Photon-stimulated desorption and ultraviolet photoemission spectroscopic study of the interaction ofH2O with a Ti(001) surface

The adsorption of ${\mathrm{H}}_2$O on a stepped Ti(001) single crystal, oriented within 4° of Ti(001) has been studied using synchrotron radiation from the Synchrotron Ultraviolet Radiation Facility at National Bureau of Standards. The species formed upon adsorption of ${\mathrm{H}}_2$O were identified through variable-wavelength ultraviolet photoemission spectroscopy. At room temperature (∼300 K), water dissociates to form O, H, and OH. At low temperature (∼90 K) and low coverage (<1 L), the same species were observed. Photon-stimulated-desorption experiments were performed under these conditions yielding predominately ${\mathrm{H}}^{+}$ ions with little or no O${\mathrm{H}}^{+}$ or ${\mathrm{O}}^{+}$. At 90 K and coverages greater than 1 L, an ice overlayer was formed suppressing the ${\mathrm{H}}^{+}$-ion desorption. Separate experiments involving the adsorption of hydrogen and coadsorption of oxygen and hydrogen showed an order of magnitude less ${\mathrm{H}}^{+}$ desorption, indicating that the ${\mathrm{H}}^{+}$ desorption was associated with the presence of OH on the surface. The ${\mathrm{H}}^{+}$-ion yield as a function of photon energy showed a threshold at 25 eV perhaps due to O $2s$ excitation. A second threshold at 33 eV, a broad peak near 45 eV, and a slow decrease toward higher photon energy suggests a correlation with the Ti $3p$ core-hole excitation although other possibilities cannot be eliminated. Possible bonding configurations are proposed to explain the origins of the ${\mathrm{H}}^{+}$ emission.