Coadsorbate effects on adsorbate vibrational properties

A quantitative analysis of infrared absorption spectra to determine coadsorbate induced relative changes of the vibrational polarizability αv of an adsorbate mode and of the dielectric screening ϵ due to this extra species is presented. Four (ternary) coadsorption systems consisting of the Ru(0 0 0 1)-(2 × 2)-(X + CO + O) layer with additional coadsorbates X = H, NO, CO, or O (all of them occupying the remaining empty fcc site) have been studied with FT-IRAS, TDS, LEED and work function change measurements. On-top CO is thereby used as a probe molecule to monitor coadsorbate effects on the dielectric properties of the layer. The vibrational polarizability αv associated with the internal C–O stretch mode (νC–O) of on-top CO is lowered by all coadsorbates. The dielectric screening ϵ within the adsorbate layer is reduced in the presence of the atomic coadsorbates O and H whereas an increase of ϵ is found for the molecular coadsorbates, threefold coordinated CO and NO. The derived changes of αv and dielectric screening ϵ, as well as the involved line shifts of νC–O and νRu–CO can be understood in terms of the standard Blyholder backbonding model, i.e. CO 5σ charge donation to the metal combined with a backdonation to electronic states with 2π∗ character.