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Reaction pathways that bypass the conventional saddle-point transition state (TS) are of considerable interest and importance. An example of such a pathway, termed 'roaming,' has been described in the photodissociation of [H.sub.2]CO. In a combined experimental and theoretical study, we show that roaming pathways are important in the 308-nm photodissociation of C[H.sub.3]CHO to C[H.sub.4] + CO. The C[H.sub.4] product is found to have extreme vibrational excitation, with the vibrational distribution peaked at [approximately equal to] 95% of the total available energy. Quasiclassical trajectory calculations on full-dimensional potential energy surfaces reproduce these results and are used to infer that the major route to C[H.sub.4] + CO products is via a roaming pathway where a C[H.sub.3] fragment abstracts an H from HCO. The conventional saddle-point TS pathway to C[H.sub.4] + CO formation plays only a minor role. H-atom roaming is also observed, but this is also a minor pathway. The dominance of the C[H.sub.3] roaming mechanism is attributed to the fact that the C[H.sub.3] + HCO radical asymptote and the TS saddle-point barrier to C[H.sub.4] + CO are nearly isoenergetic. Roaming dynamics are therefore not restricted to small molecules such as [H.sub.2]CO, nor are they limited to H atoms being the roaming fragment. The observed dominance of the roaming mechanism over the conventional TS mechanism presents a significant challenge to current reaction rate theory. reaction dynamics | roaming mechanisms | photochemistry | quasiclassical trajectories | transition state

The unique mechanical performance of animal cells and tissues is attributed mostly to their internal biopolymer meshworks. Its perplexing universality and robustness against structural modifications by drugs and mutations is an enigma in cell biology and provides formidable challenges to materials science. Recent investigations could pinpoint highly universal patterns in the soft glassy rheology and nonlinear elasticity of cells and reconstituted networks. Here, we report observations of a glass transition in semidilute F-actin solutions, which could hold the key to a unified explanation of these phenomena. Combining suitable rheological protocols with high-precision dynamic light scattering, we can establish a remarkable rheological redundancy and trace it back to a highly universal exponential stretching of the single-polymer relaxation spectrum of a 'glassy wormlike chain.' By exploiting the ensuing generalized time-temperature superposition principle, the time domain accessible to microrheometry can be extended by several orders of magnitude, thus opening promising new metrological opportunities. biopolymers | light scattering | nonlinear rheology | wormlike chain

A simple model to predict the effect of medium orientation of longitudinal media on the transition sharpness is reported. It is shown that a traditional slope model can be suitably modified by multiplying the total field gradient with the medium squareness to reflect the loss in effective field gradient due to reversible magnetization rotations. The results agree well with experimental data. Index Terms--Longitudinal recording media, oriented media, signal-to-noise ratio (SNR), slope model.

A combination of the state-to-state dynamics study and quasiclassical trajectory (QCT) calculations is employed to study the kinetics of the F + CH[D.sub.3] gas-phase abstraction reaction. The excitation function of the reaction is shown to rise from the threshold, leveling off at higher energy values.