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I have been reading about collisional excitation in Physics for Scientists and Engineers by Knight. What I am confused about is that it says that collisional excitation is the predominant method of excitation in electrical discharges such as fluorescent lights, street lights, and neon signs.

An example is used of placing low pressure mercury gas in a tube. A high voltage between electrodes at each end of the tube causes the gas to ionise, creating a current in which both ions and electrons are charge carriers. If pressure is too high then the mean free path between electrons is too short for the electrons to gain enough kinetic energy to excite the atoms.

Later in the text the example of a high pressure sodium lamp is given, where an electrical discharge is created in sodium vapour.

I thought sodium lamps worked on the principle of collisional excitation, but in this case, wouldn't we require low pressure gas? Or do high pressure sodium lamps work differently?

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  • $\begingroup$ Did you compare the ionization energies of sodium and mercury? $\endgroup$
    – Floris
    Sep 10, 2017 at 2:37
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    $\begingroup$ @Floris thank you for the hint. The ionisation energy of sodium is lower than that of mercury. This means that the electrons in the sodium example require less kinetic energy to excite the atoms, and so collisional excitation is achieved with a mean free path less than that in the mercury example. So while a relatively low pressure gas is needed, this is, of course, relative. A sodium lamp can have a greater pressure than mercury in a gas tube, if my reasoning from your hint is correct. $\endgroup$
    – L. Maynard
    Sep 10, 2017 at 2:59
  • $\begingroup$ I think so yes. The term "high pressure" is relative... see edisontechcenter.org/SodiumLamps.html for some details. $\endgroup$
    – Floris
    Sep 10, 2017 at 14:32

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