We present here a method for quantitatively determining tip effects on surf
ace diffusion during a scanning tunneling microscopy experiment. Using the
technique of atom tracking, we measure the bias voltage and tunnel current
dependencies of adsorbed Si dimer dynamics on Si(001). Throughout the range
of typical tunneling conditions, the activation barrier for diffusion vari
es by less than 3%. We also find a striking difference between the electric
-field effects on dimer diffusion and rotation, indicating the importance o
f transition states for this system. [S0163-1829(98)52744-0].