Sharp Structures Research Articles

Anion spectroscopy of uracil, thymine and the amino-oxo and amino-hydroxy tautomers of cytosine and their water clusters

In this work we investigate different forms of electron binding in the mass-selected and cooled nucleobases uracil, thymine and cytosine and their water clusters. In photodetachment–photoelectron spectra of the pyrimidine nucleobases, sharp structures were found at 86±8 meV (uracil), 62±8 meV (thymine) and 85±8 meV (cytosine), which are due to photodetachment of dipole-bound states. The photodetachment angle dependence of these states shows mostly p-wave detachment, which confirms the predicted predominant s-character of the electronic wave function of dipole-bound states. This anisotropy of electron emission and their sharp photodetachment structures can be taken for dipole-bound state recognition. Water attachment to the nucleobases results in positive valence-bound electron affinity, s-wave detachment and broad spectra, implying that the electron now is trapped inside the π* LUMO of the nucleobases, stabilized by the water dipole. The solvent shifts in dependence on water aggregation are linear and allow by extrapolation an estimation of the monomer electron affinities. All three pyrimidine nucleobases are estimated to have a very similar valence-bound electron affinity in the range of 0–200 meV. In nucleobase·(H2O)n clusters, due to the large total dipole moment, dipole-bound states also exist. Resonant excitation of these dipole-bound states with a photon of 1064 nm wavelength causes dissociation of the anion cluster, leading to monomer anions in their dipole-bound state. These monomer anions can be photodetached by a second IR photon. Whereas, for uracil and thymine, one dipole-bound state is detected, for cytosine we find two dipole-bound states (85±8, 230±8 meV) which are attributed to the dipole-bound states of the simultaneously present amino-hydroxy and amino-oxo cytosine tautomers. We also give a possible explanation why the formation of the dipole-bound state of the amino-oxo tautomer at 230 meV is improbable in the supersonic expansion.

An electrolyte electroreflectance study of ReS 2

We report an electrolyte electroreflectance (EER) study of ReS 2 single crystals in the energy range of 1.3–6.0 eV. The EER spectrum sharp derivative-like structures in the vicinity of the band-edge excitonic transitions as well as higher lying interband transitions. The transition energies are determined accurately and a probable energy-band structure is constructed.

Sharp edged silicon structures generated using atom lithography with metastable helium atoms

By combining atom lithography and plasma etching technology in a two-step process, we demonstrate the transfer of sharp edged structures into silicon with a depth of 580 nm and an inclination of better than 86°. A self-assembled monolayer resist deposited on a Au-coated Si surface is damaged by a beam of metastable helium atoms through a physical mask. A wet etching process removes Au in the damaged regions, resulting in an intermediate mask of patterned Au on Si. Low-pressure plasma etching is then used to transfer the pattern of the Au mask into the Si. This plasma etching process shows a selectivity greater than 19 with respect to the Au mask.

Ferromagnetic transitions assisted by charge localization inf-band metals: Band-structure effects in cerium

An increased f-band occupation in Ce, caused by increased temperature $(T),$ is found to contribute negatively to the electronic specific heat. The same type of charge redistribution is caused by an imposed exchange splitting $\ensuremath{\xi}.$ This leads to a modification of the Stoner criterion to allow for magnetic transitions at low T. Sharp structures in the density of states (DOS) near the Fermi energy ${(E}_{F}),$ such as for Ce, are necessary for this effect. The effect can be enhanced in compounds due to sharper DOS features.

