CO on Pt(335): Vibrational Stark effect, mode coupling, and local field effects on a stepped surface

Abstract

We have studied CO on Pt(335) using infrared diode laser spectroscopy and electric field modulation. The spectra cover the 2000–2160 cm −1 range. The Pt(335) surface is stepped, with (111) terraces four atoms wide. At low coverage only one species is observed: CO adsorbed on step edges. In terms of the applied field, the Stark tuning rate of a low coverage of this species is (6.0 ± 0.8) × 10 −7 cm −1/( V/ cm). At the same coverage, the screened dynamic dipole moment ~ e = (0.94 ± 0.11) e. The rate calculated from ~ e is (7.0 ± 1.1) × 10 −7 cm −1/( V/ cm), agreement with the measured value. At higher coverages te also observed. Terrace CO is similar to CO on Pt(111) in vibrational frequency, dynamic dipole moment and desorption temperature. However, the Stark tuning rate of terrace CO is at least an order of magnitude smaller than expected. We discuss why the tuning rate might be so small. When both species are present, the vibrational spectra are strongly affected by dipole-dipole coupling. As a result, the electroreflectance spectrum is not the derivative of the polarization modulation spectrum. The spectra are analyzed using a phenomenological coupled-oscillator model. Fitting the spectra to the model, we find that for CO adsorbed on step edges, the Stark tuning rate in terms of local field is(7.5 ± 2.0) × 10 −7 cm −1/( V/ cm), independent of coverage. The saturation coverage clean surface at 300 K is (8.3 ± 0.6) × 10 14 cm −2. With saturation coverage of CO the linewidth is very small, only 4.5–5.0 cm −1 FWHM, even though the sample is at 300 K. Also, while cleaning the surface, we observed a correlation between Sn coverage and saturation coverage of CO. One possible explanation for the correlation, although not proven, is that Sn blocks CO adsorption sites. If so, the data suggests that each Sn atom blocks 11 ± 6 CO adsorption sites, which may explain how Sn helps prevent CO poisoning of Pt electrodes in fuel cells.