A novel method for determining absolute coverages by temperature programmed desorption with application to the adsorption of CH₃I on MgO(100)

A new method is presented for conveniently determining the absolute coverage of molecular overlayers on solid surfaces using only temperature programmed desorption (TPD), i.e. no additional measurement techniques must be employed. The method is based on a calibration of the measured mass spectrometer signal during evaporation from multilayers of the adsorbate using an a priori calculation of the evaporation flux. The computation of the evaporation flux requires a knowledge of the molecule's equilibrium vapor pressure at the evaporation temperature and an assumption of near unity trapping/sticking probability for the vapor impinging on the condensed layer at this temperature. The method is illustrated using TPD of methyl iodide from the MgO(100) surface. Significant features in the TPD are discussed in light of the methyl iodide coverages obtained using the method.