Filling-in Effect Research Articles

Elliptical boundary arrays for coherent and incoherent imaging

In the imaging of far-field source distributions by a planar array of antenna elements, the quality of the image is determined by the beam pattern of the array. The physical size of the aperture ultimately determines its resolution capability, and the excitation or weighting imparted to the aperture controls the sidelobe characteristics. For a given number and spacing of antenna elements, the largest physical apertures are formed if the elements are distributed only around the boundaries of the apertures. This, however, results in beam patterns with rather high sidelobes. Thus it is of clear practical advantage to have available a technique for effectively filling in the interior of such a boundary aperture through appropriate signal processing applied to the boundary element signals. The paper builds on early results concerning such a technique, which made use of incoherent (receive-only) imaging systems. It is shown that for both incoherent imaging, the latter implemented using a transmit/receive scheme, it is possible to obtain such a filling-in or interpolation effect when imaging with an array composed of an elliptical (or circular) arrangement of array elements. Explicit synthesis theorems which give a specific constructive approach to implementing this effective interpolation to any desired degree of accuracy are given, and this approach is illustrated by giving the results of simulations, both with and without element location errors.

X-ray photoemission spectra of the 2s valence orbitals in cyclic alkanes in relation to valence bands of amorphous Group 4 and 5 elements

The crystalline elements Ge, Si, As, Sb, and Bi display a characteristic two-peak structure in the x-ray photoelectron spectra of their s-like valence bands. In the corresponding amorphous materials the gap between these two bands is filled in. It has been suggested that this is a consequence of topological disorder, i.e., of the existence of odd-membered rings (especially five-membered rings) in the amorphous cases, whereas the crystalline solids contain only six-membered rings. To test this hypothesis we have studied tetrahedrally bonded systems of different ring size in the gas-phase cyclic alkanes. The XPS spectrum of cyclohexane shows a striking resemblance to that of diamond. The other gas-phase spectra can be combined according to different ring-structure models. When this is done, we find that five- and six-membered rings alone are not sufficient to explain the ''filling-in effect'', but that five-, six-, and seven-membered rings are. The topological disorder model of bonding in amorphous semiconductors therefore appears valid. More generally, this work establishes further links between molecular orbital theory and bonding in covalent solids.

Diurnal variation of the Ring effect

The filling-in or Ring effect for the three Fraunhofer lines 4227 Å CaI, 4326 Å FeI, and 4340 Å HI has been observed in the day sky under clear and cloudy conditions. A pronounced diurnal variation for all three lines shows a midday minimum with a three-fold increase as the solar zenith angle increases from 30° to 90°.

Polarization measurements of the H β line in blue sky light

New measurements have been made of the reduced polarization in a Fraunhofer line (H β) of blue sky light. The value of the depolarization agrees well with a previously reported intensity filling-in effect. There is no evidence of a change of the azimuth of vibration of the polarization across the line profile.