A Penning mixture (Weston 1968, p. 334), (Bylander 1979, p. 65), named after Frans Michel Penning, 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 minute amount of another gas, one that has lower ionization voltage than the main constituent (or constituents).
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 the 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 or argon alone, and lowers the striking voltage at which the tube becomes conductive and starts producing light. The optimal level of argon is about 0.25%, but some of it gets adsorbed onto the borosilicate glass used for the tubes, so higher concentrations are used to take the losses into account; higher argon content is used in higher-power tubes, as hotter glass adsorbs more argon. The argon 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 small percentage 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.
Other kinds of Penning mixture include helium - xenon.
A Penning gas mixture consists of a rare gas containing impurity atoms possibly at very low concentrations. The impurity atoms have an ionization potential (Vion) which is lower than or equal to the metastable potential (Vmeta) of the parent noble gas. The Penning effect in a Penning gas mixture is the ionization by charge transfer (charge exchange) during collision between a metastable atom and a neutral atom which decreases the average energy to form an ion pair, e.g.
Cu + Armeta –> Cu++ Ar + e-1
In a glow discharge, this results in an increase of the the ionization coefficient (Townsend first coefficient), a decrease in breakdown potential and a lowering of the cathode fall potential. The magnetic Penning effect describes the increase of the ionization probability of gas in a low pressure electrical discharge resulting from the helical (spiral) movement of electrons in a magnetic field placed normal to the anode-cathode electrical field. (underline added) Source: Orange Book, p. 148
Penning gas mixtures for improving the energy resolution of proportional counters
An experimental investigation of the Penning effect in both argon and xenon has been carried out with a variety of quench gases of different ionization potentials. Acetylene with argon and trimethylamine with xenon show the strongest true Penning effect, returning very high gas gains for a given voltage and improved energy resolution over more conventional mixtures. Data for these mixtures, as well as others exhibiting weaker metastable effects, nonmetastable Penning effects, and charge transfer through photoionization, are presented. http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?reload=true&arnumber=34567
See Also
3.7 - Non-synchronized Voiding at Plane of Inertia Figure 7B.08 - Russells Periodic Chart of the Elements Figure 12.11 - Russells Locked Potential Full Ten Octave Gamut Inert Gas Ionization Mixture Motion-in-inertia Neutral Noble Gas Penning Ionization Titration