Abstract
By monitoring the first-order optical second-harmonic diffraction from monolayer coverage gratings, we measured the surface diffusion coefficient D (T, Φ) for CO on Ni (110) as a function of the sample temperature T (see Figs. 1–3) and the angle Φ between the grating orientation and the crystallographic [110] directon1,2 (see Fig. 4). We found that the migration of CO is two-dimensional and is along the two principal directions of Ni(110). We deduced Do, [110] = 7 × 10−9 cm2/s and Ediff, [110] = 1.3 kcal/mol, Do, [001] = 9 × 10−7 cm2/s and Ediff, [001] = 2.9 kcal/mol. This result shows that the binding-energy distribution of CO along Ni(110) is surprisingly smooth, compared to the binding energy variations (≥7 kcal/mol) on Ni(111) and Ni(100). We believe that such a small variation of the binding energy along [110] direction (1.3 kcal/mol or ≤0.06 eV/atom) explains why rich and sometimes inconsistent superstructures of CO were observed on Ni(110). The large difference in D0 ≈ (l/4)v012 is partly a result of a three-times-larger mean hopping distance 1 along [001]. The remaining difference is tentatively explained as arising from the relaxed or looser binding of CO between [110] rows. This leads to a larger entropy change and, therefore, a larger “trial frequency” v0 along [001].
© 1991 Optical Society of America
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