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

We relate the helium specular beam intensity during adsorbate desorption (H e 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 condit ions. We tested the combination of thermal energy atom scattering (TEAS) an d 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 inte nse adsorbate-adsorbate (and vacancy-vacancy) attractions along Ag-O-Ag row s we obtain an instantaneous coverage which is in good agreement with TDS r esults. He desorption curves confirm the extreme sharpness of the desorptio n transition further indicating that the tails of the TDS peak bear small c ontributions from oxygen which did not belong to the O(2 x 1) phase. Opport unities and limitations inherent to the use of the He desorption curve (and its first derivative) as a marker of the temperature position and sharpnes s of the desorption transition are also addressed. (C) 1999 American Instit ute of Physics. [S0021-9606(99)02004-8].