Application of photon-counting in atomic-absorption spectrophotometry

An analysis is given of the statistics of photon-counting as a means of making measurements in atomic-absorption spectrometry. It is concluded that the simple application of Poisson counting statistics may give an over-optimistic estimate of the error, since other sources of error, such as flame flicker and variable nebulization rate, may be more significant.     

Chemiluminescence of nitric oxide: The NO (C2Π−A2Σ+) rate constant

A rocket borne scanning $\text{1}\text{4}$-m spectrometer was used to observe the night airglow between 1900 and 2300 Å at a resolution of 15 Å. An attitude control system oriented the payload horizontally near apogee (150 km) and pointed the field of view to the north and parallel with the terminator. Progressions from the δ(0, v″) and γ(0, 0) band system of nitric oxide were observed. The observations of the δ-bands and γ-bands are consistent with the relative intensities produced in the preassociation of ground state atomic oxygen and nitrogen. An observation of the v′ = 0, ε-band progression is suggested and is consistent with the presence of a resonance fluorescence contribution near apogee. The A²Σ⁺ (upper level of the γ-bands) can only be populated by cascade from the C²π state (upper level of the γ-bands) in the preassociation reaction. In the absence of quenching of the A²Σ⁺ state, the emission rate of the γ-bands can be used as a measure of the C?A transition and thus to deduce the branching ratio to the A²Σ⁺ state. This branching ratio is estimated to be 0.21±0.04.     

Comparison theorems and orbit counting in hyperbolic geometry

In this article we address an interesting problem in hyperbolic geometry. This is the problem of comparing different quantities associated to the fundamental group of a hyperbolic manifold (e.g. word length, displacement in the universal cover, etc.) asymptotically. Our method involves a mixture of ideas from both ``thermodynamic' ergodic theory and the automaton associated to strongly Markov groups.

     

Temperature dependence of the cristobalite α-β inversion

The cristobalite - inversion has been studied using DSC on cristobalites produced by firing high purity quartz with and without addition of a mineraliser. If no mineraliser was used, the inversion temperatures and hysteresis on heating and cooling increased with firing temperature. Firing time had little or no effect on inversion temperature. When a mineraliser was used, the same general trend was observed with increases in firing time at low temperatures leading to splitting of the inversion peak. The amount of mineraliser added had little effect. Tridymite inversions were also observed. The results are explained in terms of the degree of order of the cristobalite structure.We are grateful for the award of an S.E.R.C. CASE award in conjunction with Specialist Refractory Services to SJS. Helpful discussions have been held with Mr. D. W. Bailey and Dr. P. Watkins.     

Compactness of the Space of Minimal Hypersurfaces with Bounded Volume and <Emphasis Type="Italic">p</Emphasis>-th Jacobi Eigenvalue

Given a closed Riemannian manifold of dimension less than eight, we prove a compactness result for the space of closed, embedded minimal hypersurfaces satisfying a volume bound and a uniform lower bound on the first eigenvalue of the stability operator. When the latter assumption is replaced by a uniform lower bound on the p-th Jacobi eigenvalue for \(p\ge 2\) one gains strong convergence to a smooth limit submanifold away from at most \(p-1\) points.     

Bryozoan metabolites: an ecological perspective

This Highlight covers the chemical ecology of bryozoans, primarily the ecological functions of bryozoan natural products. The Highlight is arranged taxonomically, according to the bryozoan Treatise classification (P. Bock, Bryozoa Homepage, 2006, http://bryozoa.net).     

Finite difference Poisson-Boltzmann electrostatic calculations: Increased accuracy achieved by harmonic dielectric smoothing and charge antialiasing

A common problem in the calculation of electrostatic potentials with the Poisson-Boltzmann equation using finite difference methods is the effect of molecular position relative to the grid. Previously a uniform charging method was shown to reduce the grid dependence substantially over the point charge model used in commercially available codes. In this article we demonstrate that smoothing the charge and dielectric values on the grid can improve the grid independence, as measured by the spread of calculated values, by another order of magnitude. Calculations of Born ion solvation energies, small molecule solvation energies, the electrostatic field of superoxide dismutase, and protein-protein binding energies are used to demonstrate that this method yields the same results as the point charge model while reducing the positional errors by several orders of magnitude. © 1997 by John Wiley & Sons, Inc.