Structure determination of ordered and disordered organic molecules on a surface from the substrate diffraction spots in low energy electron diffraction: (3×3)R30°-C2H2 and disordered CH3OH on Pd(1 1 1)

We examine the effects of chemisorbed organic molecules on the intensity versus beam energy (I/E) variation of the substrate (1 × 1) Bragg spots in low energy electron diffraction (LEED). We find that these integer-order spots alone can provide important information about the local geometries of the molecules on the surface, largely independent of the degree of long-range order of an adsorbate overlayer. This is demonstrated by comparing LEED studies of two different adsorbates on Pd(1 1 1), namely: C2H2 which forms an ordered (3×3)R30° overlayer, and thus gives rise to superstructure (or fractional-order) Bragg spots, and disordered CH3OH, which scatters diffusely into directions between the integer-order spots. This allows LEED to be used conveniently for the determination of the structures of adsorbates on surfaces as a continuous function of coverage, independent of the degree of long-range order of the adlayer.