The sample of Yb in the cathode lamp is not isotopically pure. The location of each of the isotopes' spectral lines will be slightly shifted. This paper [1] gives the isotope shift for Yb. They measured the spectral lines by using cool Yb atoms with a laser locked to an 87Rb isotope. I used the isotope shifts given in this paper and scaled them with the natural abundance of each isotope to simulate the spectrum,
Pressure broadening has a Lorentzian profile. I simulated the spectrum of the Yb isotopes for several values of broadening.
The simulation indicated that even with 100MHz line broadening, we should still be able to resolve spectral lines of individual isotopes. I included a plot of a 1GHz Lorentzian type broadening to compare with the expected 1GHz Gaussian Doppler broadening later.
Doppler broadening is described by a Gaussian distribution. Here is a plot of the results of my simulation for the expected value of the Doppler broadening.
My simulation indicates that we should be able to resolve individual isotope lines if we can compensate for the Doppler broadening.