Abstract

Molecular oxygen adsorbs on a platinum (111) single crystal surface in two distinct chemisorbed phases at different temperatures.¹ At temperatures of -197°C, molecular oxygen preferentially absorbs to the platinum surface as a superoxo-like phase (O₂⁻). However, at temperatures of −135°C, it preferentially absorbs as a peroxo-like phase with one additional electron (O₂⁻²). These two states have different static (equilibrium) photoelectron spectra, corresponding to the different oxidation states and different positions of the oxygen molecules on the Pt( 111) surface (Fig. 1.) In our work we use time-resolved photoemission spectroscopy to monitor changes in the chemical ground state of oxygen on Pt (111), after ultrafast excitation by an IR (800nm) pump pulse. The pump pulse transiently heats the electrons in the metal, promoting oxygen from the superoxo (O₂²⁻) to peroxo (O⁻²) phase. This enables us, for the first time, to directly observe changes in the chemical bond character of a molecule adsorbed on a surface, with the sub-100 femtosecond time-resolution necessary to follow the complete progress of the reaction. This is in contrast to previous work, which monitored the electron distribution on surfaces,² or which followed only the first 1/2 oscillation of a vibrating molecule on a surface by monitoring a very short-lived excited state.³

© 2001 Optical Society of America