Atomic vapor cells for chip-scale atomic clocks with improved long-term frequency stability

A novel technique for microfabricating alkali atom vapor cells is described in which alkali atoms are evaporated into a micromachined cell cavity through a glass nozzle. A cell of interior volume 1 mm3, containing 87Rb and a buffer gas, was made in this way and integrated into an atomic clock based on coherent population trapping. A fractional frequency instability of 6 x 10(-12) at 1000 s of integration was measured. The long-term drift of the F=1, mF=0-->F=2, mF=0 hyperfine frequency of atoms in these cells is below 5 x 10(-11)/day.     

Die spezifische Wärme von PMMA und PVC im Einfrierbereich

Zusammenfassung Mit Hilfe eines adiabatischen Absolutkalorimeters und der Differentialthermoanalyse wird die spezifische Wärme von PVC und PMMA im Einfrierbereich untersucht. Dabei werden Maxima beobachtet, deren Größe und Temperaturlage von der thermischen Vorgeschichte abhängig sind. Der größte Teil dieser Maxima ist auf Enthalpierelaxation zurückzuführen. In einigen Fällen wird als Ursache zusätzlich ein Aufschmelzen geordneter Strukturen angenommen. Solche Strukturen sind röntgenographisch nicht nachweisbar und bilden sich in amorphen Polymeren nur während des Temperns unterhalb der Einfriertemperatur aus.

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Summary The specific heat of polyvinylchloride (PVC) and polymethylmethacrylate (PMMA) was examined in the glass transition region by using an adiabatic absolute calorimeter and a differential scanning calorimeter (DSC). It was found that the size and position of maxima in thecp-T-chart is dependent on the thermal history. The bigger part of these maxima are due to the enthalpy relaxation. However, in a few cases, these maxima cannot be explained by enthalpy relaxation alone, but shall be interpreted by assuming the melting of ordered structures. These structures are not detectable by X-ray. They develop only in amorphous polymers during annealing below the glass temperature.

     

An Epoxy Resin Tells its Life Story: Thermal analysis as a constant companion from development to disposal

With the aid of thermal analysis, epoxy (EP) resins have been characterized from the curing of the components all the way to disposal. The methods employed for the investigations were DSC, TMA, DMA, TG and TG-QMS. The experimental results obtained will be used here to demonstrate the typical possibilities offered by these methods for characterization of an epoxy resin from cradle to grave.This revised version was published online in November 2005 with corrections to the Cover Date.     

A Doubly Bridged Bis(phenylethynyl)benzene: Different from a Twisted Tolan

The synthesis of a doubly bridged 1,4-bis(phenylethynyl)benzene is reported. The target displays photophysical properties, distinctly different from that of its congeners, the singly bridged tolans. Quantum-chemical calculations suggest a lack of planarization of the bridged bis(phenylethynyl)benzene in the first excited state.     

Odor Characteristics of Novel Non-Canonical Terpenes

Several non-canonical, methylated terpenes have been described as products of genetically modified Escherichia coli recently, and the aroma properties of 28 odor-active methylated derivatives of prenol, isoprenol, bornane, camphene, carene, citronellol, fenchol, geraniol, limonene, linalool, terpineol, and farnesol were characterized for the first time in the current study. Twelve methylated monoterpenes exhibited a particularly intense and pleasant odor and were therefore chosen for the determination of their respective odor thresholds (OTs) in comparison to their non-methylated equivalents. In addition to the determination of OTs based on the literature value for the internal standard, (2E)-decenal, the threshold values of the compounds with individually determined OTs of the participants were calculated. This enabled a more precise identification of the OTs. Among the non-canonical terpenes, the lowest OTs in the air were found for 2-methyllinalool (flowery, 1.8 ng L⁻¹), 2-methyl-α-fenchol (moldy, 3.6 ng L⁻¹), 2-methylgeraniol (flowery, 5.4 ng L⁻¹), 2-methylcitronellol (citrus-like, 7.2 ng L⁻¹), and 4-methylgeraniol (citrus-like, 16 ng L⁻¹). The derivatives of geraniol, linalool, and citronellol showed very pleasant odor impressions, which could make them interesting for use as flavoring agents in the flavor and fragrance industry.