WFPC2 Studies of the Crab Nebula. II. Ionization Structure of the Crab Filaments

Narrowband images of the Crab Nebula taken with the Hubble Space Telescope (HST) WFPC2 show the morphology and ionization structure of the filaments in great detail. At HST resolution, low- and high-ionization emission from filaments in the Crab differ in two complementary respects. First, low-ionization emission is found to be concentrated in very sharp structures, while high-ionization emission is predominantly found in a much more diffuse component. For example, approximately 80% of emission from [O I]λ6300 arises in features with scales of less than 0.5, while only 10% of [O III]λ5007 emission arises in such compact structures. Second, individual filaments are found to lie along a sequence of ionization structures, ranging from features in which all lines are concentrated in the same compact volume through features with low-ionization cores surrounded by high-ionization envelopes. Hester and coworkers proposed in 1996 that this sequence can be understood as the result of the nonlinear development of magnetic Rayleigh-Taylor (R-T) instabilities along the interface between the Crab synchrotron nebula and swept-up ejecta. We present photoionization models of cylindrically symmetrical filaments consisting of a quadratic core surrounded by an extended envelope. A good deal of the observed variation in filament structure in the Crab can be matched by varying the assumed density profiles in these models. This implies that variations in the development of R-T instabilities in the Crab account for much of the spectral variation within the remnant. We also present a photoionization model of a uniform, low-density medium, which reasonably matches the extended diffuse component that dominates the high-ionization emission. This envelope model produces strong [O III] but virtually no [O I]. While the He I/Hβ ratio remains fairly constant throughout a range of filament models, this ratio is a factor of 5 lower in the envelope model. We find that the apertures used in ground-based spectroscopy of the Crab generally include emission from several discrete filaments as well as a component of diffuse emission. This places a fundamental limit on what can be inferred reliably from comparison of spectra with one-dimensional photoionization models. Many filament cores coincide with dust extinction features seen in a continuum image of the Crab. We consider one such feature in detail and find that the extinction of about 1.2 mag suggests that the dust-to-gas mass ratio may be an order of magnitude higher than is typical in the interstellar medium.

Indicators of Reconnection Processes and Transition to Global Chaos in Nontwist Maps

Reconnection processes of twin-chains are systematically studied in the quadratic twist map. By using the reversibility and symmetry of the mapping, the location of the is theoretically determined in the phase space. The enable us to obtain useful information about the reconnect ion processes and the transition to global chaos. We succeed in deriving the general conditions for the reconnection thresholds. In addition, a new type of reconnect ion process which generates shearless curves is studied. In the past decades, enormous effort has been dedicated to the study of two­ dimensional area-preserving maps with the twist condition,l) but very few studies have been made of nontwist maps. Recent studies on nontwist maps have revealed that rich properties are generated by violating the twist condition. 2) - 6) In a previous paper, 6) we studied the properties of the quadratic twist map and numerically determined the critical boundary in the two-dimensional parame­ ter space, where the transition to global chaos occurs. The critical boundary has many sharp singular structures, and their locations seem to have a one-to-one corre­ spondence with those of the reconnect ion thresholds. The relationship between the transition to global chaos and the reconnect ion processes was first pointed out by Howard and Hohs,2) but it has not yet been thoroughly investigated. In order to investigate the detailed structure of the critical boundary, one needs accurate information regarding the reconnection processes. In this paper, we study the details of the reconnect ion processes in the quadratie twist map and propose a theoretieal method to determine the reconnection thresholds. We show that the reversibility and symmetry of the mapping guarantee the existence of the indicator points in the phase space. These enable us to study the reconnect ion processes sys­ tematieally. For twin-chains of period one and period two, the reconnect ion thresh­ olds have already been determined, either exactly or approximately. 2), 3) The method presented here reproduces results which have been previously obtained, and it pro­ vides general conditions for the reconnect ion thresholds. The quadratie twist map (QTM) is defined by

Developing novel photoresists: microstructuring of triazene containing copolyester films

The growing field of microtechnology induces an increasing demand for affordable functional materials. Several strategies exist for improving the ablation behaviour of standard polymers, e.g., doping with a chromophore. Another possibility is the synthesis of copolymers containing photosensitive moieties besides standard monomers. The specialty polymers investigated in this study contain a triazene group (Ph–NN–NR 2) as the photosensitive unit; a polyester with varying contents of triazene groups is presented. These units can be situated in either the main chain or in the side chain. Under irradiation the photosensitive chromophores are cleaved fast and irreversibly. Spectroscopic evidence shows the loss of nitrogen to be the initial photochemical decomposition step. For a comparison of the performance of copolymers with different fractions of triazene groups in the backbone, we have determined the threshold fluences and the effective absorption coefficients. With respect to these ablation parameters, only small differences were found within the set of copolymers investigated. From atomic force microscopy (AFM) and scanning electron microscopy (SEM) it becomes evident that a minimum content of triazene groups in the main chain is necessary for defined microstructures with sharp edges, whereby a resolution in the sub- μm scale has been achieved. At lower concentrations no sharp structures are formed; at high concentrations bubbles impair the quality of the structures.

LO Phonon Renormalization in Optically Excited CuCl Nanocrystals

We have revealed sharp structures in the absorption spectrum of CuCl nanocrystals using persistent hole-burning spectroscopy and observed sidebands due to phonon-assisted absorption. The LO phonon frequency in the excited state of the nanocrystal is reduced by about 10% from the free value due to mixing with the exciton. When the mixing becomes resonant, the phonon sideband anticrosses with the excited exciton state. We present a theory of the phonon renormalization in nanocrystals and obtain results in excellent agreement with the experiment.

