Passage of far-ultraviolet radiation (1050–2000 Å) through the imperfect vacuum maintained by mechanical and oil-diffusion pumps results in the buildup of films opaque to far-uv radiation. Such films quickly coat lamp windows and other optical surfaces. The films can be removed by washing with petroleum ether or by exposure of affected surfaces to a hydrogen gas discharge. Transmittance vs time studies support the view that the films are produced from photodissociated vacuum-pump oil molecules. Passage of 2×1015 photons cm−2 through a 2.5-cm gap maintained at 10−4 Torr pressure results in two films of 50% combined transmittance measured at about 1200 Å.
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