CO STRETCHING VIBRATIONS ON PT(111) AND PT(110) STUDIED BY SUM-FREQUENCY GENERATION

Optical sum-frequency generation (SFG) is used to characterize CO stre tching vibrations on Pt(111) and Pt(110) surfaces, Different adsorptio n sites (terminal bridge and step sites) are identified in the SFG spe ctra of CO on Pt(111), in good quantitative agreement with previous in frared relation-absorption experiments on this system. For CO on Pt(11 0) we only observe CO molecules on terminal sites. The measured CO str etching vibration frequencies on Pt(110), both for low and high covera ges, are at variance with the results of previous infrared studies. Ou r SFG results for CO on Pt(110) art confirmed by independent EELS meas urements which, in addition, also reveal the frustrated rotational mod e and the metal-CO vibration, The measured frequency of 2065 cm(-1) fo r low CO coverage on Pt(110)-(1x2) is consistent with a previously pro posed empirical relation between the frequency of an isolated adsorbed CO molecule and the coordination number of the binding Pt surface ato m.