COVERAGE DEPENDENCE OF CO DISSOCIATION ON CLEAN AND HYDROGEN PRESATURATED FE(100) SURFACE

A detailed thermal desorption spectroscopy (TDS) study of very low and intermediate exposures of CO on clean and H-2 presaturated Fe(100) su rfaces was conducted. At extremely low CO exposures, CO adsorbs only i n the pi-bonded CO state at low temperature. Molecules in this state e xhibit an extremely weak carbon-oxygen bond and exclusively undergo di ssociation when the temperature is raised above 440 K. At higher cover ages, both dissociation and desorption is observed around 440 K even t hough all the molecules are still bound in the pi-bonded CO state at l ower temperatures. A model describing the partitioning between desorpt ion and dissociation is proposed in which the dissociation fragments d isplace the strongly bound CO in the pi-bonded state. The stoichiometr y of this reaction allows a maximum of one half of pi-bonded CO molecu les to dissociate. Preadsorbed hydrogen strongly influences the ratio of dissociation to desorption as a function of coverage and suggests t hat hydrogen may induce island formation in the CO adlayer.