Optical second-harmonic diffraction study of anisotropic surface diffusion: CO on Ni(110)

We describe in detail a technique using optical second-harmonic (SH) diffraction from a one-dimensional laser-induced monolayer grating to probe surface diffusion of adsorbates and its anisotropy on a solid surface. The case of CO on Ni(110) is used as a demonstration. The two orthogonal and independent diffusion tensor components along [11¯0] and [001] are measured, exhibiting a strong anisotropy in both the activation energy ${\mathit{E}}_{\mathrm{diff}}$ and the preexponential factor ${\mathit{D}}_0$ in the diffusion coefficients. A compensation effect between ${\mathit{E}}_{\mathrm{diff}}$ and ${\mathit{D}}_0$ is observed. In comparison with CO/Ni(111) and CO/Ni(100), our result suggests that the Ni(110) surface seen by CO is much smoother than Ni(111) and Ni(100). Both advantages and limitations of the present technique are mentioned and possible complications in the data analysis are discussed.