A FAR-INFRARED VIBRATIONAL STUDY OF O/AG(110)

We have obtained the first far infrared measurement of an atomic adsorbate-substrate stretch on a single crystal surface. The infrared reflection-absorption technique is employed on O/Ag(110), with an apparatus consisting of a laboratory source of infrared radiation, an evacuated FTIR, and a He-cooled bolometer detector. Complementary STM measurements, performed in situ, allowed rigorous characterization of surface structure. At room temperature and the lowest detectable coverage (0.14 ML), the frequency of the O-Ag symmetric stretch is (v) over tilde(ss) = 319 +/- 1 cm(-1). This band shifts continuously upwards to 335 +/- 1 cm(-1) as coverage is increased to 0.5 ML, where the (2 x 1)-0 phase is fully developed. The intensity of this dipole transition is analyzed with a standard Lorentz oscillator model. The effective charge transfer for the oscillating dipolar complex is evaluated at just 0.036 +/- 0.005e(-), indicating that the O/Ag(110) bond is polar covalent in character. Since the observed frequency shift cannot result from simple dipole-dipole interactions alone, other physical origins are discussed.