ESR studies of 0−2 adsorbed on Ti supported surfaces: Analysis of motional dynamics

Abstract

Temperature-dependent ESR spectra of O−2 adsorbed on Ti ions supported on porous Vycor glass were observed over the range 4.2 to 400 °K. These spectra were obtained under normal high vacuum conditions as well as under UHV conditions (P⩽10−9 Torr) and are very well resolved. It was observed that the line position of the g̃ tensor component that is perpendicular to the internuclear axis of O−2 remained constant with temperature, whereas the other two components of the g̃ tensor shift in position with temperature, and are accompanied by drastic line shape changes. This observation indicates that the molecular motion of O−2 on the surface is highly anisotropic, consisting essentially of planar rotation about the axis perpendicular to the internuclear axis of O−2 and parallel to the normal to the surface. Furthermore, the observation of nonequivalent 17O hfs of O−2 suggests that the internuclear axis of O−2 might be tilted slightly from the surface and/or one oxygen is closer to the Ti4+. The ESR line shapes were simulated for the different possible models: Brownian diffusion, jump diffusion (from weak jump to strong jump), approximate free diffusion, and discrete jump. It was found that the theoretical spectra calculated using the model of weak jump rotational diffusion best fit the observed spectra in the temperature range below 57.4 °K. However, in the temperature range above 57.4 °K, although the Brownian diffusion model seems the best among the models used, none of the present models used could successfully reproduce the observed line shapes. The rotational correlation time τR∥ was found to range between 10−5 sec (below 14.5 °K) and 10−9 sec (263 °K). The values of τR∥ depend strongly on the model used in the lower temperature range, but were essentially independent of model above 100 °K. The activation energy for rotational diffusion was estimated to be 0.5 kcal/mole above 100 °K. The line shape below 15 °K is independent of temperature, although the O−2 spectrum appears to exhibit residual motional effects. This observation suggests that coherent quantum mechanical motion is predominant below 15 °K. This matter is discussed in some detail, and the appropriate theory to investigate quantum effects on the motional dynamics is outlined including possible isotope effects on the motion. Spectral observation of possible interaction between C2H4 and O−2 on the surface is presented. Also discussed are the techniques for preparing samples with strong well-resolved signals and for removing the other types of O−2 signals, which do not show significant temperature-dependent spectral changes. The show.