Near-K-Edge Double and Triple Detachment of the F

Double and triple detachment of the F^{-}(1s^{2}2s^{2}2p^{6}) negative ion by a single photon have been investigated in the photon energy range 660 to 1000 eV. The experimental data provide unambiguous evidence for the dominant role of direct photodouble detachment with a subsequent single-Auger process in the reaction channel leading to F^{2+} product ions. Absolute cross sections were determined for the direct removal of a (1s+2p) pair of electrons from F^{-} by the absorption of a single photon.

Single-photon multiple detachment in fullerene negative ions: absolute ionization cross sections and the role of the extra electron.

We have obtained experimental photo-double- and photo-triple-detachment cross sections for the fullerene negative ion using Advanced Light Source photons of 17-90 eV. The cross sections are 2 and 2.5 times larger than those for C60 and appear to be compressed and shifted in photon energy as compared to C60. Our analysis reveals that the additional electron in C60- primarily produces screening which is responsible for the modification of the spectrum. Both screening effects, the shift and the compression, can be quantitatively accounted for by a linear transformation of the energy axis. Applying the transformation allows us to map the neutral and negative ion cross sections onto each other, pointing out the close relationship of correlated few-electron dynamics in neutral and negatively charged extended systems. In contrast, dynamics of neutral and negatively charged atoms or small molecules are typically not closely related.

Bianisotropy and spatial dispersion in highly anisotropic near-infrared resonator arrays.

We measure, simulate, and analyze the optical transmission through arrays of Ag nanorod pairs and U-shaped nanostructures as a function of polarization and angle of incidence. The bianisotropic nature of the metamaterials is exhibited in data and in simulations, and we argue that the electric field rather than the magnetic field excites the low frequency "magnetic" mode. We also observe spatial dispersion in the form of frequency shifts as a function of incident angle which we attribute to coupling effects between neighboring structures. A simple model based upon coupled electromagnetic dipoles is found to provide a qualitative description for the main features observed in the spectra.

Confinement resonances in photoionization of Xe@C60+.

Experimental evidence is presented for confinement resonances associated with photoabsorption by a Xe atom in a C60 cage. The giant 4d resonance in photoionization of Xe is predicted to be redistributed into four components due to multipath interference of photoelectron waves reflected by the cage. The measurements were made in the photon energy range 60-150 eV by merging a beam of synchrotron radiation with a mass/charge selected Xe@C60+ ion beam. The phenomenon was observed in the Xe@C(58)(3+) product ion channel. [corrected]

Optical plasmonic resonances in split-ring resonator structures: an improved LC model.

We systematically investigate the resonant behavior of arrays of Ag nano-structures ranging from isolated simple rods, to U-shapes, to single split ring structures. We show that the lowest order plasmonic resonance associated with a rod red shifts as we create a U and SRR into the position normally associated with a simple LC mode. A second mode red shifts and grows in intensity as we extend the arms of the U-shape, and a third mode appears in the spectra as we close the arms and form a split ring structure. We performed simulations of the structures and examine the E-field and current density. The simulations show that the current path is different for these modes. We examine the behavior of the lowest order mode in detail, discuss the effects of skin depth, and present an improved LC model to describe this resonance.

Significant redistribution of Ce 4d oscillator strength observed in photoionization of endohedral Ce@C82+ ions.

Mass-selected beams of atomic Ceq+ ions (q = 2, 3, 4), of C82+ and of endohedral Ce@C82+ ions were employed to study photoionization of free and encaged cerium atoms. The Ce 4d inner-shell contributions to single and double ionization of the endohedral Ce@C82+ fullerene have been extracted from the data and compared with expectations based on theory and the experiments with atomic Ce ions. Dramatic reduction and redistribution of the ionization contributions to 4d photoabsorption is observed. More than half of the Ce 4d oscillator strength appears to be diverted to the additional decay channels opened by the fullerene cage surrounding the Ce atom.

Temperature-driven change in the unstable growth mode on patterned GaAs(001).

We observe a dramatic change in the unstable growth mode during GaAs molecular beam epitaxy on patterned GaAs(001) as the temperature is lowered through approximately 540 degrees C, roughly coincident with the preroughening temperature. Observations of the As2 flux dependence, however, rule out thermodynamic preroughening as driving the growth mode change. Similar observations rule out the change in surface reconstruction as the cause. Instead, we find evidence that the change in the unstable growth mode can be explained by a competition between the decreased adatom collection rate on small terraces and a small anisotropic barrier to adatom diffusion downward across step bunches.

Enhanced fluorescence from periodic arrays of silver nanoparticles.

Electron beam lithography was used to fabricate silver nanoparticle arrays and study the effects of geometrical properties of particles on metal-enhanced fluorescence. Nanoparticle size, shape, interparticle spacing, and nominal thickness were varied in a combinatorial pattern for investigation of the particle plasmon resonance effect on enhancement of fluorescence from three different fluorophores; Fluorescein, Cy3, and Cy5. A specific geometric property for optimal enhancement from each fluorophore was determined. For interparticle spacings greater or equal to 270 nm, the enhancement matched what is expected for a single-particle model. For those particles smaller than 210 nm, the enhancement was lower than for the larger spacing in the range studied. Triangular-shaped particles gave similar enhancement to those of square-shaped particles. This combinatorial pattern by e-beam lithography was found to be useful for studying how individual parameters enhance the fluorescence that are important for rational design of enhanced fluorescence sensors.

Photoexcitation of a volume plasmon in C60 ions.

