The reaction mode selectivity and regiochemistry of the photocycloaddition of 2,3-dimethylbuta-I ,3diene ( D M B D ) to the benzene ring are markedly dependent on the arene substituent. Mixtures of 1 :I adducts and large proportions of diene dimers result from parent benzene and Ph-R for R = Me, OMe, CFJ, C0,Me or F, and (2n + 27c) cycloadducts are formed between styrene and phenylacetylene and DMBD. In marked contrast, the ( 4 + 4) photocycloadduct 4 is essentially the sole product from benzonitrile and the diene. The exceptional directing influence of the cyano group towards 2.5-, 1',4'attack of the diene is little perturbed by the presence or position of Me, O M e or F substituents on the benzonitrile. The role in these (4 + 4) cycloadditions of polarisation in the S, arene on approach of the diene is discussed.

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Phenomena following a rapid mechanical quench of liquid 4He from its normal to its superfluid phase are reported and discussed. The mechanical expansion apparatus is an improved version of that described previously. It uses a double-cell geometry to effect a partial separation of the sample from the convolutions of the bellows that form the outer wall of the cell. Consistent with earlier work, no evidence is found for the production of quantized vortices via the Kibble-Zurek (KZ) mechanism. Although the expansion is complete within 15ms , the second-sound velocity and attenuation continue to increase for a further approximately 60ms ; correspondingly the temperature decreases. Subsequently, the temperature rises again toward its final value as the second-sound velocity and attenuation decrease. It is shown that this unexpected behavior is apparently associated with a large-amplitude second-sound oscillation produced by the expansion, and it is suggested that the observed vortices are created by the normal fluid-superfluid counterflow that constitutes the second-sound wave. If production of large-amplitude second sound is inherent to the mechanical expansion of liquid 4He through the superfluid transition, as appears to be the case for final temperatures more than 3mK from the lambda transition, the phenomenon sets a lower bound on the density of KZ vortices that can be detected in this type of experiment.

Polycrystalline CuSn thin films can be used as highly reproducible and easily fabricated bolometers for the temperature region of 1.3-3 K, an important range for both phonon pulse experiments and photon and particle detection. We report their fabrication and the dependence of their characteristic parameters on the evaporation melt and deposition conditions. The superconducting transition temperature of the films was determined predominantly by the alloy composition, the normal state resistance by the film thickness, and the bolometer sensitivity by the deposition rate. X-ray diffraction showed that the films were composites of Cu 6 Sn 5 complexes and pure tin regions.