Comparison of exchange mechanisms in πN → ϱN and pion photo- and electroproduction

Pho-, photo- and electroproduction are compared and contrasted from a t-channel exchange point of view. A common exchange mechanism is evident. Systematic differences associated with the variable mass of the vector particle are found - in particular, the mass dependence of the non-pole-like contributions is reminiscent of that seen in higher mass resonance production. Naive vector meson dominance arguments which do not allow for these mass-dependent effects are shown to disagree both qualitatively and quantitatively with the data. We emphasize the implications of this, and similar studies, for an understanding of absorption effects in two-body scattering.     

The Stark and Zeeman effects in methyl silane

The Stark and Zeeman effects in methyl silane in the ground vibronic state have been studied in detail using the molecular-beam electric-resonance method. For a symmetric rotor without internal rotation, the rotational dependence of the effective dipole moment for matrix elements diagonal in J has been shown by Watson, Takami, and Oka to have the form μ_Q = μ₀ + μ_JJ(J + 1) + μ_KK². It is shown here that, to this order, a complete characterization of the Stark effect requires only one more parameter, namely, the effective anisotropy (α_| - α_⊥)_eff in the polarizability. From Stark measurements alone, the true anisotropy cannot be separated from the additional dipole distortion constant shown by Aliev and Mikhaylov to enter dipole matrix elements off-diagonal in J. By studying nine different transitions (J, K, m_J) → (J, K, m_J ± 1) in CH₃²⁸SiH₃, values were obtained for the four Stark parameters: μ₀ = 0.7345600(33) D, μ_J = 8.83(35) μD, μ_K = ?32.82(37) μD, and (α_| - α_⊥)_eff = 1.99(16) × 10^?24 cm³. These errors reflect only the internal consistency in the data; the absolute error in μ₀ is 32 μD. The modification of the Stark effect by internal rotation is discussed; it is shown that the only significant effect here is to modify the interpretation of μ₀. The change in μ₀ upon isotopic substitution of ³⁰Si for ²⁸Si was determined: $\text{μ}{}_0\text{(}{}^{30}\text{Si) - μ}{}_0\text{(}{}^{28}\text{Si) = 67.0(2.0) μD}$. A study of molecular magnetic effects in CH₃²⁸SiH₃ has yielded the two molecular g factors, g_⊥ = ?0.036391(21) nm and g_| = ?0.10667(13) nm, as well as the anisotropy in the susceptibility $\text{(χ}{}_{\mathrm{|}}\text{- χ}{}_{\mathrm{\perp }}\text{) = ?44.9(2.3) × 10}{}^{\mathrm{?30}}\text{J}\text{T}{}^2$. The molecular quadrupole moment has been calculated.     

Jumping solitary waves in an autonomous reaction-diffusion system with subcritical wave instability

We describe a new type of solitary waves, which propagate in such a manner that the pulse periodically disappears from its original position and reemerges at a fixed distance. We find such jumping waves as solutions to a reaction-diffusion system with a subcritical short-wavelength instability. We demonstrate closely related solitary wave solutions in the quintic complex Ginzburg-Landau equation. We study the characteristics of and interactions between these solitary waves and the dynamics of related wave trains and standing waves.     

Diffusion on a self-assembled monolayer: molecular modeling of a bound + mobile lubricant

The diffusion of tricresyl phosphate molecules on an octadecyltrichlorosilane self-assembled monolayer (SAM) was characterized using molecular dynamics simulations. The simulations predict that when placed on the top of a close-packed SAM, the molecules remain mobile on the surface with an isotropic diffusion activation energy of approximately 9 kJ/mol. In contrast, an anisotropic barrier that results from chain tilt within the SAM is predicted for diffusion into a defect created by reducing the alkane chain length within a cylinderical region of the surface. Once in the defect, the molecules become trapped by embedding part of the molecule into the side of the SAM.     

Isomerism due to the rotation of an alkyl side-chain attached to a heterocyclic ring: Crystal and molecular structure of two forms of 6-nitro-2,4-bis(1′,1′,2′-trichloropropyl)-1,3-benzdioxin

Compound I crystallizes in the space groupP2₁/c withZ=4,a=5.889(5),b=30.755(10),c=10.815(3) Å,=92.95(6)°. Compound II crystallizes in the space groupP2₁/n withZ=4,a=10.235(2),b=10.144(1),c=18.346(2) Å,=92.00(1)°. The structures were solved by direct methods and refined by full-matrix least squares, from room-temperature data obtained with an Enraf-Nonius CAD4 diffractometer, to conventionalR factors of 0.041 for I and 0.048 for II.In both isomers the dioxin ring has an approximate envelope conformation with a pseudoequatorial –CC1₂·CHCl·CH₃ group in the 2-position and a pseudoaxial –CCl₂·CHCl·CH₃ group in the 4-position. The molecular structures of I and II differ in that the group at the 2-position is rotated by about 120° in one isomer relative to its position in the other. Both compounds have packing patterns with a wave motif. In II there are interactions of the –NO₂ group with other atoms both between molecules in the same wave and also between molecules in adjacent waves. In I the interactions of the –NO₂ group with other atoms are in the same wave only with adjacent waves being packed together by van der Waals forces alone.