Thermal desorption from ordered chemisorbed phases studied by helium scattering: Oxygen on Ag(110)

We relate the helium specular beam intensity during adsorbate desorption (He desorption curve! to the instantaneous surface coverage. In this way the He desorption curve is shown to provide a picture of the desorption process which can be fruitfully compared to the one coming from thermal desorption mass spectra (TDS), obtained under strictly comparable experimental conditions. We tested the combination of thermal energy atom scattering (TEAS) and TDS in the case of the associative desorption from long range ordered O(2 x 1)-Ag(110) phase. Using the so-called overlap approach and assuming intense adsorbate-adsorbate (and vacancy-vacancy) attractions along Ag-O-Ag rows we obtain an instantaneous coverage which is in good agreement with TDS results. He desorption curves confirm the extreme sharpness of the desorption transition further indicating that the tails of the TDS peak bear small contributions from oxygen which did not belong to the O(2 x 1) phase. Opportunities and limitations inherent to the use of the He desorption curve (and its first derivative) as a marker of the temperature position and sharpness of the desorption transition are also addressed. (C) 1999 American Institute of Physics. [S0021-9606(99)02004-8].