Coadsorbate effects on adsorbate vibrational properties

A quantitative analysis of infrared absorption spectra to determine coadsorbate induced relative changes of the vibrational polarizability α_v of an adsorbate mode and of the dielectric screening ? 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 α_v associated with the internal C-O stretch mode (ν_C-O) of on-top CO is lowered by all coadsorbates. The dielectric screening ? within the adsorbate layer is reduced in the presence of the atomic coadsorbates O and H whereas an increase of ? is found for the molecular coadsorbates, threefold coordinated CO and NO. The derived changes of α_v and dielectric screening ?, as well as the involved line shifts of ν_C-O and ν_Ru-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.