Green plasma in xenon-nitrogen mixture
Channel: Carl Willis
Date: 2013-02-27

A low-pressure xenon arc discharge turns bright emerald green in the presence of a small amount of nitrogen gas.  The glass tube shown in this video is 1.5 inches diameter and 30 inches long, evacuated to a high vacuum with a turbomolecular pump and then backfilled with 1.00 torr high-purity N2 gas and 100 torr xenon gas.  The gas pressures are measured by two capacitance manometers--a low-range (1 torr) and a high-range (1000 torr).  Achieving a stable green color in an electric discharge has been a challenge to plasma artists for a long time.  This mixture of N2-Xe is one compelling solution.  I have found no other mixtures with anything resembling this color, including tests with oxygen, argon, perchloroethylene, ethane, carbon dioxide, and alcohol vapors and xenon.  (Perchloroethylene produces a dingy greenish-yellow color, but it decomposes in the discharge to form brown residues.) Air contains mostly nitrogen and will produce a less-saturated version of this green color with xenon.  Interestingly, N2-Kr and N2-Ar mixes do NOT show the green color.  In the N2-Xe green mixture, 1 torr is about right for the N2 partial pressure in my opinion.  At partial pressures below about 0.5 torr, the green substantially fades as the pressure drops.  At partial pressures of 3-4 torr N2, the filamentary arc characteristic of xenon becomes more diffuse and takes on a pinkish hue, which I think detracts from the green phenomenon.  Xe pressures between 50-70 torr are probably right for most decorative applications; Xe at 100 torr is harder to ignite but makes a very kinetic pattern as seen here.  Lower pressures of Xe tend to be dominated by some beautiful green diffuse glows.  Lastly, the green glow is also accompanied by an "afterglow" in this mixture.  It is fleeting rather than the long-lived glow of an N2-O2 mixture, but still easy to discern in a dark room.  The color appears identical to the green in the plasma.