Search Results - (Author, Cooperation:C. Cerjan)

2014-02-14

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

A model study is presented which describes the interaction of a gas with a crystalline surface. Both the surface and gas are treated quantum mechanically using a type of mean field approximation for the coupling of the translating particles to the surface. By varying the angle of incidence of the incoming particle, a large diffraction effect is observed for a narrow range of angles. Significant differences between the static and thermal features of the gas–surface scattering are observed.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The molecular photodissociation dynamics of a model, collinear CO2 system is investigated using the time-dependent wave packet method. Resonance structures in the absorption line shape, found previously in time-independent studies, are correlated to particular oscillatory motions in the dissociating wave packet dynamics. Dramatic changes in the absorption line shape are predicted for this system for short pulse excitation due to the removal of the effects of one class of resonances. Three different methods of solving the time-dependent Schrödinger equation were tested and the optimal scheme was used in these calculations.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Numerical simulations of submicron Co extruded elliptical dots were performed to illustrate the relative importance of different physical parameters on the switching behavior in the easy direction. Shape, size, magnetic moment magnitude, and the magnitude and distribution of the crystalline anisotropicity were varied. The simulation represents magnetostatic, exchange, and crystalline anisotropicity fields on a structured mesh using finite difference techniques. The smooth boundary of the dots is accurately represented by use of the embedded curve boundary method. Agreement with experimental hysteresis measurements of submicron dot arrays is obtained when an appropriate angular distribution of the grain anisotropicity axes is invoked. © 2000 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We describe experiments on arrays of polycrystalline Co structures fabricated by interference lithography. The dots are thin (15–40 nm), submicron in size, and are patterned with a uniaxial, in-plane, shape anisotropy axis. We use magnetic force microscopy (MFM) in the presence of an applied field to directly observe magnetic reversal in the dots. These experiments reveal that reversal occurs predominantly through the nucleation and annihilation of a single magnetic vortex in each dot. Hysteresis loop measurements indicate that the vortices are stable over a wide range of applied fields and that the limits of this range depend on the size and thickness of the dots. Using the MFM data, we determine the statistical distribution of the single-vortex nucleation field for several different arrays. We attribute the observed variance to the random orientation of the polycrystalline grains. Finally, we show that the average vortex nucleation and annihilations fields are linearly correlated to the demagnetization field of a uniformly magnetized structure. © 2000 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The radiative emission from a soft x-ray laser-produced plasma source is examined in detail for the particular case of a Sn solid target under moderate illumination conditions: 1010–1012 W/cm2. The prominent spectral features which contribute to the relatively large conversion efficiency in the range 124–155 A(ring) are identified using extensive multi-configuration Dirac–Fock calculations. These data are used to generate synthetic spectra with plasma characteristics derived from a two-dimensional simulation of the ablating, radiating plasma. The comparison to the experimental results is generally favorable.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The conversion efficiency of spectral emission from laser-irradiated solid targets was investigated for short wavelength source development. The plasma brightness was quantified using absolutely calibrated detectors for 20 materials and spectra were obtained between 50 and 200 A(ring). Laser parameters such as wavelength, pulse length, intensity, and spot size were systematically varied to establish a comprehensive database for source optimization. Qualitative differences in the underlying dominant emission features as a function of atomic number and laser wavelength were observed that accounted for the relatively high spectral conversion efficiencies produced. In the specific case of Sn, a conversion efficiency greater than 0.8%/eV has been observed in the technologically important region of λ=134.0 A(ring) using a laser intensity of 1–2×1011 W/cm2.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The influence of base pressure, Pb, on physical properties of FeMn exchange biased spin-valve films was studied by varying Pb over two decades from 3×10−8 to 7×10−6 Torr. Giant magnetoresistive value shows a slight increase with increasing Pb until a large decrease occurs at Pb〉3.3×10−6 Torr. Exchange bias field and blocking temperature remain constant in the base pressure range between 3×10−8 and 5×10−7 Torr before a large reduction appears. Our results indicate an upper limit for base pressure, uPb(approximate)5×10−7 Torr, above which significant spin-valve performance modification and deterioration in the crystallographic texture begin as a result of the contamination both at the ferromagnetic/antiferromagnetic interface and in the bulk of FeMn layer. © 2000 American Institute of Physics.

