Real-time STM observations of atomic equilibrium fluctuations in an adsorbate system: O/Ru(0001)

For dynamic studies of surface processes a fast scanning tunneling microscope (STM) was developed, by which imaging rates of up to 20 frames s(-1) are achieved. First studies were performed with O/Ru(0001). At 300 K and small coverages the adsorbed oxygen atoms are very mobile, but video sequences recorded with the fast STM allow the random walk of individual oxygen atoms to be monitored. A hopping rate of 14+/-3 s(-1) is derived. Longer lifetimes of atom pairs indicate the operation of oxygen-oxygen interactions of the order of kT. At larger coverages these lead to the formation and decay of islands by attachment and detachment of oxygen atoms. Over longer periods of time the atomic configurations change completely, however, without improvement of the order. It is concluded that the system has reached thermodynamic equilibrium and that equilibrium fluctuations are observed. The atomic configurations are reproduced by Monte Carlo simulations with weak attractive interactions between oxygen atoms on (2x2) sites. (C) 1997 Elsevier Science B.V.