Interaction potentials for soft and hard ellipsoids

Using results from colloid science we derive interaction potentials for computer simulations of mixtures of soft or hard ellipsoids of arbitrary shape and size. Our results are in many respects reminicent of potentials of the Gay-Berne type but have a well-defined microscopic interpretation and no adjustable parameters. Since our potentials require the calculation of similar variables, the modification of existing simulation codes for Gay-Berne potentials is straightforward. The computational performance should remain unaffected.Comment: 8 pages, 4 figure

Collisional Dynamics of Bi\u3csub\u3e2\u3c/sub\u3e A(0\u3csub\u3eu\u3c/sub\u3e\u3csup\u3e+\u3c/sup\u3e). II. State-to-state Rotational Energy Transfer

Rotational-to-translational (R–T) energy transfer within v′=1 of the A(0+u) state of Bi2 has been investigated using spectrally resolved, laser induced fluorescence techniques. Spectrally resolved emissions from collisionally populated rotational levels of Bi2(A,v′=1) were observed for helium, neon, and argon collision partners after laser excitation of the high rotational levels J′=171, 201, and 231. Total rotational removal rates from the initially prepared state range from 2.8–8.9×10−10 cm3/molecule s. Collisional population of rotational states with |ΔJ|⩽56 was observed at pressures of 0.09–1.4 Torr. The state-to-state rates are adequately modeled by the energy based statistical power gap law

Optimized Periodic Coulomb Potential in Two Dimension

The 1/r Coulomb potential is calculated for a two dimensional system with periodic boundary conditions. Using polynomial splines in real space and a summation in reciprocal space we obtain numerically optimized potentials which allow us efficient calculations of any periodic (long-ranged) potential up to high precision. We discuss the parameter space of the optimized potential for the periodic Coulomb potential. Compared to the analytic Ewald potential, the optimized potentials can reach higher precisions by up to several orders of magnitude. We explicitly give simple expressions for fast calculations of the periodic Coulomb potential where the summation in reciprocal space is reduced to a few terms

Spin-Orbit Relaxation of Cesium 7 \u3csup\u3e2\u3c/sup\u3eD in Mixtures of Helium and Argon

Pulsed excitation on the two-photon Cs 62S1/2 -\u3e 72D3/2,5/2 transition results in time-resolved fluorescence at 697 and 672 nm. The rates for fine-structure mixing between the 72D3/2,5/2 states have been measured for helium and argon rare-gas collision partners. The mixing rates are very fast, 1.26±0.05×10−9 cm3/atom s for He and 1.52±0.05×10−10 cm3/atom s for Ar, driven by the small energy splitting and large radial distribution for the valence electron. The quenching rates are considerably slower, 6.84±0.09×10−11 and 2.65±0.04×10−11 cm3/atom s for He and Ar, respectively. The current results are placed in context with similar rates for other alkali-metal–rare-gas collision pairs using adiabaticity arguments

Collisional Dynamics of Bi\u3csub\u3e2\u3c/sub\u3e A(0\u3csub\u3eu\u3c/sub\u3e\u3csup\u3e+\u3c/sup\u3e). I. Quantum-resolved Vibrational Energy Transfer for v′=0–4

Vibrational-to-translational energy transfer between the lowest vibrational levels (v′=0–4) of the A(0+u) state of Bi2 has been investigated using spectrally resolved, laser-induced fluorescence techniques. The small vibrational spacing (ω′e≃132 cm−1) leads to highly nonadiabatic conditions, particularly for the Bi2(A)–He collision pair. However, the Δv=−1 transition probabilities for collisions with the rare gases range from 0.75% to 1.75% per collision, considerably lower than would be anticipated from standard vibrational energy transfer theory. Multiquantum (Δv′=±2) transfer rates are low, consistent with the low anharmonicity of the A(0+u) state. The rates for Δv′=±1 transitions scale linearly with vibrational quantum number as expected near the bottom of this nearly harmonic potential

Spatial and spectral performance of a chromotomosynthetic hyperspectral imaging system

The spatial and spectral resolutions achievable by a prototype rotating prism chromotomosynthetic imaging (CTI) system operating in the visible spectrum are described. The instrument creates hyperspectral imagery by collecting a set of 2D images with each spectrally projected at a different rotation angle of the prism. Mathematical reconstruction techniques that have been well tested in the field of medical physics are used to reconstruct the data to produce the 3D hyperspectral image. The instrument operates with a 100 mm focusing lens in the spectral range of 400–900 nm with a field of view of 71.6 mrad and angular resolution of 0.8–1.6 μrad. The spectral resolution is 0.6 nm at the shortest wavelengths, degrading to over 10 nm at the longest wavelengths. Measurements using a point-like target show that performance is limited by chromatic aberration. The system model is slightly inaccurate due to poor estimation of detector spatial resolution, this is corrected based on results improving model performance. As with traditional dispersion technology, calibration of the transformed wavelength axis is required, though with this technology calibration improves both spectral and spatial resolution. While this prototype does not operate at high speeds, components exist which will allow for CTI systems to generate hyperspectral video imagery at rates greater than 100 Hz

"The numerical accuracy of truncated Ewald sums for periodic systems with long-range Coulomb interactions"

Ewald summation is widely used to calculate electrostatic interactions in computer simulations of condensed-matter systems. We present an analysis of the errors arising from truncating the infinite real- and Fourier-space lattice sums in the Ewald formulation. We derive an optimal choice for the Fourier-space cutoff given a screening parameter $\eta$. We find that the number of vectors in Fourier space required to achieve a given accuracy scales with $\eta^3$. The proposed method can be used to determine computationally efficient parameters for Ewald sums, to assess the quality of Ewald-sum implementations, and to compare different implementations.Comment: 6 pages, 3 figures (Encapsulated PostScript), LaTe

Comparison of Plume Dynamics for Laser Ablated Metals: Al and Ti

Emissive plumes from pulsed laser ablation of bulk Ti and Al from KrF laser irradiation at laser fluence up to 3.5 J/cm2 and argon background pressures of 0–1 Torr have been observed using gated intensified charged-coupled device imagery. Mass loss for Ti increases from 0.1 to 0.8 μg/pulse as pulse energy increase from 174 to 282 mJ/pulse (35–170 photons/atom) and decreases by ∼30% as pressure increases from vacuum to 1 Torr. Early plume energies are described by the free expansion velocities of 1.57 ± 0.02 and of 1.81 ± 0.07 cm/μs for Ti and Al, respectively, and up to 90% of the incoming laser energy can be attributed to the Al shock front in the mid-field. The ablation thresholds of 90 ± 27 mJ (1.12 ± 0.34 J/cm2) for Ti and 126 ± 13 mJ (1.58 ± 0.16 J/cm2) for Al also represent 30%–70% of the incident laser energy. The decrease in mass loss at higher pressures is attributed to plasma shielding of the target surface

Periodic Motion Planning for Virtually Constrained (Hybrid) Mechanical Systems

The paper presents sufficient and almost necessary conditions for the presence of periodic solutions for zero dynamics of virtually constrained under-actuated Euler-Lagrange system. This result is further extended to detect periodic solutions for a class of hybrid systems in the plane and analyze their orbital stability and instability. Illustrative examples are given