Linear stability theory and bifurcation analysis are used to investigate the role of gravity in shear-band formation in granular Couette flow, considering a kinetictheory rheological model. We show that the only possible state, at low shear rates, corresponds to a 'plug' near the bottom wall, in which the particles are densely packed and the shear rate is close to zero, and a uniformly sheared dilute region above it. The origin of such plugged states is shown to be tied to the spontaneous symmetry-breaking instabilities of the gravity-free uniform shear flow, leading to the formation of ordered bands of alternating dilute and dense regions in the transverse direction, via an infinite hierarchy of pitchfork bifurcations. Gravity plays the role of an 'imperfection', thus destroying the 'perfect' bifurcation structure of uniform shear. The present bifurcation problem admits universal unfolding of pitchfork bifurcations which subsequently leads to the formation of a sequence of a countably infinite number of 'isolas', with the solution structures being a modulated version of their gravity-free counterpart. While the solution with a plug near the bottom wall looks remarkably similar to the shear-banding phenomenon in dense slow granular Couette flows, a 'floating' plug near the top wall is also a solution of these equations at high shear rates. A two-dimensional linear stability analysis suggests that these floating plugged states are unstable to long-wave travelling disturbances.The unique solution having a bottom plug can also be unstable to long waves, but remains stable at sufficiently low shear rates. The implications and realizability of the present results are discussed in the light of shear-cell experiments under 'microgravity' conditions.

A number of heterocyclic acceptor excited-state (exciplex) complexes with N,N-diethylaniline have been examined. In general their behaviour is consistent with the accepted theories. Spectral shift sand intensity changes with solvent and radiationless deactivation appear to be more complicated than with aromatics. The bipyridyls are discussed in some detail.

The confidence with which fluorescence decay data can be interpreted is dependent upon the accuracy and precision of these data. This dependence is especially critical when more than one exponential is involved in the decay.1,2 Unfortunately decay curve parameters have often been presented without any indication of either their accuracy or precision. An analysis is presented which suggests that the covariance ellipsoid is the most satisfactory statistic with which to define the precision.

Lifetime studies of the three azabenzenes s-triazine, pyridazine and pyrazine are reported. They show quite different behaviour. Lifetimes of four single vibronic levels of s-triazine were measured and show that an additional decay channel is operating. The fluorescence from pyridazine is so fast that it was indistinguishable from the exciting lamp pulse. Lifetimes of six single vibronic levels of pyrazine were measured and were shown to be pressure-dependent. These results fit quite well into the current understanding of the behaviour of the azabenzenes.

A reported variation of the electronic absorption spectra of diaza-aromatics in non-polar solvents with temperature appears to be explicable in terms of three effects. Hydrogen bonding interactions occur in all but exhaustively dried non-polar solvents. Microcrystals form readily at low temperatures at even quite low concentrations (c. 10-5M). The solutions are sensitive to exposure to light. The diaza-aromatics undergo photochemical change forming, in some cases, photoadducts with the solvent.