UNUSUAL ADSORPTION SITE OCCUPATION SEQUENCE - NO ADSORPTION ON STEPPED PD(112)

The adsorption of NO on a stepped Pd(112) surface has been investigate d at 300-373 K with high resolution electron energy loss spectroscopy (HREELS) and electron stimulated desorption ion angular distribution ( ESDIAD) techniques. At exposures below 5.7 X 10(14) NO/cm(2), NO molec ules preferentially adsorb on the terrace sites, with an NO stretching mode observed by HREELS at 1535-1550 cm(-1), and an O+ ESDIAD beam di rected near the (111) terrace-normal direction. For NO exposures above similar to 6 x 10(14) molecules/cm(2), step sites are occupied in add ition to the terrace sites, with an NO stretching mode observed in HRE ELS at similar to 1655-1670 cm(-1) and an additional O+ ESDIAD beam or iented in the ''downstairs'' direction. This is an unusual reversal of the site occupation sequence since one usually expects stronger adsor bate bonding on the step sites causing the steps to fill first. The N- O bond stretching frequencies observed in HREELS are indicative of wea ker bonding of NO molecules on the step sites than on the terrace site s.