Thermal desorption mass spectrometry of alkali metal atoms from transition metal surfaces. The influence of coadsorbed oxygen

Abstract

The coverage dependence of the activation energy of desorption for a planar array of electrical dipoles has been calculated in order to understand thermal desorption traces of alkali metal atoms from transition metal surfaces. The successful comparison between the computed spectra and available experimental data for K/Fe(110), K/Fe(100), K/Fe(polycrystalline), and K/Pt(111) allows us to demonstrate that the mutual dipolar repulsion within the adlayer is responsible for the coverage dependence of the thermal desorption spectra (TDS) characteristic of the above mentioned systems. Shifts of the TDS peaks up to about 450 K due to the lowering of ∼50% in the energy of desorption are well described by the proposed model. Based on these results a simple and accurate method to assess the coverage dependence of the desorption energy is presented. The coadsorption of electropositive and electronegative atoms has been modeled assuming a planar array of electrical dipoles of opposite directions. It is demonstrated that oxygen atoms substantially reduce the repulsion energy on the alkali adlayer which becomes thermally stabilized in agreement with published experimental results. In fact, for the systems K+O/Fe(polycrystalline) and K+O/Pt(111) the onset of K desorption is found to be shifted by about 200 K due to the presence of coadsorbed oxygen atoms.