsorption kinetics.     

The laser pulse from a rf waveguide CO2 laser

The behaviour of a rf-excited waveguide CO₂ laser in the pulse regime is studied experimentally. The specific time sections of the pulse have been defined and described in detail. The output pulse evolution versus input power, pressure of the laser mixture, output coupling, repetition frequency and pulse width has been investigated. The spectral content of the pulse has been analysed. A model explaining the effect of laser tuning during the pulse duration is given. The ultrahigh pulse (UHP), defined as the pulse for which the bulge effect occurs, is predicted. Received: 22 October 2002 / Revised version: 13 January 2003 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. Fax: +48-71/3203-189, E-mail: eda@zr.ita.pwr.wroc.pl