CLEAN AND WATER-COVERED SAPPHIRE (1(1)OVER-BAR-02) SURFACES - STRUCTURE AND LASER-INDUCED DESORPTION

The atomic and electronic structure of clean and water-covered sapphire (1102BAR) surfaces have been studied by low energy electron diffraction (LEED), electron energy loss spectroscopy (ELS), and temperature programmed desorption (TPD). A surface electronic state with a loss energy of 4.1 eV is measured in the bulk bandgap on the clean, reconstructed (2 x 1) surface. Exposure of the surface to water at room temperature (RT) causes the surface electronic state to shift to lower loss energies reaching a value of approximately 3.6 eV at saturation coverage. In addition a higher energy loss feature appears at approximately 7.5 eV loss and is attributed to a hydroxyl adlayer. Adsorption of water on the surface at RT was observed to proceed with near unity sticking probability. Two desorption states were seen. At low coverage a state appears with a peak temperature for desorption of 525 K. With increasing exposure the peak shifts to lower temperature, reaching 450 K near one monolayer, defined as one water molecule per (1 x 1) unit cell. At higher coverages a second state becomes populated. The water adlayer removed the surface reconstruction only after exposure to an electron beam, yielding a (1 x 1) LEED pattern. The laser-induced desorption of aluminum ions from clean and water-covered sapphire (1102BAR) at laser wavelengths of 1064 nm (1.17 eV), 355 nm (3.51 eV), and 266 nm (4.66 eV) was investigated by time-of-flight mass spectrometry. Below the laser ablation threshold, predominately Al+ ions desorbed with an average kinetic energy of 7.0 +/- 0.7 eV at all three wavelengths from both clean and water-covered surfaces. For 1064 nm light at fluences just below the ablation threshold, AlO- desorbed with kinetic energy of 1.7 +/- 0.5 eV from the clean (2 x 1) surface. The observed aluminum desorption at high kinetic energy is consistent with the previously proposed aluminum-localized exciton-mediated desorption mechanism. In sharp contrast to the laser-surface interactions, a 300 eV electron beam desorbed O+ from the clean (2 x 1) surface and H+ and OH+ from the water-covered surface.