Second Harmonic Generation Study of Photodynamics and Adsorption/desorption on Rutile Titanium Dioxide Surfaces

Surface second harmonic generation (SSHG) has been applied to study the adsorption and photodesorption properties of rough and flat (001) as well as (110) surfaces of rutile rm TiO_2 single crystals. The centrosymmetric rutile structure enables SSHG to provide surface-specific sensitivity under ultra high vacuum (UHV) and atmospheric environments. In a UHV environment, photodesorption of oxygen occurs via excitation at 266 nm and much less efficiently at 532 nm. The phenomenon can be explained as photo-ionization of surface rm O_2-species which are created when rm O_2 binds to rm Ti^{3+} surface defects. Thermal reoxidation at 300 K of the photoreduced surfaces is apparent at oxygen pressure above 10^{-6} torr. At oxygen pressure between 5 torr and 1 atm, rm O_2 is physisorbed only on rough surfaces at 300 K. The 266 nm photodesorption of these molecules has been observed, and the subsequent readsorption has been found to occur over several seconds at 100 torr. Optical damage thresholds have been measured and related to the observed photodynamics. Surface symmetry is discussed in terms of the surface second order susceptibility tensor elements. The ability of SSHG to span the UHV to atmospheric range provides a unique view of rutile rm TiO_2 surfaces.