Efficient subpicosecond photoinduced surface chemistry: The ultrafast photooxidation of CO on palladium

Ultrafast near-infrared photoexcitation of mixed adlayers of carbon monoxide and atomic oxygen on the Pd(111) surface results in the efficient associative desorption of carbon dioxide. The oxidized fraction of the desorbed molecules ranges from ∼15% to 40%, approximately an order of magnitude greater than observed from Ru(001) under similar conditions. Time-resolved correlation measurements reveal fast dynamics that indicate the photochemistry is mediated by substrate electrons. The dynamics are strongly dependent on the initial CO coverage in a manner consistent with an activation energy for oxidation that depends on the initial CO binding site. These results, in addition to comparison of the ultrafast photooxidation to thermal oxidation, are consistent with the photooxidation occurring rapidly (within one to several picoseconds) from an initial mixed CO+O adlayer structure without time for diffusional phase segregation. The observed efficient ultrafast photoexcitation of associative desorption of CO₂ from Pd(111) thus provides the opportunity to follow the picosecond dynamics of the bimolecular surface chemical reaction of CO+O from a well-defined initial geometry.