Neutral C60 is well known to exhibit a giant resonance in its photon absorption spectrum near 20 eV. This is associated with a surface plasmon, where delocalized electrons oscillate as a whole relative to the ionic cage. Absolute photoionization cross-section measurements for C+60, C2+60, and C3+60 ions in the 17-75 eV energy range show an additional resonance near 40 eV. Time-dependent density functional calculations confirm the collective nature of this feature, which is characterized as a dipole-excited volume plasmon made possible by the special fullerene geometry.

Transient evolution of surface roughness on patterned GaAs(001) during homoepitaxial growth.

We have investigated the length scale dependence of the transient evolution of surface roughness during homoepitaxial growth on GaAs(100), patterning the surface lithographically with an array of cylindrical pits of systematically varied sizes and spacings. Our atomic force microscopy measurements show that the amplitude of the surface corrugation has nonmonotonic behavior in both the length scale dependence and time evolution. This behavior allows us to rule out a number of existing continuum models of growth.

Experimental link of photoionization of Sc2+ to photorecombination of Sc3+: an application of detailed balance in a unique atomic system.

The principle of microscopic time reversal of physical processes, detailed balance, is widely used and depended upon in the theoretical community as a connection between photorecombination (PR) and photoionization (PI). This paper reports on a novel use of detailed balance and the comparison of experimental results obtained by two very distinct techniques to determine both the metastable fraction of an ion beam and partial Sc3+ PR cross sections and partial Sc2+ PI cross sections for the ground state and for two metastable states. The Sc2+ to Sc3+ system presents a unique opportunity to obtain comprehensive state-selective information by combining results from PR and PI experiments.

Photoionization of metastable O+ ions: experiment and theory.

High-resolution absolute experimental measurements and two independent theoretical calculations were performed for photoionization of O+ ions from the 2P(o) and 2D(o) metastable levels and from the 4S(o) ground state in the photon energy range 30-35.5 eV. This is believed to be the first comparison of experiment and theory to be reported for photoionization from metastable states of ions. While there is correspondence between the predicted and measured positions and relative strengths of the resonances, the cross-section magnitudes and fine structure are sensitive to the choice of basis states.

New Concepts for X-Ray, Soft X-Ray, and EUV Optical Instrumentation Including Applications in Spectroscopy, Plasma Diagnostics, and Biomedical Microscopy: A Status Report.

In this article, we review current progress in the development of several techniques for extreme ultraviolet, soft x-ray, and x-ray optical instrumentation. Applications of these concepts include diagnostics of hot plasmas, spectroscopic studies of the interaction of multicharged ion beams with matter (atoms, ions, molecules, microstructures, surfaces, solids), and biomedical x-ray microscopy. Novel applications of components include the use of glass capillary converters (GCCs) and multilayer mirrors (MLMs) or crystals. GCC devices provide guiding, focusing, and polarization analysis of short wavelength radiation over a wide bandwidth. The MLM or crystal optical elements can be used for dispersing, focusing, and polarization-sensitive studies of radiation within a narrow bandwidth. In this report we focus on the development and testing of optical diagnostic devices for the short wavelength spectral region 0.1 nm < λ < 100 nm.

Surveillance of a community's fat consumption using a food frequency questionnaire.

The objective of this study was to initiate a community surveillance of fat consumption, using a food frequency questionnaire. We surveyed 584 French speaking adults, using a telephone-administered questionnaire. A response rate of 84% was obtained, and the sample was representative of Lanaudière's population in terms of age and sex. The index was computed by determining the daily consumption of fat in grams from 13 food items. The index's median value was 39.5 grams (g) of fat per day, the upper quartile 58.5% g/day the lower quartile 26.5 g/day of fat. Using a weighted least squares regression analysis we tested whether people residing in the northern zone had a higher index value. Sex, level of education, age and residency in the northern (predominantly rural) zone of the territory were independently associated with the index. These results confirm a prior perception that residents of the more rural zone would have a higher fat consumption index value. The results suggest that this index is sufficiently sensitive for community surveillance purposes.

An epidemiological perspective of the causes of hearing loss among industrial workers.

An etiologic paradigm of hearing loss for industrial workers is introduced as having two major components, one being occupational and the other extra-occupational. The extra-occupational factors include age, noise exposure outside the work place, trauma, ear disease and ototoxic drugs. The work-related factors include occupational noise, whole body vibration, work-related diseases and toxic exposures. Within this framework, a review of the epidemiology of hearing loss examines the possible relationships between hearing loss and these factors.

A Bayesian approach for predicting judged hearing disability.

A method of determining the cutoff point for an administrative decision to award compensation is proposed. To construct the predictive system a Bayesian approach and discriminant analysis were employed. Judged hearing disability was used as the criterion with audiometric scores as the determining variables. The common law doctrine of the balance of probability was used as the criterion, namely the 50th centile, on which to propose a cutoff point. The highest precision in predicting judged hearing disability was obtained with an average audiometric score at 1,000, 2,000, 3,000, 4,000 Hz in the worse ear. Assuming that judged hearing disability is a valid predictor of handicap, the cutoff point based on the balance of probability (50th centile) was obtained at 25 dB. The study also confirmed results from previous studies: hearing sensitivity in frequencies higher than 2,000 Hz is required to predict hearing disability and handicap, judged hearing disability is better correlated with hearing sensitivity in the worst ear, and the audiometric cutoff point for a medical legal definition of impairment should be lower than what certain technical groups have proposed in the past.