A surface/gas-phase reaction on Ti${\mathrm{O}}_2$(110) was visualized in situ by scanning tunneling microscopy. When a vacuum annealed $\left(1\times 1\right)$ surface heated at 800 K was exposed to an ${\mathrm{O}}_2$ ambient of $1\times {10}^{-5\mathrm{}}\mathrm{Pa}$, hill-like structures were randomly nucleated over terraces. Then they were transformed into new terraces, with added rows comprising double strands. We proposed a reoxidation scheme to interpret the dynamics; partially reduced ${\mathrm{Ti}}^{n+}$ ions $\left(n\le 3\right)$, which had been accumulated at interstitial positions in the vacuum annealed crystal, were oxidized at the surface to form the hills, added rows, and new terraces.

• Received 21 July 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.791