To essentially wrap up our work on the home-made Fabry-Perot project, we improved the optics a bit so that we could observe the Zeeman effect in mercury. Mercury was harder than the previous study of helium because the structure of the splitting is more complicated, with many closely spaced lines.
The spectrum of mercury on its own is quite simple with a small number of bright lines in the visible: blue, green and a pair of closely spaced yellow lines. The spectrum below was taken with our home-made grism spectrometer and the same Canon 100D camera used in the Fabry-Perot experiments (so the same spectral response). The principal lines of mercury are (in Angstroms): blue 4358, green 5461, yellow 5770 and 5791.
To improve the collimation we used a 1/4-20 washer as an aperture (6.5mm diameter) at the front of the permanent magnet / discharge tube setup previously described. This aperture was positioned 35cm away from a 1.2mm-wide cardboard slit (i.e. approximately f54). The slit was positioned at the focal point of a 100m f4 bellows lens to give a collimated beam to the Fabry-Perot. A 600 line/mm transmission grating was positioned between the etalon and the imaging camera lens. A linear polarising filter was attached to the front of the camera lens. Two focal lengths were used, depending on how wide a spectrum was to be recorded (100m f2.8 and 150mm f4).
With the polariser parallel to the magnetic field, the blue and green mercury spectral lines split into two and three lines, as expected. The two closely spaced yellow lines did not show splitting, also as expected.
We created a gif that combines the parallel and perpendicular polarisations. The perpendicular field splitting agrees approximately with expectations, although our resolution may not be quite high enough to resolve the blue lines. The Fabry-Perot mirror spacing may need to be to be adjusted to see the splitting in the blue; the spacing was set in order to see the red helium line.
Here is a gif with the helium Zeeman splitting done with the same setup as for the mercury (with the exception that the helium was taken with the 100mm lens and the mercury with 150mm). The helium lines appear to show only the "normal Zeeman effect", i.e. splitting into a triplet. However, the broadening of the yellow and violet lines may in fact be showing something more complex: the transition to the Paschen-Back effect in the moderately high field of approximately 1 Tesla.
In terms of things left to do, we may eventually attempt to see the longitudinal Zeeman effect; this will require drilling a small hole through our 25mm thick steel magnetic yoke. We already have a 25mm diameter neodymium magnet with a 6mm hole along its axis. When viewed longitudinally, some of the Zeeman lines will be polarised circularly instead of linearly (in fact, the linear polarisation is just the circular rotation viewed 90 degrees rotated).
Zeeman effect in mercury using the home-made Fabry-Perot (with gifs!)
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Re: Zeeman effect in mercury using the home-made Fabry-Perot (with gifs!)
For those who want to understand the mercury Fabry-Perot lines better, I attach some diagrams taken from an excellent student science poster [Casey Wall, The Zeeman Effect in Mercury (2011). Summer Research. Paper 83]. The work was done with a high-resolution spectrometer using a 20-cm wide echelle grating and a 12-inch f8 mirror in the Ebert configuration [see Optical Spectroscopy and the Zeeman Effect, (2012) Greg Elliott, University of Puget Sound].
Note to Peter Z: I believe the two closely spaced yellow lines provide a built-in calibration that you had previously suggested doing with the sodium doublet.
Note to Peter Z: I believe the two closely spaced yellow lines provide a built-in calibration that you had previously suggested doing with the sodium doublet.
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Re: Zeeman effect in mercury using the home-made Fabry-Perot (with gifs!)
Wonderful stuff! What a bonanza of results.
Where to go from here? It would be interesting to plot out a few of these interferograms and see how well the observed Zeeman pattern matches the theory. The green and yellow Hg patterns seem especially sharp in the gif. Do you have full resolution image files for these? Maybe it would make sense to try to measure the B field accurately from the He data then use this field strength to calculate the more complex Hg patterns and compare with observations. With the yellow Hg interferogram available as calibration data, converting the observed splittings in pixels to wavelength splittings should be straightforward.
Nicely done!
Peter
Where to go from here? It would be interesting to plot out a few of these interferograms and see how well the observed Zeeman pattern matches the theory. The green and yellow Hg patterns seem especially sharp in the gif. Do you have full resolution image files for these? Maybe it would make sense to try to measure the B field accurately from the He data then use this field strength to calculate the more complex Hg patterns and compare with observations. With the yellow Hg interferogram available as calibration data, converting the observed splittings in pixels to wavelength splittings should be straightforward.
Nicely done!
Peter
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Re: Zeeman effect in mercury using the home-made Fabry-Perot (with gifs!)
We did a little more work on the optics by positioning a plano-convex lens between the discharge tube and the slit. This allowed more light to be gathered in a shorter time, improving the signal to noise a bit. We also took the images in Canon RAW format, which turned out to have much less colour noise than jpg. Here is the full size image of the mercury Zeeman spectrum (with a polariser parallel to the magnetic field):
It is now possible to see a bit of splitting in the 5770 yellow line versus the 5791 orange line (see energy level diagrams above). For completeness, here is an image of the helium spectrum taken under the same conditions as the mercury (with no polariser):
With the better resolution it is more obvious that the red and green lines show the "normal" Zeeman effect while yellow and blue lines show the "Paschen-Back" Zeeman effect.