Characterisation of a Low Temperature Surface Phase of CO on Pt{110}

Abstract

Adsorption of CO on a Pt{110} (1 × 2) surface precooled to 160 K is found to proceed without loss of the (1 × 2) reconstruction, in contrast to adsorption at higher temperatures. Using high resolution electron energy loss spectroscopy, we find that adsorption occurs initially into a linear species, with the additional occupation of bridged sites at fractional coverages above 0.2. At saturation, the ratio of bridged to linear species is 1:3. The absorption intensity from the linear species is shown to be 2.5 times that of the bridged species. Heating the saturated surface to temperatures between 280 and 340 K results in an irreversible transformation of the bridged species to linear, and a lifting of the (1 × 2) reconstruction to (1 × 1). The orientation of CO molecules in the low temperature metastable phase on the (1 × 2) surface was determined using angle-resolved photoemission, with electron collection in the plane orthogonal to the incidence plane. The CO molecules are found to be tilted away from the macroscopic surface normal, along the [100] azimuth, by 15 ± 5°. This contrasts with our previous findings for the stable phase of CO on the annealed (1 × 1) surface, where the tilt angle was found to be 26 ± 2° along the [211] azimuth.