Patent Number: 
Section: claims

1. Mirror with an optically effective surface, comprising:a mirror substrate;a reflection layer system configured to reflect electromagnetic radiation that is incident on the optically effective surface; anda capping layer, which is arranged on a side of the reflection layer system that faces the optically effective surface and which is produced from a first material;wherein, either individually or in clusters, particles of a second material are applied onto the capping layer, wherein the second material differs from the first material, and wherein the second material is selected from the group consisting of noble metals and sulphur (S). 2. Mirror according to claim 1, wherein the noble metals comprise at least one of gold (Au), silver (Ag), palladium (Pd) and platinum (Pt). 3. Mirror according to claim 1, wherein the capping layer has a surface structure that comprises defects, and wherein the particles of the second material colonize the defects in the surface structure of the capping layer. 4. Mirror according to claim 1, wherein the particles are applied onto the capping layer such that, when compared to a mirror as claimed in claim 1 but not comprising the particles as recited in claim 1, a contamination of the capping layer during operation of or during transport of the mirror is less. 5. Mirror according to claim 1, wherein the first material is selected from the group consisting of metals, oxides, carbides, borides, nitrides and mixtures thereof. 6. Mirror according to claim 1, wherein the first material is ruthenium (Ru). 7. Mirror according to claim 1, wherein the capping layer has a thickness between 0.5 nm and 10 nm. 8. Mirror according to claim 1, wherein the clusters of the second material comprise no more than 25 atoms. 9. Mirror according to claim 8, wherein the clusters of the second material comprise no more than 15 atoms. 10. Mirror according to claim 1, wherein the reflection layer system is a multiple layer system. 11. Mirror according to claim 1, wherein the mirror has a reflectivity of at least 0.5 for electromagnetic radiation with a predetermined operating wavelength incident on the optically effective surface at an angle of incidence in relation to the respective surface normal of at least 65°. 12. Mirror according to claim 1, wherein the mirror is configured for an operating wavelength of less than 30 nm. 13. Mirror according to claim 1, configured for operation in a microlithographic projection exposure apparatus. 14. Optical system of a microlithographic projection exposure apparatus comprising an illumination device and a projection lens, wherein at least one of the illumination device or the projection lens comprises the mirror according to claim 1. 15. A particle-treated mirror with an optically effective surface, comprising:a mirror substrate;a reflection layer system configured to reflect electromagnetic radiation that is incident on the optically effective surface; anda capping layer, which is arranged on the side of the reflection layer system facing the optically effective surface and which is produced from a first material;wherein, either individually or in clusters, particles of a second material are applied onto the capping layer, wherein the second material differs from the first material, and wherein the particles are applied onto the capping layer such that, when compared to a further, untreated mirror configured as the particle-treated mirror but not comprising the applied particles, a contamination of the capping layer during operation of or during transport of the particle-treated mirror is less. 16. The particle-treated mirror according to claim 15, wherein the capping layer has a surface structure that comprises defects, and wherein the particles of the second material colonize the defects in the surface structure of the capping layer. 17. Mirror according to claim 15, wherein the clusters of the second material comprise no more than 25 atoms.