Electronic Resource

1089-7666

AIP Digital Archive

Physics

Emission from oxygen in the vacuum ultraviolet (VUV) spectral range from 140 to 260 A(ring) is evaluated as a diagnostic for optically thin plasmas. A one-dimensional Lagrangian, two-fluid hydrodynamic simulation code, which is self-consistently coupled to a comprehensive atomic rate equation model is used for the analysis. This model with the associated atomic rates is described and compared to recent data from laboratory experiments.

Electronic Resource

1089-7666

AIP Digital Archive

Physics

The simulation of ion extraction from a quasineutral plasma is described using a two-dimensional time-dependent numerical fluid equation code, flow. Ion motion is treated using the cold ion continuity and momentum conservation equations in the potential field of complex structures. An upwind differencing scheme in Cartesian coordinates is used to solve these fluid equations. The plasma electrons are modeled as having a Maxwell–Boltzmann distribution, which allows for ion acoustic effects inside the plasma. The growth and evolution of two-dimensional ion sheaths is treated while evaluating the ion flux collected on the extractor surface. It is found that spatial grids very much coarser than the Debye length (of order ten times larger or greater) can be successfully used to accurately model ion extraction. Although the transition region between the high density quasi-neutral plasma and the ion sheath is not resolved, the ion flux across this region is still obtained correctly. The assumption of a Maxwell–Boltzmann distribution for the electrons requires the solution of a highly nonlinear Poisson equation. Previous approaches to this problem have relied upon a Newton–Raphson root search with standard iterative techniques for the resulting linearized equations. A new methodology is described here using the recently developed strongly implicit procedure (SIP), which has been optimized for the discretized two-dimensional Laplace operator.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

We describe experiments that measured the angle resolved intensity of He (Ei=18 and 66 meV) elastically scattering from the surfaces of rare gas overlayers physisorbed on Ag(111). These studies were done on a layer-by-layer basis for 1, 2, 3, and ∼25 ordered overlayers of Ar, Kr, and Xe. Two types of experiments are described. The first is diffraction, where the scattered He intensity was measured as a function of the detector angle, with the incident polar and azimuthal angles held constant. In the second type of experiment, selective adsorption, we measured the specular intensity as a function of incident angle. The purpose of these experiments was to examine the He–surface potential, to assess the relative contributions that various He–rare gas pair potentials, nonadditive multibody terms, and He–substrate interactions make to the systems studied. The experiments are compared with the results of accurate close-coupling calculations, in order to quantitatively perform these assessments. The comparisons between the selective adsorption data and scattering calculations demonstrate the extreme sensitivity that such measurements have to the He–surface potential. In particular, observable changes in the calculated selective adsorption spectra appear when different He–rare gas potentials are tested, or when various nonadditive terms are included in the potential. The results suggest that further refinements in the He-heavy–rare gas pair potentials may be in order.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The vibrational predissociation dynamics of a collinear model of the I2(v)He cluster is studied by numerically exact time-dependent quantum mechanics, and by the time-dependent self-consistent field (TDSCF) approximation. The time evolution for the initial excitation levels v=5, 11, 22 is explored. Excellent agreement is found between the TDSCF and the exact evolution of the wave packet; in particular the approximation reproduces well the dephasing events in the dynamics, and the measurable predissociation lifetimes. The results are very encouraging as to the applicability of quantum TDSCF as a quantitative tool in the study of van der Waals predissociation dynamics.

Electronic Resource

0021-9991

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Computer Science

Physics

Electronic Resource

0021-9991

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Computer Science

Physics

Electronic Resource

0021-9991

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Computer Science

Physics

Electronic Resource

0010-4655

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Computer Science

Physics

Electronic Resource

0010-4655

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Computer Science

Physics

Electronic Resource

0009-2614

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Chemistry and Pharmacology

Physics

Electronic Resource

0020-7608

Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics

Wiley InterScience Backfile Collection 1832-2000

Chemistry and Pharmacology

The theories of the dilatation, r → r eiθ, and translation, x → x + iq, transformations as related to the Stark problem are reviewed, and new results obtained. Results for the hydrogen atom n = 1 and n = 2 levels and the 1P0, 2s2p H- shape resonance in dc fields are presented, and the extension to the ac Stark effect made. Spectral estimates are made using the technique of the numerical range and via discussion of several model problems, using both coordinate rotation and coordinate translation.

16 Ill.

Electronic Resource