672 research outputs found
Measurement of focusing properties for high numerical aperture optics using an automated submicron beamprofiler
The focusing properties of three aspheric lenses with numerical aperture (NA)
between 0.53 and 0.68 were directly measured using an interferometrically
referenced scanning knife-edge beam profiler with sub-micron resolution. The
results obtained for two of the three lenses tested were in agreement with
paraxial gaussian beam theory. It was also found that the highest NA aspheric
lens which was designed for 830nm was not diffraction limited at 633nm. This
process was automated using motorized translation stages and provides a direct
method for testing the design specifications of high numerical aperture optics.Comment: 6 pages 4 figure
Photoluminescence and photoluminescence excitation studies of lateral size effects in Zn_{1-x}Mn_xSe/ZnSe quantum disc samples of different radii
Quantum disc structures (with diameters of 200 nm and 100 nm) were prepared
from a Zn_{0.72}Mn_{0.28}Se/ZnSe single quantum well structure by electron beam
lithography followed by an etching procedure which combined dry and wet etching
techniques. The quantum disc structures and the parent structure were studied
by photoluminescence and photoluminescence excitation spectroscopy. For the
light-hole excitons in the quantum well region, shifts of the energy positions
are observed following fabrication of the discs, confirming that strain
relaxation occurs in the pillars. The light-hole exciton lines also sharpen
following disc fabrication: this is due to an interplay between strain effects
(related to dislocations) and the lateral size of the discs. A further
consequence of the small lateral sizes of the discs is that the intensity of
the donor-bound exciton emission from the disc is found to decrease with the
disc radius. These size-related effects occur before the disc radius is reduced
to dimensions necessary for lateral quantum confinement to occur but will
remain important when the discs are made small enough to be considered as
quantum dots.Comment: LaTeX2e, 13 pages, 6 figures (epsfig
Electron attachment to SF6 and lifetimes of SF6- negative ions
We study the process of low-energy electron capture by the SF6 molecule. Our
approach is based on the model of Gauyacq and Herzenberg [J. Phys. B 17, 1155
(1984)] in which the electron motion is coupled to the fully symmetric
vibrational mode through a weakly bound or virtual s state. By tuning the two
free parameters of the model, we achieve an accurate description of the
measured electron attachment cross section and good agreement with vibrational
excitation cross sections of the fully symmetric mode. An extension of the
model provides a limit on the characteristic time of intramolecular vibrational
relaxation in highly-excited SF6-. By evaluating the total vibrational spectrum
density of SF6-, we estimate the widths of the vibrational Feshbach resonances
of the long-lived negative ion. We also analyse the possible distribution of
the widths and its effect on the lifetime measurements, and investigate
nonexponential decay features in metastable SF6-.Comment: 22 pages, 10 figures, submitted to Phys. Rev.
Oscillatory Size-Dependence of the Surface Plasmon Linewidth in Metallic Nanoparticles
We study the linewidth of the surface plasmon resonance in the optical
absorption spectrum of metallic nanoparticles, when the decay into
electron-hole pairs is the dominant channel. Within a semiclassical approach,
we find that the electron-hole density-density correlation oscillates as a
function of the size of the particles, leading to oscillations of the
linewidth. This result is confirmed numerically for alkali and noble metal
particles. While the linewidth can increase strongly, the oscillations persist
when the particles are embedded in a matrix.Comment: RevTeX4, 5 pages, 2 figures, final versio
Cascaded Optical Field Enhancement in Composite Plasmonic Nanostructures
Copyright © 2010 The American Physical SocietyWe present composite plasmonic nanostructures designed to achieve cascaded enhancement of electromagnetic fields at optical frequencies. Our structures were made with the help of electron-beam lithography and comprise a set of metallic nanodisks placed one above another. The optical properties of reproducible arrays of these structures were studied by using scanning confocal Raman spectroscopy. We show that our composite nanostructures robustly demonstrate dramatic enhancement of the Raman signals when compared to those measured from constituent elements
