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

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Discussion Overview

The discussion centers around the Franck-Hertz experiment conducted with mercury, specifically addressing the production of visible light despite theoretical predictions indicating emission at 254 nm, which is ultraviolet. Participants explore the implications of energy levels and the nature of light emitted during the experiment.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes the observation of bluish glowing regions during their Franck-Hertz experiment and questions the presence of other energy levels that could account for visible light emission.
  • Another participant references historical use of mercury vapor rectifiers, explaining that the light emitted by mercury ions at low pressure appears pale blue-violet and contains ultraviolet light, which could relate to the experiment's findings.
  • A question is raised about the relationship between the visible blue "layers" and the invisible emissions at 254 nm, particularly regarding their impact on the current measurements and the observed voltage drops.
  • One participant discusses the emission spectrum of mercury, detailing specific wavelengths of visible light emitted by excited mercury ions and suggesting that while the initial excitation may produce UV light, subsequent emissions can be in the visible range.
  • The possibility of using neon instead of mercury in the experiment is mentioned, highlighting that neon also produces visible light while avoiding the hazards associated with mercury vapor.

Areas of Agreement / Disagreement

Participants express differing views on the mechanisms behind the visible light production in the Franck-Hertz experiment with mercury. There is no consensus on the exact relationship between the observed light and the theoretical predictions regarding energy levels and emissions.

Contextual Notes

Participants reference various sources and papers to support their claims, but there are unresolved questions regarding the interaction between different energy levels and the implications for current measurements in the experiment.

Who May Find This Useful

This discussion may be of interest to those studying atomic physics, experimental methods in physics, or the historical context of the Franck-Hertz experiment.

greypilgrim
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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)?
 
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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:
----------------------------
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.
 
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?
 
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
 

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