Observations of the Kinematic Interaction of Surface Steps during Evaporation of NaCl

In the Burton—Cabrera—Frank theory [1], the speed of a surface step is dependent on the step separation on account of the interaction by surface diffusion (molecules leaving one step diffuse to another). This is essentially the kinematic theory of crystallization, and the interaction is kinematic. The theory is usually applied to macroscopic effects, e.g., etch-pit shape. Here we describe direct electron-microscope observations on kinematic interaction between elementary steps with gold decoration [2, 3]. During evaporation from a NaCl crystal under vacuum, pits are produced around dislocations on {100} surfaces, which have a characteristic step structure, step height do/2 = 2.81 A or do = 5.62 A [4]. The steps are equidistant in steady-state evaporation, and the separation is determined by the conditions at the center. Kinematic interaction is most prominent in unsteady states, e.g., when the sequences are not indefinitely long ones with equal step heights.