Improvement of spatial resolution of keyhole effect images.

An algorithm is proposed which improves the spatial resolution of difference or effect images acquired with a keyhole sampling strategy. This new reconstruction algorithm uses a priori information about sharp structures in the observed signal changes from a high resolution reference scan. The potential of this algorithm even being able to deal with noisy effect images is illustrated by application to functional MRI data.

Wet Etching of Al2 O 3 for Selective Patterning of Microstructures Using Ar + Ion Implantation and H 3 PO 4

A new etching method for single‐crystal is proposed using implantation and chemical etching. Photolitographically defined patterns are transferred into sharp structures on the wafer surface by selectively removing the damaged material. A double implantation of at 50 keV followed by 180 keV was performed on all samples, using a dose of for the low‐energy implant. Doses of , and were used for the 180 keV implant. Scanning electron microscopy and atomic force microscopy were used to characterize the pattern obtained, while transmission electron microscopy and Rutherford backscattering channeling analysis were employed to study the implanted layer. Under the conditions investigated, both the etchable range (90 to 160 nm) and the etching rate (1 to 7 nm/min) were found to be strongly correlated with the implanted dose. Sharp and uniform interfaces between etched and unetched regions were obtained, forming steps with an average slope of 30 to 45°. The proposed method offers high selectivity, lack of contamination of the substrate, and compatibility with standard processing.

Intrinsic exciton transitions in GaN

Intrinsic excitonic transitions in GaN have been studied using a variety of spectroscopic measurements. Sharp spectral structures associated with intrinsic free excitons could be observed in photoluminescence, reflection, and absorption spectra. The energy positions of excitonic transitions in GaN epitaxial layers were found to be influenced by the residual strain resulting from lattice-parameter and thermal-expansion mismatches between the epilayers and the substrates. The values of the four principal deformation potentials of wurtzite GaN were derived by using the strain tensor components determined by x-ray measurements. The observation of spectral features involving the emission of LO phonons in absorption and photoluminescence excitation spectra at energies above exciton resonances indicate that a phonon-assisted indirect excitation process, which simultaneously generates a free exciton and a LO phonon, is a very significant and efficient process in GaN. The lifetime of the free excitons is found to be longer than the relaxation time of LO-phonon emission but much shorter than that of acoustic-phonon emission.

High Resolution TEM Observation of Si Nanoparticle Interfaces Fabricated by SIMOX

Highly-oriented Si nanoparticles (SNP) were precipitated in amorphous SiO2 using separation by implanted oxygen (SIMOX) method and showed visible luminescence at room temperature. In order to investigate the microstructure of the SNPs, we observed the Si/SiO2 interface by cross-sectional high-resolution transmission electron microscopy (HRTEM). The three-dimensional shape of the SNPs observed from different directions is an {111}-surrounded octahedral structure, each apex of which has an {100} facet. The upper (surface-side) interface of SiO2/Si(100) contains missing atomic rows, while the lower (substrate-side) interface of Si(100)/SiO2 is atomically flat except for the steps at the intersection with (111) plane. Similar atomically sharp structures were observed at the Si{111}/SiO2 interfaces. Image-simulation of the interfaces well reproduced the microstructures observed by HRTEM and proved that the interfaces are those of amorphous SiO2 and single-crystal Si without any intermediate layer.

The stability of asperities on W(001)

Instrumental as the electron sources in electron microscopes, the FIM, the STM, flat panel displays, and other technologies, sharp tungsten structures nevertheless remain for the most part unexamined on the atomic scale. We have modeled a sharp tungsten asperity in order to investigate its stability, upon which these applications are critically dependent. Using the extended embeddedatom method, we have determined the energy barriers for several rearrangements which blunt the asperity. We also obtain the lowest-energy pathways for such rearrangements, and these highlight the importance of directional bonding tendencies for elements from the center of the transition series.

Quasiparticle momentum distributions in the t- J model

A detailed analysis is made for the momentum distribution n( k ) of the low-energy states of doped two-dimensional t- J model obtained from small-cluster diagonalizationns. We show that the smoothly part of n( k) is practically identical for all the low-energy states with a given hole number and that this part can be obtained by a simple scaling procedure from the single-hole results. We thereby demonstrate that the coherent part in n( k) obtained by subtraction show sharp structures in k-space for all the low-energy states and thus provide the ‘quasiparticle’ momentum distribution of the t- J model.