Age Differences in the Association of Obstructive Sleep Apnea Risk with Cognition and Quality of Life
Using a sample of 2925 stroke-free participants drawn from a national population-based study, we examined cross-sectional associations of obstructive sleep apnea risk (OSA) with cognition and quality of life and whether these vary with age, while controlling for demographics and co-morbidities. Included participants from the REasons for Geographic And Racial Differences in Stroke Study were aged 47-93. OSA risk was categorized as high or low based on responses to the Berlin Sleep Questionnaire. Cognitive function was assessed with standardized fluency and recall measures. Depressive symptoms were assessed with the four-item Center for Epidemiologic Studies Depression Scale. Health-related Quality of Life (HRQoL) was assessed with the Medical Outcomes Study Short Form-12 (SF-12). MANCOVA statistics were applied separately to the cognitive and quality of life dependent variables while accounting for potential confounders (demographics, co-morbidities). In fully adjusted models, those at high risk for OSA had significantly lower cognitive scores (Wilks’ Lambda = 0.996, F(3, 2786) = 3.31, p < .05) and lower quality of life (depressive symptoms and HRQoL) (Wilks’ Lambda = 0.989, F(3, 2786) = 10.02, p < .0001). However, some of the associations were age-dependent. Differences in cognition and quality of life between those at high and low obstructive sleep apnea risk were most pronounced during middle age, with attenuated effects after age 70
Structure, dielectric relaxation and electrical conductivity of 2,3,7,8-Tetramethoxychalcogenanthrene-2,3-dichloro-5,6-dicyano-1,4-benzoquinone 1:1 charger-transfer complexes
[EN] 2,3,7,8-Tetramethoxychalcogenanthrenes (5,10-chalcogena-cyclo-diveratrylenes, 'Vn(2)E(2)', E = S, Se) form isotypical 1:1 charge-transfer (CT) complexes with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). X-ray analysis of Vn(2)S(2) . DDQ shows the compound to have a columnar structure with segregated stacks of donors and acceptors. The donors are virtually planar in accordance with a formulation of [Vn(2)E(2)](+)[DDQ](-). Donor cations and acceptor anions are equidistant in their respective stacks, but in each case they inclined to the stacking axis, nevertheless guaranteeing an optimum overlap of the half-filled frontier orbitals which are of pi-type character according to MNDO calculations. Dielectric ac measurements of permittivity epsilon' and loss factor E '' clearly reveal two processes, a dielectric one at low temperatures and a conductive one at high temperatures. The dielectric process can be described by the Havriliak-Negami (HN) and the Kohlrausch-Williams-Watts (KWW) model, and the conductive process by a Debye-type plot. Using these methods, the relevant parameters are evaluated. The de conductivities of polycrystalline samples moulded at 10(8) Pa show a temperature dependence in the plots of ln sigma vs. T-1, which is typical of semiconductors. Two slopes are found; that in the low-temperature region (<285 K) is explained by an easy-path model (intragrain conductivity with low activation energies), whereas in the high-temperature region conduction across the grain boundaries (with higher activation energies) is becoming predominant. The activation energies for the intrinsic conductivities obtained by the ac and de measurements are similar. Despite the columnar structure with segregated stacks, due to stoichiometric oxidation states of the components, the absolute values of conductivity are low ten. 10(-6) S cm(-1) at 293 K), though higher (by a factor of ca. 10(3)) than those of compounds like Vn(2)E(2) . TCNQ with stacks in which donor and acceptor molecules alternate.Behrens, U.; DĂaz Calleja, R.; Dötze, M.; Franke, U.; Gunsser, W.; Klar, G.; Kudnig, J.... (1996). Structure, dielectric relaxation and electrical conductivity of 2,3,7,8-Tetramethoxychalcogenanthrene-2,3-dichloro-5,6-dicyano-1,4-benzoquinone 1:1 charger-transfer complexes. Journal of Materials Chemistry. 