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The rotational spectrum of formic anhydride (C 2 H 2 O 3 ) has been studied and assigned in the frequency range from 8 to 40 GHz. The rotational constants, centrifugal distortion constants and the electric dipole moment have been determined for the ground vibrational state. The most stable conformation of the molecule has been determined.

Small single crystals of the transparent ferromagnet Eu3SiO5 have been synthesized by high temperature chemical transport. Magnetic measurements indicate Tc = 9°K and a saturation magnetization very close to 7 μB/Eu ion. Crystalline samples show a very low residual optical absorption and an absorption edge near 2 eV which displays a small red shift of 20 meV on cooling below Tc. Samples, containing a small percentage of dissolved EuO clusters, show in addition an absorption band at lower energies with a temperature dependence and magnetic behavior typical for EuO. The photoluminescence of the pure compound has a single emission band near 1·9 eV with a high quantum yield. At low temperatures also the fluorescence displays a red shift similar to that of the absorption edge. The fluorescence is accompanied with photoconductivity.

In relation to several postulates of the ozonolysis mechanism, the stability and conformation of the hypothetical methylene peroxide molecule (CH 2 O 2 ) is investigated by an ab initio SCF MO calculation using a gaussian lobe function basis set of double zeta accuracy. The structure which has a three membered ring is found to be the most stable one amongst the conformations considered. The molecular electronic structure is discussed with the help of the electronic charge distribution in this species.

Polycrystalline powders and small single crystals of the transparent ferromagnet Eu3Si05 have been synthesized by high temperature chemical transport. Initial susceptibility and magnetization indicate a Curie temperature of 9K and a saturation moment very close to 7μB/Eu‐ion. Characteristic crystalline samples show a very low residual optical absorption and an absorption edge near 0.6/μ, which displays a small red shift of 20meV on cooling below Tc. Reemission measurements on powder samples show in addition an absorption band at longer wavelengths with a temperature dependence and magnetic behaviour typical for EuO; the magnetic measurements support the assumption of an EuO‐admixture in these samples. The photoluminescence, which we expect to be typical of the pure compound, shows a single emission band peaking near 0.65μ and an excitation edge close to the intrinsic absorption edge. At low temperatures the fluorescence yield is high, and a magnetic red shift is observed similar‐ to that of the absorption edge. The small crystals are found to be photoconducting at room temperature when illuminated with light of wavelengths shorter than the intrinsic absorption edge.

A new method of embedding micro crystals in a casting resin is described. The resin proved to withstand liquid helium temperatures, thus optical transmission experiments could be performed at various temperatures.