Surface states and oxygen chemisorption on Ti(0001)

The system ${\mathrm{O}}_2$ on Ti(0001) has been investigated using appearance potential spectroscopy (APS), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and work-function measurements obtained by the field-emission retarding potential technique. A strong band of states just above the Fermi level was found to be localized near the surface. This band of surface states broadened with increasing temperature and was suppressed by exposure to oxygen. Although weaker, a similar band of surface states was previously observed on polycrystalline titanium samples and more recently predicted for the (0001) surface to occur at the Fermi level and to extend a few tenths of an eV on either side of it [Peter J. Feibelman, J. A. Appelbaum, and D. R. Hamann, Phys. Rev. B 20, 1433 (1979)]. The sensitivity of the surface-state signal to sampling depth was demonstrated by continuously varying the sampling depth between the 5-10 Å inelastic mean free path of the incident electrons in Auger electron appearance potential spectroscopy and approximately half that value for disappearance potential spectroscopy. The work function remained approximately constant for very low oxygen coverage and then steadily increased with additional exposure from a clean-surface value of 4.58±0.05 eV, leveling off at approximately 5.3 eV as the surface became saturated with oxygen. The initially diffuse low-coverage $p(2\mathrm{}\times 2)$ LEED superstructure formed upon exposure at room temperature was significantly improved upon flashing the sample to 250°C. This was accompanied by an abrupt decrease of 0.15 eV in the work function, although AES revealed no decrease in the amount of oxygen on the surface. A similar decrease in the work function upon a 250°C flash anneal was observed for other exposures as well, suggesting some reordering of the surface adatoms. These results are interpreted in terms of two chemisorption states for the oxygen: a tightly bound $\alpha$ state at low coverages characterized by a well-ordered $p(2\mathrm{}\times 2)$ LEED pattern and a work function below that of the clean surface, and a $\beta$ state characterized by a disordered structure and a work function higher than the clean surface value. Heating converts the $\beta$ state to the $\alpha$ state.