Surface diffusion of individual atoms and dimers: Re on W(211)

Abstract

Quantitative observations of diffusion on the W(211) surface have been made both for rhenium atoms and for rhenium dimers formed from atoms in adjacent [1̄11] channels, using the field ion microscope to reveal the surface on the atomic level. From measurements of the dispersion of individual atomic displacements over the range 290⩽T⩽350 °K, the diffusion parameters for Re atoms are found as D₀=2.2(×2.8^±1) ×10⁻³ cm² sec⁻¹ and ΔE^‡_m=19.8±0.7 kcal mole⁻¹. The diffusion coefficient of dimers is obtained from observations of center of mass motion over the range 260⩽T⩽350 °K; these yield the parameters D₀=4.5(×1.7^±1) ×10⁻⁴ cm² sec⁻¹ and 〈ΔE^‡〉_c.m.=18.0±0.3 kcal mole⁻¹. Rhenium dimers are found in either a straight or a staggered state, with the latter favored. Dimers diffuse by making single atom jumps, in a two step process in which the straight configuration converts to the staggered at the rate a, and the staggered reverts to the straight at the rate b. Quantitative observations of the frequency of occurrence for the two states, combined with diffusion measurements, yield ν_a=1.1(×2^±1) ×10¹² sec⁻¹, ΔE^‡_a =17.5±0.4 kcal mole⁻¹ and ν_b=2.5(×1.7^±1) ×10¹² sec⁻¹, ΔE^‡_b=18.3±0.3 kcal mole⁻¹. This is to be compared with the values ν=5.9(×2.8^±1) ×10¹² sec⁻¹ and ΔE^‡_m=19.8±0.7 kcal mole⁻¹ found for jumps of single rhenium atoms. Interactions between rhenium atoms on the W(211) surface are small, on the order of van der Waals forces. Nevertheless, comparison with previous work on iridium dimers reveals a pronounced chemical specificity.