SHIFTS IN ADSORBATE VIBRATIONAL FREQUENCIES DUE TO INTERNAL ELECTRIC-FIELDS

A new physical mechanism is proposed to explain the shifts in vibrational frequency of negatively adsorbed species on a metal surface. Ab initio cluster model calculations for NO adsorbed Dn Ag(111) in two different orientations, N-down and O-down, suggest that the low-coverage HREELS peak appearing at 1282 cm-1 is due to adsorbed NO in an O-down orientation. The large shift in vibrational frequency relative to free NO is shown to be due to the internal electric field produced by the negative charge on the adsorbate and the metal image charge. While external electric fields are known to produce vibrational shifts this is the first evidence that the same effect can be produced by internal electric fields which have their origin in the ionic nature of the chemisorption bond between NO and Ag(111).