CO ON PT(335) - VIBRATIONAL OVERTONES AND SITE DEPENDENCE OF THE VIBRATIONAL STARK-EFFECT

Electron energy loss spectroscopy is used to compare atop CO at two si tes on Pt(335): on the flat (111) terrace and on the step edge. The cr oss section for the C-O stretch overtone of the terrace species is not significantly larger than that of the edge species. Previous experime nts have shown that the vibrational frequency of terrace CO responds m uch less to applied electrostatic field than does edge CO, even though their ir cross sections are about the same. The present experiment sh ows that CO has similar molecular properties at the two sites: a chemi cal explanation for the different Stark tuning rates is ruled out. The difference could be due to spatial variation of the electric field. L ocal screening of almost all the static electric field at terrace site s is one possibility, but a simple jellium model is unable to explain the observed effect. Interaction of the molecular quadrupole moment wi th the spatially varying field near the surface is also important.