# I Plasma ball, energy levels of neon and xenon

#### spareine

I was wondering about the filament colors in a plasma ball that I have got. The main part of each filament is blue, but its end part is pink. Possibly the gas is 95% neon and 5% xenon, the pink part is light emitted by neon, and blue part light emitted by xenon. Using a hand spectroscope I watched the spectrum of the pink color, which was identical to the spectrum of a neon lamp, between 585 and 700 nm. These spectral lines correspond in the energy level diagram of neon to the transition from level 2p (18.5 eV above ground state) to 1s (16.5 eV), so the colliding free electrons in the filament have been accelerated to a kinetic energy of 18.5 eV or more. To keep the filament alive some neon atoms have to be ionised (22 eV), so there are free electrons with a kinetic energy of 22 eV or more as well.

Questions:
1) is the direct deexcitation from ionised neon to ground state likely, so that extreme ultraviolet (22 eV, 60 nm) would be emitted inside the glass ball? Would the 1 mm glass wall block the emitted ultraviolet entirely?
2) is it likely that most neon atoms that made the transition from 2p to 1s were fully ionised before arriving at 2p? Is it likely that the transition from ionization to 2p does not emit light? And is it likely that the final transition from 1s to ground state does not emit light?
3) why are neon atoms emitting light only at the end part of the filament, versus xenon in the main part? May be it has to do with the lower ionisation energy of xenon, 12 eV, but I don't see how.

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This is interesting. To answer part 3, I believe the voltage between the electrodes would need to be just over 22 volts to have any appreciable amount of neon get ionized. The xenon probably needed to be in lower concentration in order to make this happen, or higher currents would result with attempts to increase the voltage, that wouldn't produce much neon ionization, and instead, a substantial increase in xenon ionization would occur. The voltage of the arc would be then limited to something in the neighborhood of 12 volts. With a low concentration of xenon, and 22+ volts between electrodes, the electric fields could be such that the 22+ volt region w.r.t. the cathode is achieved in the vicinity of the anode, so that if an electron was able to go from cathode to near the anode without yet undergoing a major collision, it would be able to ionize the neon. The actual physics inside the arc lamp may be somewhat more complex than this, but I think I have a satisfactory answer. $\\$ One additional comment: Perhaps the neon doesn't need to get ionized at all=the xenon could be furnishing the vast majority of the ionized electrons. It is possible it runs at about 19 volts. $\\$ Also a google of the neon spectrum shows you may have observed the 632.8 nm transition from 5s to 3p. See https://en.wikipedia.org/wiki/Helium–neon_laser#mediaviewer/Fileene-2.png $\\$ See also page 4 of http://web.physics.ucsb.edu/~phys128/experiments/laser/LaserFall06.pdf $\\$ Alittle more googling showed the following results, so apparently the 632.8 (6328 Angstroms) line is not one of the brightest lines of a neon arc lamp: https://physics.nist.gov/PhysRefData/Handbook/Tables/neontable2.htm This listing shows about 4 rather intense lines between 6900 and 7300 Angstroms. It would appear the energy diagrams of the first two "links" are rather incomplete and simply emphasize the 632.8 nm, 1.15 um, and 3.39 um transitions.

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