6(4):547-553. https://doi.org/10.1039/JM9960600547S54755364Behrens, J., Hinrichs, W., Link, T., Schiffling, C., & Klar, G. (1995). SELFSTACKING SYSTEMS, PART 6.1HOST LATTICE FUNCTION OF 2,3,8,9-TETRAMETHOXYDIBENZO[c,e][1,2]-DICHALCOGENINS IN THEIR ELECTRICALLY CONDUCTING IODINE COMPLEXES. Phosphorus, Sulfur, and Silicon and the Related Elements, 101(1-4), 235-244. doi:10.1080/10426509508042522Berges, P., Kudnig, J., Klar, G., MartĂnez, E. S., & Calleja, R. D. (1989). Elementorganische Verbindungen mit o-Phenylenresten, XVI . 2:1-Komplexe von 2,3,7,8-Tetramethoxychalcogenanthrenen mit Tetracyanethen / Organometallic Compounds with o-Phenylene Substituents, Part XVI 2:1-Complexes of 2,3,7,8-Tetramethoxychalcogenanthrenes with Tetracyanoethene. Zeitschrift fĂĽr Naturforschung B, 44(2), 211-219. doi:10.1515/znb-1989-0219Hinrichs, W., Berges, P., Klar, G., Sánchez-MartĂnez, E., & Gunsser, W. (1987). Structure and electrical conductivity of TCNQ-2,3,7,8-tetramethoxychalcogenanthrene complexes. Synthetic Metals, 20(3), 357-364. doi:10.1016/0379-6779(87)90832-0Sánchez MartĂnez, E., DĂaz Calleja, R., Gunsser, W., Berges, P., & Klar, G. (1989). Structure and dielectric relaxation of 2,3,7,8-tetramethoxychalcogenanthrene-TCNQ complexes. Synthetic Metals, 30(1), 67-78. doi:10.1016/0379-6779(89)90642-5GunĂźer, W., Henning, J. H., Klar, G., & MartĂnez, E. S. (1989). Spin Density and Magnetic Susceptibility of Charge-Transfer Complexes with Chalkogenanthrene Donors. Berichte der Bunsengesellschaft fĂĽr physikalische Chemie, 93(11), 1370-1373. doi:10.1002/bbpc.19890931148G. M. Sheldrick , SHELXTL-PLUS, Release 4.21/0, Siemens Analytical X-Ray Instruments, 1990.Bock, H., Rauschenbach, A., Näther, C., Havlas, Z., Gavezzotti, A., & Filippini, G. (1995). Orthorhombisches und monoklines 2,3,7,8-Tetramethoxythianthren: kleiner Strukturunterschied – groĂźe Gitteränderung. Angewandte Chemie, 107(1), 120-122. doi:10.1002/ange.19951070132Bock, H., Rauschenbach, A., Näther, C., Havlas, Z., Gavezzotti, A., & Filippini, G. (1995). Orthorhombic and Monoclinic 2,3,7,8-Tetramethoxythianthrene: Small Structural Difference–Large Lattice Change. Angewandte Chemie International Edition in English, 34(1), 76-78. doi:10.1002/anie.199500761Hinrichs, W., Berges, P., & Klar, G. (1987). Selbststapelnde Systeme, IV 2,3,7,8-Tetramethoxythianthreniumsalze/Selfstacking Systems, Part IV 2.3.7.8-Tetramethoxythianthrenium Salts. Zeitschrift fĂĽr Naturforschung B, 42(2), 169-176. doi:10.1515/znb-1987-0209Peover, M. E. (1962). 879. A polarographic investigation into the redox behaviour of quinones: the roles of electron affinity and solvent. Journal of the Chemical Society (Resumed), 4540. doi:10.1039/jr9620004540Wheland, R. C., & Gillson, J. L. (1976). Synthesis of electrically conductive organic solids. Journal of the American Chemical Society, 98(13), 3916-3925. doi:10.1021/ja00429a030Zanotti, G., Del Pra, A., & Bozio, R. (1982). Structure of tetraethylammonium–2,3-dichloro-5,6-dicyano-p-benzoquinone. Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry, 38(4), 1225-1229. doi:10.1107/s0567740882005330Zanotti, G., Bardi, R., & Del Pra, A. (1980). Structure of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry, 36(1), 168-171. doi:10.1107/s0567740880002750Handbook of Chemistry and Physics, ed. R. C. Weast, CRC Press, Cleveland, OH, 1977–1978, 58th edn., p. D–178.Sánchez MartĂnez, E., DĂaz Calleja, R., Berges, P., Kudnig, J., & Klar, G. (1989). Structure, electrical conductivity and dielectric relaxation of a 1,2-dimethoxybenzene-tetracyanoethene 1:1 complex. Synthetic Metals, 32(1), 79-89. doi:10.1016/0379-6779(89)90831-xĂ…sbrink, L., Fridh, C., & Lindholm, E. (1977). HAM/3, a semi-empirical MO theory. I. The SCF method. Chemical Physics Letters, 52(1), 63-68. doi:10.1016/0009-2614(77)85121-xĂ…sbrink, L., Fridh, C., & Lindholm, E. (1977). HAM/3, a semi-empirical MO theory. III. Unoccupied orbitals. Chemical Physics Letters, 52(1), 72-75. doi:10.1016/0009-2614(77)85123-3Dewar, M. J. S., & Thiel, W. (1977). Ground states of molecules. 38. The MNDO method. Approximations and parameters. Journal of the American Chemical Society, 99(15), 4899-4907. doi:10.1021/ja00457a004Dewar, M. J. S., & Thiel, W. (1977). Ground states of molecules. 