We passed a 399nm laser through the cathode tube and into a photodiode. The signal from the photodiode was sent to a pre-amp and into an oscilloscope. We frequency scanned the laser between two known frequencies and captured the resulting signal on the oscilloscope.
On our first attempt we did not use any averaging or filters. Here is a plot of the data we managed to gather.
Next we attempted to get a better signal. We used a 30MHz low-pass filter and averaged the signal 4 times. We were able to obtain a much cleaner signal.
It is interesting to note the location of the 168Yb isotope. It has a natural abundance of roughly 3% and has an isotope shift that puts it outside of the broadened spectral lines of the other isotopes. If more work were done on trying to observe the amount of absorption that is due to that one isotope it may be possible, using the Beer-Lambert law, to get a good estimate of the total density of Yb in the tube.
I overlaid the over the data obtained in the experiment. The simulated 1GHz Doppler broadening seems to fit quite well with the data. I attempted to fit a similar Lorentzian curve to the data.
The Gaussian curves seem to fit the data better than the Lorentzian curves. This means that the spectral broadening is most likely due to Doppler effects rather than pressure and collisional effects.