A new technique was described which involved the use of second-harmonic optical diffraction, from a one-dimensional laser-induced monolayer grating, to probe the anisotropic surface diffusion of adsorbates. The 2 orthogonal and independent CO diffusion tensor components were measured along the [1¯10] and [001] directions of the (110) Ni surface. It was found that the results (table 14) could be described by the expressions:

[1¯10] D (cm2/s) = 3.8 x 10-9 exp[-1.1(kcal/mol)/RT]

[001] D (cm2/s) = 4.8 x 10-6 exp[-3.1(kcal/mol)/RT]

thus revealing the existence of a marked anisotropy in both the activation energy and the frequency factor. The diffusion parameters were related by the compensation effect. The results suggested that the (110) Ni surface, as seen by CO, was much smoother than the (111) and (100) Ni surfaces.

X.D.Xiao, X.D.Zhu, W.Daum, Y.R.Shen: Physical Review B, 1992, 46[15], 9732-43

Table 14

Diffusivity of CO on the (110) Surface of Ni

(45º bisects [001] and [1¯10])