39. MNDO results for molecules containing hydrogen, carbon, nitrogen, and oxygen. Journal of the American Chemical Society, 99(15), 4907-4917. doi:10.1021/ja00457a005Ă…sbrink, L., Fridh, C., & Lindholm, E. (1978). Valence excitation of linear molecules.I. Excitation and UV spectra of N2, Co, acetylene and HCN. Chemical Physics, 27(2), 159-168. doi:10.1016/0301-0104(78)88001-xFridh, C., Ă…sbrink, L., & Lindholm, E. (1978). Valence excitation of linear molecules. II. Excitation and UV spectra of C2N2, CO2 and N2O. Chemical Physics, 27(2), 169-181. doi:10.1016/0301-0104(78)88002-1Lindholm, E., Bieri, G., Ă…sbrink, L., & Fridh, C. (1978). Interpretation of electron spectra. III. Spectra of formamide, studied withHAM/3. International Journal of Quantum Chemistry, 14(6), 737-740. doi:10.1002/qua.560140605Starkweather, H. W. (1981). Simple and complex relaxations. Macromolecules, 14(5), 1277-1281. doi:10.1021/ma50006a025Starkweather, H. W. (1990). Distribution of activation enthalpies in viscoelastic relaxations. Macromolecules, 23(1), 328-332. doi:10.1021/ma00203a056Havriliak, S., & Negami, S. (1967). A complex plane representation of dielectric and mechanical relaxation processes in some polymers. Polymer, 8, 161-210. doi:10.1016/0032-3861(67)90021-3J. Ross McDonald , Complex Nonlinear Least Squares Immitance Fitting Program, LEVM6, 1993;Impedance Spectroscopy, Wiley-Interscience, New York, 1987.Williams, G. (1978). Time-correlation functions and molecular motion. Chemical Society Reviews, 7(1), 89. doi:10.1039/cs9780700089Williams, G., & Watts, D. C. (1970). Non-symmetrical dielectric relaxation behaviour arising from a simple empirical decay function. Transactions of the Faraday Society, 66, 80. doi:10.1039/tf9706600080A. R. West , Solid State Chemistry and its Applications, Wiley, Chichester, 1984, ch. 13.Sánchez MartĂnez, E., DĂaz Calleja, R., & Klar, G. (1990). Self-stacking systems 5. Electrical and dielectric properties of 5,5-dibromo-2,3,7,8-tetramethoxyselenanthrene. Synthetic Metals, 38(1), 93-98. doi:10.1016/0379-6779(90)90071-
Positron-molecule interactions: resonant attachment, annihilation, and bound states
This article presents an overview of current understanding of the interaction
of low-energy positrons with molecules with emphasis on resonances, positron
attachment and annihilation. Annihilation rates measured as a function of
positron energy reveal the presence of vibrational Feshbach resonances (VFR)
for many polyatomic molecules. These resonances lead to strong enhancement of
the annihilation rates. They also provide evidence that positrons bind to many
molecular species. A quantitative theory of VFR-mediated attachment to small
molecules is presented. It is tested successfully for selected molecules (e.g.,
methyl halides and methanol) where all modes couple to the positron continuum.
Combination and overtone resonances are observed and their role is elucidated.
In larger molecules, annihilation rates from VFR far exceed those explicable on
the basis of single-mode resonances. These enhancements increase rapidly with
the number of vibrational degrees of freedom. While the details are as yet
unclear, intramolecular vibrational energy redistribution to states that do not
couple directly to the positron continuum appears to be responsible for these
enhanced annihilation rates. Downshifts of the VFR from the vibrational mode
energies have provided binding energies for thirty species. Their dependence
upon molecular parameters and their relationship to positron-atom and
positron-molecule binding energy calculations are discussed. Feshbach
resonances and positron binding to molecules are compared with the analogous
electron-molecule (negative ion) cases. The relationship of VFR-mediated
annihilation to other phenomena such as Doppler-broadening of the gamma-ray
annihilation spectra, annihilation of thermalized positrons in gases, and
annihilation-induced fragmentation of molecules is discussed.Comment: 50 pages, 40 figure
Threshold Laws for the Break-up of Atomic Particles into Several Charged Fragments
The processes with three or more charged particles in the final state exhibit
particular threshold behavior, as inferred by the famous Wannier law for (2e +
ion) system. We formulate a general solution which determines the threshold
behavior of the cross section for multiple fragmentation. Applications to
several systems of particular importance with three, four and five leptons
(electrons and positrons) in the field of charged core; and two pairs of
identical particles with opposite charges are presented. New threshold
exponents for these systems are predicted, while some previously suggested
threshold laws are revised.Comment: 40 pages, Revtex, scheduled for the July issue of Phys.Rev.A (1998
- …