Energy-Dependent Cancellation of Diffraction Spots due to Surface Roughening

The low-energy electron diffraction (LEED) pattern of the step-kinked $\mathrm{Pt}\{531\}$ surface at 200 K shows energy-dependent cancellation of diffraction spots over unusually large energy ranges, up to 100 eV. This cannot be reproduced theoretically when a flat surface geometry is assumed. A relatively simple model of roughening, however, involving 0.25 ML of vacancies and adatoms leads to very good agreement with the experiment. The cancellation of intensities within a very narrow range of adatom or vacancy coverages is caused by the interference of electrons emerging from different heights but similar local environments. This is a rare example where the energy dependence of integrated LEED spot intensities is dramatically affected by the long-range arrangement of atoms.

Figure 1

Schematic drawing of (a) an ideal $\mathrm{Pt}\{531\}$ surface and (b) islands of adatoms and vacancies on the same surface. The islands are marked by dashed lines.

Figure 2

Experimental (solid lines) and best fit theoretical IV curves of selected beams for adatom or vacancy coverages of 0.0 (dotted lines) and 0.25 (dashed lines).

Figure 3

Top: Envelope functions of the $(2,-2)$ beam for different values of ${\Theta }_{\mathrm{ad}}$ for a three-level model and a two-level model for ${\Theta }_{\mathrm{ad}}=0.50$ (offset with respect to the others). The horizontal dashed lines indicate the intensity range between 0% and 5% of the maximum intensity. Bottom: Experimental $(2,-2)$ IV curve (solid line), original theoretical IV curve for flat surface (dot-dashed line), and multiplied with envelope function for ${\Theta }_{\mathrm{ad}}=0.25$ (dashed line). The horizontal dashed line is placed at 5% of the maximum intensity.