Cluster calculations imply that the only stable state of H2 on simple and noble metals is fully dissociated. Calculations using a jellium substrate predict a stable state of molecular chemisorption. A plausible reason for the difference is proposed. By use of an energy diagram constructed on the basis of small cluster calculations, it is shown how molecules with very low kinetic energy can utilize vibrational energy to overcome a high entrance channel energy barrier to dissociation.

The microwave cavity technique for measuring electron concentration is considered, an experimental study of the effects of holes in the cavity on the field distribution in the cavity is reported and a method is described which enables the relationship between electron concentration and frequency change to be established empirically. A method for continuously monitoring the frequency changes due to ionization is described, and the effect of electron collisions is treated theoretically. A technique allowing instantaneous measurements of electron concentration and rate of decay of concentration is described.The decay of ionization in afterglows in caesium vapour has been studied, giving recombination coefficients which vary with pressure from about 0·3×10−6 cm3 s−1 at 0·5 torr to about 2×10−6 cm3 s−1 at 3 torr, consistent with a dissociative process. At late stages in afterglows, where diffusion loss dominates, estimates of reduced mobilities of diffusing ions suggest that at 0·1 torr the Cs+ ion dominates, whereas at pressures of about 10 torr the Cs2+ ion is more important. Measurements of electron collision frequency as a function of vapour pressure between 0·1 and 4 torr are reported.

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