No abstract

A simple statistical theory is proposed which gives quantitative values for ion–polar neutral momentum transfer collision frequencies. The calculated collision frequencies depend on the rotational temperature of the neutrals, the translational temperature of the ions and neutrals, the masses of the ion and neutral, and the ratio of the neutral dipole moment to the square root of the neutral polarizability. Agreement with available experimental collision frequencies is very good. The results are consistent with available experimental rate constants for exothermic proton transfer reactions. The effects of nonorbiting collisions and finite ion–neutral hard sphere radius are examined.

The chemisorption of hydrogen on tungsten (001) induces displacive rearrangements (’’reconstruction’’) of the substrate surface, depending on the temperature T and the adsorbate coverage ϑ. By means of LEED and other techniques, the surface phase diagram in the T–ϑ plane (ϑ<0.7, T<500 K) has been studied. The predominant structure is (√2×√2) which is stabilized by hydrogen in the region ϑ<0.3, but is transformed to an incommensurate structure at higher coverages. Adsorption below ?200 K produces no ordered phase. The LEED patterns from a terraced surface exhibit reduced symmetry and reveal the direction of the lattice distortion; the atomic displacements are found to be along 〈10〉 for the H-induced structures, in contrast to the 〈11〉 displacements on clean W(001). The substrate rearrangement is believed to be a major cause of the observed coverage dependence of the desorption energy and other adsorbate properties.