CO ON PT(335) AND PT(111) - VIBRATIONAL STARK-EFFECT AND ELECTRON-ENERGY LOSS INVESTIGATION

An earlier study of CO on the stepped Pt(335) surface found a surprisingly large difference between the Stark tuning rates of CO on the step edge and CO on the (111) terrace. We report results of two experiments designed to clarify the source of this difference. In one experiment IR spectroscopy in the range 1820-2020 cm-1 was used to measure the Stark tuning rate of CO on the flat Pt(111) surface. The external-field tuning rate of on-top CO at saturation coverage is (2.56 +/- 0.12) x 10(-7) cm-1/(V/cm), a factor two smaller than that of edge CO on Pt(335) at low coverage. The external-field Stark tuning rate of bridge-bonded CO on Pt(111) is about 50% larger than that of on-top CO. If one assumes the static and IR fields are screened equally, the local-field Stark tuning rate of on-top CO is (3.4 +/- 0.9) x 10(-7) cm-1(V/cm), a factor four smaller than for CO on Pt(111) at the aqueous double layer. One explanation for the smaller than expected Stark tuning rates of CO on Pt in UHV is excess screening of the static electric field. In another experiment, electron energy loss spectroscopy was used to study Co on Pt(335). The spectra show bridge-bonded CO on Pt(335) at saturation coverage and at 40% of saturation. This is the first observation of bridge-bonded CO on Pt(335). A combination overtone and double-loss peak was observed at saturation coverage, with intensity relative to the fundamental of 0.017 +/- 0.003, about a factor two larger than observed by Steininger et al. on Pt(111).