The intramolecular mode in isotopically substituted NO+ β- and β″-alumina

Single crystals of ¹⁵N-substituted NO⁺ β- and β″-alumina were prepared and Raman scattering spectra of the nitrosonium intramolecular stretching mode were measured. The harmonic frequency and the cubic anharmonic force constant were calculated using these and similar data from the naturally abundant nitrosonium aluminas. Differences in the calculated values between the β and β″ system were attributed to structural differences in the conduction planes.     

Detection by low temperature photoluminescence of oxygen recoils in “through-oxide” arsenic implanted silicon

The technique of low-temperature photoluminescence is used to study the damage induced by arsenic implantations through SiO₂ layers. The presence of an oxygen dependent damage center in only the “through-oxide” implanted samples and in none of the samples implanted into bare surfaces is interpreted as indicating the presence of oxygen recoils from the oxide. It is also seen that the in-diffusion of oxygen during thermal growth of the SiO₂ is insufficient to produce detectable oxygen-dependent luminescence after irradiation.     

Properties of the conjugate-gradient and Davidon methods

Two quadratically convergent gradient methods for minimizing an unconstrained function of several variables are examined. The heart of the Fletcher and Powell reformulation of Davidon's method is a variableH-matrix. The eigenvalues and eigenvectors of this matrix for a quadratic function are explored, leading to a proof that the gradient vectors at each step are mutually orthogonal. From this, a geometric interpretation of theH-matrix in terms of the projection of the gradient into a solution subspace is derived. These properties are then used to arrive at the main result, which states that, for a quadratic function, the direction vectors generated by the Davidon algorithm and the conjugate-gradient algorithm of Hestenes and Stiefel are scalar multiples of each other, provided the initial step each takes is in the direction of steepest descent. If one assumes no round-off error and a perfect one-dimensional search, the methods generate identical steps leading to the minimum.It is also shown that, for a quadratic function, the Davidon algorithm has a simplified version which searches in the same directions. However, the unique advantage of the original scheme, that it yields the curvature of the function at the minimum, is sacrificed for simplicity.Although these results apply to the case of a quadratic function, a comparative study of the same algorithms for a general function can be found in a companion paper.This research was carried out under Contract No. NAS 9-4036, NASA-Manned Spacecraft Center, Houston, Texas. The author is indebted to Dr. H. J. Kelley, whose suggestions and encouragement provided the inspiration for this paper.     

Pathway selection by growth cones of identified motoneurones in live zebra fish embryos

How is the adult pattern of connections between motoneurones and the muscles that they innervate established during vertebrate development? Populations of motoneurones are thought to follow one of two patterns of development: (1) motor axons initially follow stereotyped pathways and project to appropriate regions of the developing muscle or (2) motor axons initially project to some regions that are incorrect, the inappropriate projections being eliminated subsequently. Here we observed individually identified motoneurones in live zebra fish embryos as they formed growth cones and as their growth cones navigated towards their targets. We report that from axogenesis, each motor axon followed a stereotyped pathway and projected only to the specific region of the muscle appropriate for its adult function. In addition, the peripheral arbor established by each motoneurone was restricted to a stereotyped region of its own segment and did not overlap with the peripheral arbor of the other motoneurones in that segment. We conclude that the highly stereotyped pattern of innervation seen in the adult is due to initial selection of the appropriate pathway, rather than elimination of incorrect projections.     

A computational study of the chromatographic separation of racemic mixtures

Summary A model is devised using molecular mechanics to simulate chromatographic separations of enantiomers. Theoretical results derived from this model are compared with experimental findings obtained using supercritical fluid chromatography. The model is then developed to incorporate the effects of binding the stationary phase to a matrix. Computed results show that addition of the matrix into the model has significant effects on the ability of the stationary phase to separate racemic mixtures.     

Adjustability as a requirement of ideal clocks

A coordinate frame is considered as an arrangement of clocks that meet certain criteria of synchronization. Einstein's ideal clock is compared with the behavior required for clocks to maintain synchrony in the group of coordinate frames that leaves Maxwell's equations invariant. The required clock rates differ from the rate of Einstein's clock. An ideal adjustable clock is defined as an Einstein clock augmented by variable “gearing” that can offset its rate. Ongoing adjustment of these clocks enables them to meet all synchronization criteria in the group of coordinate frames. The need for adjustment is due to the well known invariance of Maxwell's equations under a group of coordinate transformations larger than the Lorentz group, and has nothing to do with imperfections in clocks. It is shown that the adjustments needed by ideal adjustable clocks to maintain synchrony can be measurably separated from additional adjustments that may be needed to compensate for random imperfections. The necessity for adjustment brings with it the necessity for ongoing measurement of the light signals whose exchange defines synchronization. Implications are discussed, both for the interpretation of Maxwell's equations and for the role of measuring instruments.     

Determination of rhenium in molybdenite by X-ray fluorescence: A combined chemical-spectrometric technique

Rhenium in molybdenite is separated from molybdenum by distillation of rhenium heptoxide from a perchloric-sulphuric acid mixture. It is concentrated by precipitation of the sulphide and then determined by X-ray fluorescence. From 3 to 1000 microg of rhenium can be measured with a precision generally within 2%. The procedure tolerates larger amounts of molybdenum than the usual colorimetric methods.     

An XPS study of the valence bands in solid and liquid bismuth

High resolution XPS measurements on bismuth show the valence band structure of the liquid phase to be very similar to that of the solid and very different from that of free electrons. The spectra are in general accord with recent calculations of the densities of states for crystalline and amorphous group V elements.     

Some investigations of fusing and dissolving samples

When both Si-H and Si-vinyl are present, the sum of these groups can be determined bromometrically. For Si-H a gasvolumetric method is applied and Si-vinyl results from the difference. The bromometric determination is carried out by dissolving the sample in a mixture of carbon tetrachloride/acetic acid, addition of excess solution of bromine in acetic acid and back-titration of the unconsumed bromine. Mercuric chloride serves as a catalyst for the bromine addition. The Si-H determination is carried out by measuring the hydrogen gas evolved after hydrolysis of the sample in wet n-butanol in the presence of sodium n-butanolate. The bromometric method proved to be also useful for the rapid and sensitive determination of either Si-vinyl or Si-H. The time required is 45 min, the lower limit is 0.02 mmole per g.