Long hops of an adatom on a surface

In a recent experiment, Senft and Ehrlich (Phys. Rev. Lett. 74(1995) 294) reported the observation of long hops in the migration of Pd on W(211). In this Letter rye consider a possible microscopic mechanism responsible for such hops. We demonstrate that (a) Kramer's theory provides a good framework for analysis of the experimental data; (b) that the measured temperature dependence may be reasonably well accounted for; (c) the experimental results do not unequivocally demonstrate a substantial amount uf long jumps.