Coverage grating template for the study of surface diffusion: K coadsorbed with CO on Re(001)

Abstract

The coverage grating method as a template for coadsorbates is introduced for the study of surface diffusion. The effect of coadsorbed CO inside coverage troughs formed by laser induced thermal desorption (LITD) on potassium surface diffusion on Re(001) has been investigated using the coverage grating optical second harmonic diffraction method. Enhancement of the first-order diffraction peak at a certain CO coadsorption coverage, observed for the first time, demonstrates the very strong electronic interaction between these coadsorbates. The activation energy for K surface diffusion and the pre-exponential factor significantly increase with CO coverage. At initial potassium coverage of 1.0 ML, the activation energy for potassium diffusion increases from 5.0 kcal/mol on the clean rhenium surface to 15.0 kcal/mol in the presence of 0.065 ML CO, and the pre-exponential factor increases from 5.6×10 −3 to 2.0×10 2 cm 2/s. The activation energy doubles for potassium coverages of 0.9 ML and 0.8 ML as a result of the same CO coverage. TPD measurements indicate that strong attractive interactions exist between CO and K on Re(001). Coadsorbed CO and K stabilize each other while forming K x –CO surface complexes which slow down the potassium surface diffusion predominantly by site-blocking effect. This site-blocking effect increases as the number of potassium atoms ( x) interacting with a single CO coadsorbate increases at higher initial K coverages. The nature of this site-blocking is discussed.