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 co verage of molecular overlayers on solid surfaces using only temperatur e programmed desorption (TPD), i.e. no additional measurement techniqu es must be employed. The method is based on a calibration of the measu red mass spectrometer signal during evaporation from multilayers of th e adsorbate using an a priori calculation of the evaporation flux. The computation of the evaporation flux requires a knowledge of the molec ule'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 il lustrated using TPD of methyl iodide from the MgO(100) surface. Signif icant features in the TPD are discussed in light of the methyl iodide coverages obtained using the method.