Penning mixture
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A Penning mixture is a mixture of gases used in electric lighting or displaying fixtures. Although the popular phrase for the most common of these is a neon lamp, it's more efficient to have the glass tube filled not with pure neon, but with a Penning mixture, which is defined as a mixture of one inert gas with a tiny amount of another gas, one that has lower ionization voltage than the main constituent (or constituents).
[edit] Explanation
The other gas, a quench gas, has to have lower ionization potential than the first excited state of the noble gas. The energy of the excited noble gas atoms then can ionize the quench gas particles by energy transfer via collisions; known as Penning effect.
A very common Penning mixture of about 98-99.5% of neon with 0.5-2% of argon is used in some neon lamps, especially those rated at 110 volts. The mixture is easier to ionize than either neon and argon alone, and lowers the striking voltage at which the tube becomes conductive and starts producing light. The optimal amount of argon is about 0.25%, but some of it gets absorbed into the borosilicate glass used for the tubes, so higher concentrations are used to take the losses in account; higher amounts are used for higher-power tubes, as hot glass absorbs argon better. This argon content changes the color of the "neon light", making it slightly more yellowish.
A Penning mixture of neon and argon is also used as a starter gas in sodium vapor lamps, where it is responsible for the faint reddish glow before the sodium emission kicks in.
The Penning mixture used in plasma displays is usually helium or neon with few percents of xenon, at several hundred torr.
Penning mixtures with the formulas of argon-xenon, neon-argon, argon-acetylene, and xenon-TMA are used as filler gases in gaseous ionization detectors.
