Why does a mercury Franck-Hertz tube produce visible light?

[greypilgrim]

Hi.
I've recently conducted a Franck-Hertz experiment with mercury. I was able to see bluish glowing regions just as here:

However, theory predicts 254 nm, which is far below visible. Are there other energy levels at play here? Wouldn't that mess up the 4.9-V-spaces of the drops in the current vs. acceleration voltage diagram (they were clearly visible on the oscilloscope)?

 

[Klystron]

Mercury vapor rectifiers were used to convert high-current AC to DC before being replaced by solid state devices in the 1970's. From wikipedia:
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The mercury ions emit light at characteristic wavelengths, the relative intensities of which are determined by the pressure of the vapor. At the low pressure within a rectifier, the light appears pale blue-violet and contains much ultraviolet light.
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Technicians could use the visible light to measure rectifier conditions without disturbing operation.

 

[greypilgrim]

So is the number of those blue "layers" different from the invisible ones at 254 nm? Why do only the latter show up in the measured current, corresponding to the drops every 4.9 V?

 

[Klystron]

After reading several published versions of the Franck-Hertz 1914 experiment, your results appear to agree. My understanding is that the excited electron emits UV at ~254 nm, below the spectrum visible to human eye but can react with skin cells. The post-collision mercury vapor ions emit photons in visible light:

Mercury Hg Emission line spectrum
Wavelength (nm) Name Color
404.7 H-line violet
435.8 G-line blue
546.1 green
578.2 ...

This paper from Rutgers defines mercury vapor excited states https://www.physics.rutgers.edu/grad/506/franck-hertz.pdf

Replacing mercury vapor with neon in the experiment produces visible light photons while also eliminating harmful Hg vapors. Neon values appear after Hg in this paper http://hyperphysics.phy-astr.gsu.edu/hbase/FrHz.html#disc