AMBIGUITIES IN ADSORBATE SITE ASSIGNMENT FROM VIBRATIONAL FREQUENCIES - A TLEED STRUCTURAL STUDY OF (2X1)CO-PD(110)

The structure of the (2 X 1)CO-Pd(110) surface phase has been determined by LEED intensity analysis. The CO molecule is found to be adsorbed in an atop site, tilted by 11-degrees +/- 4-degrees with respect to the surface normal, with a C-O bond length of 1.16 +/- 0.04 angstrom. Interestingly, the C-O vibrational frequency for this system (2003 cm-1) is virtually identical to the frequency observed for the (2 X 1)CO-Ni(110) surface phase (1998 cm-1) which a previous LEED study has shown involves bridge bound CO molecules. The result indicates that care must be taken in assigning site symmetries on the basis of C-O stretching frequencies alone.