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Surface enhanced Raman scattering (SERS) enhancement factors G for nanoparticles consisting of a single Ag sphere or a dimer of Ag nanospheres are calculated using a T-matrix method that rigorously incorporates dipole reradiation (DR) effects. A comparison with the commonly used plane wave (PW) approximation, |Eloc(ω)|2|Eloc(ω′)|2, which for zero Stokes shift is |Eloc(ω)|4, is made so as to determine the error associated with using the PW enhancement factor instead of DR in modeling SERS intensities. Calculations for the single sphere are performed for various molecule locations, detector locations, and sphere sizes, while the dimer calculations consider the effects of molecule and detector locations for 50 nm diameter spheres with a 2 nm gap. In both the single sphere and dimer calculations, excellent agreement (<0.3%) is found between the PW approximation and DR calculations when the molecule is located along the incident field polarization direction and with the detector along an axis that is orthogonal both to the polarization and wave vector directions. The errors for other molecule locations, different detector locations, and larger sphere sizes can be considerably larger. A qualitative description of the nature of the errors is developed based on interferences between radiation emitted by the sphere and by the molecule and on quadrupole excitation in the metal spheres. An average over molecule and detector locations for both the single sphere and dimer results leads to DR enhancement factors that differ by factors of 2–3 (sometimes higher, sometimes lower) from the PW results. This indicates that for quantitative field enhancement factor calculations, the more rigorous DR result is important. [ABSTRACT FROM AUTHOR]

The composition and structure of the NiPd(111) alloy surface are measured using low energy electron diffraction (LEED). The diffracted intensities are analyzed using tensor-LEED methods with the average t-matrix approximation. The major composition results of the study are that the surface is enriched in palladium (≈80% Pd) while the second layer is enriched in nickel (≈28% Pd). No significant structural changes were found, with all measured interlayer displacements being smaller than the corresponding measurement uncertainties. Comparisons of these results are made with literature for other NiPd low-index surfaces, for the well-studied NiPt(111) surface, and with the growing body of theoretical literature for NiPd alloy surfaces. [ABSTRACT FROM AUTHOR]

Multiple scattering of acoustic waves in a planar horizontal waveguide by finite-length cylinders is considered. Cylinder height equals the waveguide depth, and both are vertically constrained by the pressure-release boundaries. An analytically exact solution is obtained via normal mode expansion method in conjunction with the concept of the T matrix. The problem is decomposed into an infinite number of two-dimensional multiple scattering problems, modulated by waveguide mode shapes. Examples are presented for an isovelocity waveguide in which the medium is uniform and the waveguide depth is constant. It is found that, in numerical computations, including one or two evanescent modes captures the essence of the evanescent modes. Multiple scattering in the waveguide is compared with the corresponding two-dimensional case. It is concluded that, in low frequencies, the wave patterns in the two cases are very similar, with a shift in the frequency. The similarity diminishes when there are multiple propagating modes. Despite the mode mixing, some key features in the scattering as observed in the two-dimensional problem remain observable in the waveguide case. [ABSTRACT FROM AUTHOR]

A T-matrix based method of solution of the multiple scattering problem was presented by the authors [J. Acoust Soc. Am. 112, 2688–2701 (2002)]. This method can be applied to the computation of relatively small problems, since the number of operations required grows with the number of spheres N as O(N3), and with the sixth power of the wave number. The use of iterative techniques accelerated using the fast multipole method (FMM) can accelerate this solution, as presented by Koc and Chew [J. Acoust. Soc. Am. 103, 721–734 (1998)] originally. In this study we present a method that combines preconditioned Krylov subspace iterative techniques, FMM accelerated matrix vector products, a novel FMM-based preconditioner, and fast translation techniques that enable us to achieve an overall algorithm in which the cost of the matrix-vector multiplication grows with N as O(N log N) and with the third power of the wave number. We discuss the convergence of the iterative techniques, selection of the truncation number, errors in the solution, and other issues. The results of the solution of test problems obtained with the method for N∼102–104 for different wave numbers are presented. © 2005 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

A recently developed method [Semkat et al., Phys. Rev. E 59, 1557 (1999); Kremp et al., in Progress in Nonequilibrium Green's Functions (World Scientific, Singapore, 2000), p. 34] for incorporating initial binary correlations into the Kadanoff-Baym equations (KBE) is used to derive a generalized T-matrix approximation for the self-energies. It is shown that the T-matrix obtains additional contributions arising from initial correlations. Using these results and taking the time-diagonal limit of the KBE, a generalized quantum kinetic equation in binary collision approximation is derived. This equation is a far-reaching generalization of Boltzmann-type kinetic equations: It self-consistently includes memory effects (retardation, off-shell T-matrices) as well as many-particle effects (damping, in-medium T-matrices) and spin-statistics effects (Pauli-blocking). © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]