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

Abstract

Low-energy electrons (65 eV) were used to produce anion vacancies (colour centres) at temperatures below 100 K in epitaxial thin films of NaCl(100) which were grown about ten monolayers thick on a Ge(100) substrate. A low-energy electron diffraction profile analysis of the NaCl diffraction spots during electron bombardment revealed that the defects created are randomly distributed across the NaCl surface at these temperatures. Whereas at low electron exposures predominantly colour centres are produced, at high exposures molecular vacancies are also formed. We present data of the desorption kinetics of physisorbates on both the undamaged surface and the damaged surface in the regime of dominant colour centre formation carried out by isosteric heat analysis and the leading-edge method. Zeroth-order desorption kinetics 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 centres causes a change in desorption kinetics for the physisorbed noble gases Ar, Kr and Xe and of CO 2 from zeroth- to first-order desorption in the low coverage regime up to 0.2 ML, providing evidence that the equilibrium between the two-dimensional (2D) gas and the 2D condensate is effectively destroyed. On the defect sites the atoms and also CO 2 are bound more strongly by 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.