Reduction of aqueous silver nitrate by hydrazine dihydrochloride in weakly alkaline solution results in a polydisperse colloid that is stable for many months without addition of any stabilizing compounds. The average size of the predominantly spherical particles depends on the initial concentration of silver ions, ranging between 40 and 70 nm in diameter. The colloidal solutions exhibit a characteristic absorption in the blue region of the visible spectrum and are not turbid below a formal silver concentration of 4.5 × 10 -4 M. With colloids prepared from 1.5 × 10 -4 M silver(I), the SERS spectra of dyes such as nile blue A could be recorded from a solution with concentrations as low as 10 -10 M, whereas no SERS signal was observed for dye concentrations higher than 10 -4 M. The maximum signal intensity was obtained at a concentration of about 10 -7 M. With colloids prepared from g3 × 10 -4 M silver(I), no SERS signal was obtained from highly diluted solutions, but the concentration limit for the maximum signal intensity of around 10 -7 M became even sharper. The thus prepared silver colloids can therefore be recommended for qualitative detection of certain organic compounds in the parts per billion range as well as for a semiquantitative determination in the parts per million range.

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Monopulse is an established technique for radar angle estimation. One can show that monopulse estimation is based on a general approximation derived from maximum likelihood (ML) estimation. This tutorial provides a derivation of this relation and presents extensions of this monopulse principle to multi-dimensional array and parameter estimation problems, in particular to space-time adaptive processing (STAP) with reduced dimension, subarrays and generalized sidelobe canceller (GSLC) configurations. The performance of these monopulse applications can be predicted by exploiting the distribution of the monopulse ratio. It is demonstrated that this distribution is more realistic than the Cramér-Rao bound (CRB). Several examples of performance of monopulse estimators are given for thinned and fully filled planar arrays, adaptive beamforming with and without low sidelobes, GSLC and STAP. Finally conditions for estimates with low variance are discussed

This paper describes a new method for the preparation of silver colloids with a narrow range of particle size to be used in surface‐enhanced Raman spectroscopy. Using malachite green as a strongly adsorbing dye, it can be shown that colloids from different preparation batches exhibit the same enhancement factor within an error margin of about 15%. By varying the number of nucleation centers, the particle size can be determined at will. An increase in particle diameter from about 38 to about 76 nm leads to an estimated five‐fold increase in surface enhancement.

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