A model to describe surface physics in extreme ultraviolet lithography (EUVL) optics mirror contamination is proposed. Photon-induced desorption is a key process in determining the surface areal density of the related adsorbed gas species when irradiating power is high. Scaling law for acceleration test is analyzed and experiment was performed by using synchrotron undulator radiation. It appears that oxidation on a Si capping layer is almost anti-linearly dependent on photon intensity and acceleration test can be conducted by increasing the H2O pressure linearly with photon intensity increase. In contrast, it was found that carbon deposition 5oes not linearly scale to photon intensity. Special care is necessary in understanding results conducted in accelerated conditions as well as under different pulse duty conditions. (c) 2005 American Vacuum Society.