Modifications in desorption kinetics of physisorbed species induced by colour centres on NaCl(100)

Low-energy electrons (65 eV) were used to produce anion vacancies (colour c entres) at temperatures below 100 K in epitaxial thin films of NaCl(100) wh ich were grown about ten monolayers thick on a Ge(100) substrate. A low-ene rgy electron diffraction profile analysis of the NaCl diffraction spots dur ing electron bombardment revealed that the defects created are randomly dis tributed across the NaCl surface at these temperatures. Whereas at low elec tron exposures predominantly colour centres are produced, at high exposures molecular vacancies are also formed. We present data of the desorption kin etics of physisorbates on both the undamaged surface and the damaged surfac e in the regime of dominant colour centre formation carried out by isosteri c heat analysis and the leading-edge method. Zeroth-order desorption kineti cs is found in the full range up to one monolayer, except for the first few percent, with constant parameters of desorption. The presence of colour ce ntres causes a change in desorption kinetics for the physisorbed noble gase s Ar, Kr and Xe and of CO2 from zeroth- to first-order desorption in the lo w coverage regime up to 0.2 ML, providing evidence that the equilibrium bet ween the two-dimensional (2D) gas and the 2D condensate is effectively dest royed. On the defect sites the atoms and also CO2 are bound more strongly b y 20-25% compared with the perfect NaCl(100) surface. From the experimental data and from simulations, a saturation concentration of colour centres on this surface of 10% is deduced. (C) 2000 Elsevier Science B.V. All rights reserved.