Why are the spectral lines weak in my hydrogen Geissler tube?

[Orthoceras]

TL;DR

Why are the spectral lines weak in the light from my hydrogen geissler tube, compared to the continuous background? What is the source of this continuous background?

Using a hand spectroscope, I looked at the spectrum of two geissler tubes, helium and hydrogen. The helium spectrum is what I expected, dominated by five distinct lines. The hydrogen spectrum, however, is somewhat disappointing. It is dominated by an almost continuous spectrum, it is not evident where the H-α, H-β, H-γ lines are. Is this normal for a hydrogen geissler tube? Is it because hydrogen is composed of molecules instead of atoms?

 

[Drakkith]

Is this normal for a hydrogen geissler tube? Is it because hydrogen is composed of molecules instead of atoms?

My best guess is that the spectrum is normal for molecular hydrogen, but I can't say why you also have a background of broad-spectrum light. Perhaps it is scattering of the light from the spectral lines from the spectroscope? Even you helium spectrogram is showing significant 'scattered' light from its handful of strong spectral lines. Is your spectroscope clean and free of damage on its grating/prism and other clear pieces that the light travels through?

 

[Orthoceras]

A website selling geissler tubes compares their "new style geissler tube" to the "old style tubes". It says the old style tubes are inferior because "spurious emissions in the spectrum are of considerable number and intensity". Unfortunately, the website does not specify the cause of the spurious emissions. The spurious emissions look like the background in my hydrogen tube. However, at the same time I am starting to doubt the H-α is really present in my tube. I guess my tube has been damaged somehow, and doesn't behave like a hydrogen tube anymore.

 

[Drakkith]

However, at the same time I am starting to doubt the H-α is really present in my tube. I guess my tube has been damaged somehow, and doesn't behave like a hydrogen tube anymore.

What is the voltage of your tube? Note that H-alpha is an atomic transition, not a molecular one. The hydrogen in your tube has to be split apart and then excited if you want to see the alpha, beta, and gamma spectral lines of atomic hydrogen. It might be that a difference in voltage or pressure favors molecular emissions over atomic emissions.

Also, I think you have the new-style and old-style spectra reversed. The new style, according to the link, has greatly reduced 'spurious' emissions, and should be the first image.

 

[Orthoceras]

I used a cheap high voltage generator from ebay. When connected to the Geissler tube, it produces 5 kV pulses, and each pulse results in a discharge through the gas, at 60 mA during 2 μs. Maybe a traditional Ruhmkorff coil generator would produce a higher current and another pulse duration.

You are right, I accidentally reversed the new/old labels"

 

[Drakkith]

Well, I'm afraid that I can't help you any further, as I'm not familiar with these tubes. Best of luck though.

 

[Orthoceras]

Thanks

 

[tech99]

Maybe the glass fluoresces due to bombardment with particles.

 

[Tom.G]

Maybe the glass fluoresces due to bombardment with particles.

Not as far fetched as it sounds.

Many years ago I had access to a 10mW Argon Laser, 488nm Blue emission.

I was playing with some photographic filters that screw on to the front of a camera lens
.
There were a few UV-block filters (Skylight Filters) included in the mix. The higher-priced ones were fine, two of the bargain-basement cheapy filters glowed Red under the Blue Laser light!

Take-Away: The bargain-basement varieties should stay locked away in the basement.

Cheers,
Tom