Adsorption of H₂O on a Single-Crystal α-Al₂O₃(0001) Surface

The adsorption of H 2 O on a single-crystal α-Al 2 O 3 (0001) surface was examined using laser-induced thermal desorption (LITD) and temperature-programmed desorption (TPD) techniques. α-Al 2 O 3 (0001) models the surface of Al 2 O 3 exhaust particles generated by solid rocket motors that may affect the stratospheric ozone layer. After cleaning and annealing to 1100 K, the α-Al 2 O 3 (0001) surface displayed a well-defined hexagonal (1 × 1) low-energy electron diffraction (LEED) pattern. Absolute hydroxyl coverages on this α-Al 2 O 3 (0001) single-crystal surface were determined using H 2 O LITD signals. Hydroxylation by the dissociative adsorption of H 2 O was differentiated from molecular H 2 O adsorption using TPD studies with isotopically labeled H 2 18 O. For H 2 O dissociative adsorption at 300 K, the initial sticking coefficient was S ≈ 0.1. The H 2 O sticking coefficient decreased nearly exponentially with hydroxyl coverage, and the hydroxyl coverage saturated at ϑ OH = 0.5 × 10 15 OH groups/cm 2 after a H 2 O exposure of >10 10 langmuir. For constant H 2 O exposures performed at different H 2 O pressures, the resulting hydroxyl coverage also increased with H 2 O pressure suggesting collisionally activated H 2 O adsorption. On the basis of these H 2 O adsorption results, α-Al 2 O 3 rocket exhaust particles in the stratosphere should be hydroxylated at coverages of ϑ OH ≈ 0.3 × 10 15 OH groups/cm 2 . H 2 O adsorption on α-Al 2 O 3 (0001) was also investigated using a H 2 O plasma. Plasma hydroxylation yielded much larger hydroxyl coverages of ϑ OH = 3.6 × 10 15 OH groups/cm 2 and destroyed the hexagonal LEED pattern after one plasma exposure. Larger hydroxyl coverages were measured after consecutive H 2 O plasma exposures indicating that plasma hydroxylation progressively roughens the α-Al 2 O 3 (0001) surface.