Kinetics and atomic structure of O adsorption on W(110) from time- and state-resolved photoelectron spectroscopy and full-solid-angle photoelectron diffraction

We have studied the kinetics of the low-pressure adsorption of oxygen on W(110) via time- and chemical-state-resolved photoelectron spectroscopy (PS) and diffraction (PD). Using high-brightness third-generation synchrotron radiation from the Advanced Light Source, together with a new photoelectron spectrometer/diffractometer system, we are able to resolve four distinct chemical states in W 4f spectra (clean surface, bulk, W bound to two O atoms=O2, and W bound to three O atoms=O3) and to measure such spectra in about 20 s each so as to follow the kinetics of oxygen adsorption at 3×10−9 Torr from the clean surface to near saturation. The time-dependent transformations from one state of the surface W atoms to another have been determined at three temperatures of 298, 360, and 593 K. We also find that, for this adsorption pressure on our surface, no long-range-ordered structures are observable in LEED, even though the previously observed ordered structures of p(2×1), p(2×2), and (1×1)×12 are formed at higher pressures of approximately 10−6 Torr. The room-temperature state-resolved PS data are modelled using a simple Monte Carlo approach which assumes no mobility after molecular dissociation, and these calculations are found to describe the experimental data very well. Combining experiment and theory also permits deriving the sticking coefficient as a function of time, yielding results which agree with prior work. Full-solid-angle PD patterns have also been determined at the end of 298 K oxygen exposure for the O2 and O3 W atoms, and these have been analyzed using multiple scattering theory and R-factor analysis. The final local pseudo-threefold hollow geometries for oxygen are found to be very similar to those for a saturated one-monolayer structure of O on W(110) (the (1×1)×12 structure), including a lateral shift of O away from the position corresponding to three equal bond distances, but with some contraction of the OW vertical separation in going from O2 to O3 suggested. This study indicates considerable potential of such time- and state-resolved PS and diffraction for investigating surface reaction kinetics and structure, particularly for the large number of systems that do not exhibit long-range order and in view of future instrumentation improvements that should lead to much shorter data accumulation times and/or higher ambient pressures of measurement.