A method for in situ protection of a surface (7a) of an aluminum layer (7) of a VUV radiation reflecting coating (6) of an optical element (4), arranged in an interior of an optical arrangement, against the growth of an aluminum oxide layer (8), including carrying out an atomic layer etching process for layer-by-layer removal of the aluminum oxide layer from the surface. The etching process includes a surface modification step and a material detachment step. At least one boron halide is supplied as a surface modifying reactant to the interior in pulsed fashion during the surface modification step. A plasma is generated at a surface (8a) of the aluminum oxide layer, at least during the material detachment step. The atomic layer etching process is performed until the aluminum oxide layer reaches a given thickness (D), or the aluminum oxide layer is kept below that thickness (D) by the process.
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