The reflectance and optical constants of evaporated osmium films were measured in the wavelength region from 300 to 2000 Å. The films were evaporated by electron bombardment in an ion-titanium-pumped vacuum system and deposited at a rate of about 50 Å/s onto glass and super-polished quartz substrates that were at 40 and 300 °C. The optical constants were determined from reflectance measurements made at several angles of incidence. Reflectance losses during extended exposure to air were rather small, indicating that oxide films formed at room temperature are very thin. Films made on substrates at 300 °C had only slightly higher reflectances than those made at 40 °C. Owing to interference effects, semitransparent films 150–250 Å thick on glass or quartz substrates showed higher reflectances than opaque films, throughout most of the vacuum-ultraviolet region. Osmium films prepared under optimum conditions had a reflectance of 34% at 584 Å. The fact that high reflectance can be obtained with films deposited onto unheated substrates makes osmium a very useful coating material for replica gratings, which cannot be heated.
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