ON THE TIME STRUCTURE OF TUNNELING IMAGES OF STEPS

Because of the limited time resolution of the scanning tunneling micro scope (STM), tunneling images of monatomic steps on surfaces display s udden jumps in the position of steps from one scan line to the next. W e have studied the time structure of the step jump events experimental ly as well as theoretically by invoking Monte Carlo simulations of the motion of atoms alongside steps and also analytical modeling. The dis cussion focuses on a time and temperature regime where the mean time b etween step jumps is comparable to, though larger than, the time betwe en two scan lines. Then, two regimes can be distinguished in the proba bility distribution to find a particular time interval between step ju mps. On a short time scale, the time structure is dominated by the ret urn of adatoms to the same kink from where they had been emitted. Step jumps with a long time interval between them correspond to a random w alk of kinks where the adatom on the average does not return to the sa me kink within the time necessary to emit the next adatom. Analytical expressions for the probability of step jumps in the two time regimes are derived and compared to the results of the Monte Carlo simulations . The experimental data are analyzed with the help of the analytical t heory and the activation energies for the emission of adatoms from kin ks and for the hopping of adatoms alongside the steps are determined.