Torques on a nearly rigid body in a relativistic gravitational field

The effect of post-Newtonian potentials on the rotation of a perfect fluid, nearly rigid body is shown to consist of a precession and a torque. The frequency of the precession can be exactly represented by means of suitable differential operators. The relativistic torques, in the quadrupole approximation, depend on the instantaneous orientation of the principal axes of one body with respect to the position-like the classical torque-and velocity of the other. For a relatively low-mass body, such as a gyroscope, these velocity-dependent torques have no observable consequences.National Research Council Resident Research Associate.     

Interactions in water-alcohol mixtures: Conductance of lithium picrate in solutions of water and isomeric pentanols at 25°C

Conductance measurements of lithium picrate in solutions of water in n-pentanol, 2-pentanol, 3-pentanol, 2-methyl-1-butanol and 3-methyl-1-butanol have been carried out at 25°C. Ionic association and conductance were found to change with water content and with the molecular structure of the alcohols (i.e. position of the OH group and degree of branching of the alkyl chain). These results indicate that both conductance and ion pair formation are not the consequence of the simple motion of ions in the electrical field as required by the continuum model. A more realistic approach, involving the internal structure of the solvent mixtures, has been considered.     

Nucleophilic addition of the thiocyanate ion to ferrocenyl-stabilised carbocations

The reaction of ferrocenylmethylcarbocations with the ambident thiocyanate ion produces only the isothiocyanate isomer.     

Structural and morphological investigation of ceria-promoted Al(2)O(3) under severe reducing/oxidizing conditions

A series of CeO(2)/Al(2)O(3) samples with different ceria loadings in the range 0-25 wt % (0, 2, 5, 7.5, 15, and 25%) were prepared by incipient wetness and studied using several complementary techniques such as Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), temperature-programmed reduction (TPR), Raman, high-resolution transmission electron microscopy (HRTEM), and extended X-ray absorption fine structure (EXAFS). The aim of the investigation was to understand the behavior of ceria when deposited on alumina and treated under oxidizing and reducing conditions at high temperature (T >/= 1273 K). It is shown that ceria can partially stabilize alumina toward the formation of low-surface-area phases up to 1373 K under oxidizing conditions, while enhanced stabilization is observed under reducing conditions, being effective up to 1473 K. A detailed quantitative temperature-programmed reduction (TPR) analysis made at different loadings and calcination temperatures allowed us to identify three characteristic regions where the reduction of small and large ceria crystallites occurs with the formation of CeAlO(3) crystallites at high temperature. These are likely responsible for surface-area stabilization. For dispersed ceria samples, reduction takes place almost exclusively at low temperature (<700 K), while a shift to higher temperatures is observed upon increasing the ceria particle size. A fraction of Ce, in samples at low loadings, is stable in the lower oxidation state, even if subjected to strongly oxidizing conditions.     

Azimuthally polarized spatial dark solitons: exact solutions of Maxwell's equations in a Kerr medium

Spatial Kerr solitons, typically associated with the standard paraxial nonlinear Schro dinger equation, are shown to exist to all nonparaxial orders as exact solutions of Maxwell's equations in the presence of the vectorial Kerr effect. More precisely, we prove the existence of azimuthally polarized, spatial, dark soliton solutions of Maxwell's equations, while exact linearly polarized (2 + 1)D solitons do not exist. Our ab initio approach predicts the existence of dark solitons up to an upper value of the maximum field amplitude, corresponding to a minimum soliton width of about one-fourth of the wavelength.     

Nonparaxial dark solitons in optical Kerr media

We show that the nonlinear equation that describes nonparaxial Kerr propagation, together with the already reported bright-soliton solutions, admits of (1 + 1)D dark-soliton solutions. Unlike their paraxial counterparts, dark solitons can be excited only if their asymptotic normalized intensity u2infinity is below 3/7; their width becomes constant when u2infinity approaches this value.     

Topological origin of the phase transition in a model of DNA denaturation

The hypothesis that phase transitions originate from some topological change of the critical level hypersurface of the potential energy receives direct evident support by our study of the Bishop-Peyrard model of DNA thermal denaturation.