LVII.The clean-up of hydrogen by magnesium

Abstract

1. A clean surface of magnesium will absorb at room-temperature something like one out of every one and a half million molecules of hydrogen incident on it. The hydrogen thus absorbed (“contact-gettered”) only temporarily contaminates the surface, diffusing inwards at an appreciable rate. While the contamination lasts, however, it retards the absorption of further hydrogen. 2. Hydrogen is also cleaned up during slow dispersal of magnesium from a heated source within the vacuum vessel. There is no evidence that this is anything other than contact-gettering by a series of freshly formed surfaces. 3. Magnesium contact-getters atomic hydrogen enormously faster than ordinary molecular hydrogen. 4. The clean-up of hydrogen on to a magnesium film may be greatly accelerated by passing an ionizing electric discharge through the gas. This effect is due to the formation of atomic hydrogen from the H₂ ⁺ ions when they are neutralized and to the relatively high rate at which atomic hydrogen is contact-gettered. 5. The quantity of hydrogen that may be taken up by magnesium is very great. At the end of a series of experiments carried out with one getter deposit it contained about one-sixth or one-seventh as many hydrogen atoms as magnesium atoms. 6. Gettered hydrogen may be re evolved from magnesium owing to a “vapour pressure” exerted by the cleaned-up gas. This vapour pressure varies rapidly with concentration and temperature. 7. Gettered hydrogen is also reliberated when the magnesium is bombarded by electrons or by CO⁺ ions. In the case of electron bombardment, the effect has been observed with velocities of impact as low as 4 volts.