Photolysis of hexamethyldisilane at 206 nm

The photolysis of Me₆Si₂ at 206 nm results in two main decomposition processes: simple Si---Si bond breaking with a quantum yield of Φ = 0.21 ± 0.03, and Me₃SiH elimination with the concomitant formation of Me₂SiCH₂ with Φ = 0.18 ± 0.01. There is also a minor decomposition channel with a very small quantum yield, Φ = (5.6 ± 0.2) × 10⁻³, which results in the formation of Me₄Si and Me₂Si. The main fate of the excited Me₆Si₂ molecule produced during photolysis is stabilization by collisional deactivation. The end products observed indicate that the reaction pathways followed by the main intermediates, Me₃Si and Me₂SiCH₂, are the same as those found in the photolysis of Me₄Si (Ahmed et al., J. Photochem. Photobiol. A: Chem. 86 (1995) 33).     

Beryllium diffusion across GaAs/(Al,Ga)As heterojunctions and GaAs/AlAs superlattices during MBE growth

Beryllium diffusion during MBE growth of (Al, Ga)As layers, (Al, Ga)As/GaAs heterojunctions and GaAs/AlAs superlattices has been studied by electrochemical C-V and secondary ion mass spectrometry (SIMS) concentration profiling, in conjunction with transmission electron microscopy. Diffusion times were comparatively short since they were limited to part of the growth sequence, so non-equilibrium effects had a significant influence. The results are consistent with an interstitial-substitutional mechanism in which lattice site incorporation becomes more difficult with increasing band gap enthalpy. Incorporation involves a kick-out reaction which leads to the observed disordering of the superlattices.     

Sampling and sample preparation

Summary Problems, techniques and means of water sampling are reviewed. Applicability and performance are discussed with regard to individual and composite samples, and also to automatic sampling. Storage and preservation of samples are dealt with.

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Probenahme und Behandlung von ProbenWasser

Zusammenfassung Probleme, Verfahren und Hilfsmittel bei der Probenahme von Wasser werden behandelt. Ausführung und Anwendbarkeit von Einzel und Mischproben sowie der automatischen Probenahme werden diskutiert, ebenso die Aufbewahrung und Konservierung von Proben.

Start of discussion held at the 6th Annual Symposium on Recent Advances in the Analytical Chemistry of Pollutants, April 21–23, 1976; Vienna, Austria.     

New pressure-density-temperature measurements and new rational equations for the saturated liquid and vapour densities of oxygen

The results of pressure, density, temperature (p, ?, T) measurements in the temperature range from 65 K to 300 K, for pressures up to 7.2 MPa, and for densities from 0.3 mol dm^?3 to 39 mol dm^?3, are presented for pure oxygen. Using the experimental results, new values for the densities of saturated liquid and vapour are evaluated. To check the accuracy of these results, corresponding sets reported in the literature are critically analysed to determine the most reliable p, ?, T set for oxygen. Finally, new equations for the densities of saturated liquid and vapour are developed using a statistical procedure.     

Radiolysis of liquid propylene: Ion-molecule condensation

Radiolysis of propylene gives mainly hydrogen, and dimeric, trimeric, and other low molecular weight polymeric hydrocarbons.

Detailed analysis of the dimer shows the products to be, in order of concentration, 4-methyl-1-pentene, 1,5-hexadiene, 1-hexene, 2-methylpentane, 2,3-dimethylbutane, 4-methyl-2-pentene, 2-methyl-1-pentene, 2-hexene, and n-hexane.

The relative product concentrations, and the isotope species distribution in the products obtained from radiolysis of a 50:50 mixture of propylene and propylene-d₆, demonstrate that the alkanes, the diene, and much of the olefinic products are formed by combinations of n-propyl, isopropyl, and allyl radicals.

Isotopic species distributions in 4-methyl-1-pentene, 1-hexene, and 2-hexene demonstrate that appreciable fractions of each of these products are formed by a direct condensation of two propylene molecules with intramolecular hydrogen rearrangement. The previously postulated direct dimerization is thus verified, and the idea of its being an ion-molecule condensation receives further support.