Abstract: An optical element of the present invention comprises a fluorite substrate 1, and a lanthanum fluoride film 2 formed directly on the substrate 1. The substrate 1, on which the lanthanum fluoride film 2 is formed, has a plane which is a (111) plane or a plane inclined by an angle within ±30 degrees, preferably within 15 degrees from the (111) plane. Therefore, the lanthanum fluoride film undergoes the crystal growth subjected to the C-axis orientation on the optical substrate. Since the lanthanum fluoride film is dense and has a small surface area, it scarcely involves the oxidation and hydroxylation areas as well as the structural defect. Therefore, it is possible to reduce the optical loss of the optical element in the vacuum ultraviolet region.

Abstract: According to the invention, a fluorination treated substance remarkably reduced in the light absorption loss of a substance to be treated, such as a fluoride thin film, a fluorination treatment apparatus capable of producing the fluorination treated substance, and a process for producing a fluorination treated substance can be provided. Disclosed is a process for producing a fluorination treated substance, comprising an enclosure step of enclosing a substance to be treated in a reactor and a fluorination reaction step of introducing a fluorine gas into the reactor to bring the substance to be treated into contact with the fluorine gas and thereby carry out fluorination reaction.

Abstract: An optical element of the present invention comprises a fluorite substrate 1, and a lanthanum fluoride film 2 formed directly on the substrate 1. The substrate 1, on which the lanthanum fluoride film 2 is formed, has a plane which is a (111) plane or a plane inclined by an angle within ±30 degrees, preferably within 15 degrees from the (111) plane. Therefore, the lanthanum fluoride film undergoes the crystal growth subjected to the C-axis orientation on the optical substrate. Since the lanthanum fluoride film is dense and has a small surface area, it scarcely involves the oxidation and hydroxylation areas as well as the structural defect. Therefore, it is possible to reduce the optical loss of the optical element in the vacuum ultraviolet region.

Abstract: A method for manufacturing an optical element includes replacing fluoride atoms at a surface of the optical element. The surface is imparted with properties of substantial prevention of charge retention and/or substantial prevention of charge build-up to help maintain transmissivity in the optical element.