Measuring the Helium Spectrum: Friederich Paschen 1916

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SUMMARY

The forum discussion centers on the measurement of the fine structure of singly ionized Helium (He II) at approximately 4686 Angstroms, specifically identifying the 3d 5/2 to 4f 7/2 transition. The user suggests that the transition labeled Ia, IIc at 4685.803 A is likely the one in question. Despite extensive searches, the user finds a lack of serious measurements since Friederich Paschen's work in 1916, and they express interest in whether stellar spectra can accurately resolve the 4686 fine structure. The discussion also references recent measurements available through the NIST database.

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  • Investigate the NIST database for updated measurements of Helium spectral lines.
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  • Explore historical measurements of Helium spectra, focusing on Paschen's 1916 findings.
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neilparker62
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Need help interpreting measurement.
Re page 14 of following reference:

https://zenodo.org/record/1447321#.XSyx3z9LjIU
If I understand correctly this page is showing a set of measurements for the fine structure of singly ionized Helium at +- 4686 Angstrom. Can we figure out from this which specific transitions are being measured ? In particular I'm interested in establishing which represents the 3d 5/2 to 4f 7/2 transition. I think it's most likely the one labelled Ia, IIc at 4685.803 A.

Alternatively if there's another source of measured (not calculated!) values for this fine structure complex, please point me in the right direction. I have hunted high and low but it seems no serious measurement on the Helium Spectrum has been done since Paschen's in 1916 ?!

Or from astronomy experts: are we able to measure stellar spectra accurately enough to resolve the 4686 fine structure ? As well as other of "Bohr's Helium lines" which are generally transitions of He+ from energy levels above 4 to level 4. Paschen has measurements at +- 6560 A, 5411 A, 4859 A, 4541 A, 4338 A, 4199 A and 4100 A showing transitions from levels 6 to 12 (to level 4). Each of these is probably a fine structure "forest" itself but I would guess the readings taken are of the dominant component or perhaps an average of dominant components.
 
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phyzguy said:
Of course there have been more recent measurements. A good place to look is at NIST. The link below shows measurements of the lines you are asking about, and there are links to the references where the data was taken, as well as the energy levels for HeII.

https://physics.nist.gov/PhysRefData/Handbook/Tables/heliumtable2.htm
Many thanks but had already trawled NIST looking for data. If you follow the refs on 4686 lines, all you find is a reference to "unpublished calculations" - not measurements. I even confirmed that via an email to NIST.
 

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