UHV-AFM study of MBE-grown 10 nm scale ridge quantum wires

We evaluate the shape of ridge quantum wires (RQWIs) with nm-scale resolution by using SEM and ultra-high-vacuum (UHV) AFM system which is connected to MBE chamber. This MBE-AFM system provides us a detailed information about the evolution of size and uniformity of ridges grown under various conditions. In this report, we investigate systematically how the growth temperature T s and As flux affect the width and morphology of GaAs ridge structure. The results show that a very sharp and uniform GaAs ridge structures (W < 10 nm) can be obtained. On the basis of this understanding, we fabricated the ridge quantum wire of 10 nm width.

Reflection High Energy Electron Diffraction Intensity Oscillations in Fe3O4/MgO Superlattices Grown by Molecular Beam Epitaxy

Fe3O4/MgO superlattices are grown on MgO(001) substrates. Modulation coherency and sharp interface structures are observed by X-ray diffraction and reflection high energy electron diffraction (RHEED). The RHEED intensity oscillations of the specular and Bragg scattering provide direct evidence that structurally smooth interfaces can be maintained over many bilayers. Similar superlattices are also grown on an Fe-coated MgO(001) substrate for comparison and although the interface is relatively rough due to the misfit between the film and substrate, the modulation coherency is still maintained. RHEED intensity oscillations allow direct microscopic observations of the growth mode which is important for the further development of oxide superlattices.

Ultrahigh vacuum atomic force microscope study of 10–30 nm scale GaAs ridge structure formation by molecular beam epitaxy

The evolution of GaAs ridge structure formation by molecular beam epitaxy on a patterned substrate has been investigated using an ultrahigh vacuum atomic force microscope. It is found that the morphology of ridges can be quite irregular with random formation of various facets in the intermediate phase of growth, but self-smoothing processes of the lateral facets take place later on, leading to very sharp and smooth ridge structures in the end. The ridge top is quite sharp and straight with the height fluctuation of within 1–2 nm over the length of 1.4 μm. The role of the Ga atom flows from the side (111)B surfaces to the top (001) surface and their local modulations are considered to account for these observations.

Patchiness in plankton populations

We consider and contrast two mechanisms for the production of spatial pattern in an excitable medium model for plankton populations. The first is Turing or diffusion-driven instability. We find that, since in a turbulent environment the effective diffusivities of phytoplankton and zooplankton are similar, this mechanism is unlikely to produce observable spatial pattern in the ocean. The second mechanism is spatially varying forcing of the system. In order to display the sensitivity to small spatial variation in forcing, we consider the dynamics of an ordinary differential equation system with spatial perturbations to parameters and initial conditions. In the absence of diffusion the excitable nature of the system means that small perturbations can produce very sharp spatial structures, when diffusion is introduced, we find that this patchiness can persist on realistic scales.

Imaging of Saddle Point Electron Emission in Slow p-He Collisions.

The two-dimensional velocity distribution of electrons emitted in 5--15 keV $p\ensuremath{-}\mathrm{H}\mathrm{e}$ collisions has been measured for completely determined motion of the nuclei, that is, as a function of the impact parameter and in a well defined scattering plane. The electrons are emitted preferentially in the scattering plane and in the forward direction. The velocity distributions show sharp structures that vary strongly with impact parameter and projectile velocity. The results are compared to classical trajectory calculations.

Steady-state and time-resolved fluorescence study of some dyes in polymer microspheres showing morphology dependent resonances

Fluorescence emission spectra of N,N′-bis(2,5-di-tert-butylphenyl)-3,4:9,10- Perylenebis(dicarboximide) (DBPI), rhodamine 6G (R6G), and cresyl violet (CV) in spherical polymer beads of less than 20 μm diameter show sharp ripple structures. The observed peak positions and the intervals of the structures are consistent with the calculations of the morphology dependent resonances (MDR). Observed intensities of the MDR in the fluorescence emission spectra are found to show excitation energy dependence. The fluorescence spectra have also been measured as a function of the refractive indexes of the medium and the bead. These MDR in the beads up to 4 μm diameter do not appear to affect the fluorescence decay of the dyes, since the fluorescence lifetime remains constant irrespective of the size of the bead and the refractive index of a surrounding medium. Simulations based on the Lorentz–Mie theory for the microspheres of different refractive indexes have been used to quantify the observed effect on the basis of the available data on the homogeneous widths of the dye molecules. A fluorescence study of microcrystals of DBPI is also presented here from the point of view of comparison with fluorescence decay of dye impregnated beads. The microcrystals exhibit a size effect in the fluorescence decay which has been attributed mainly to the self-absorption effect.